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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

IMPROVING THE EPA MULTI-SECTOR

GENERAL PERMIT FOR

INDUSTRIAL
STORMWATER
DISCHARGES
Committee on Improving the
Next-Generation EPA Multi-Sector General Permit
for Industrial Stormwater Discharges

Water Science and Technology Board

Division on Earth and Life Studies

A Consensus Study Report of

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

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Support for this activity was provided by the Environmental Protection Agency. Any opinions,
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the views of any organization or agency that provided support for the project.

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Suggested citation: National Academies of Sciences, Engineering, and Medicine. 2019. Improving the
EPA Multi-Sector General Permit for Industrial Stormwater Discharges. Washington, DC: The National
Academies Press. doi: https://doi.org/10.17226/25355.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

The National Academy of Sciences was established in 1863 by an Act of Congress,

signed by President Lincoln, as a private, nongovernmental institution to advise
the nation on issues related to science and technology. Members are elected
by their peers for outstanding contributions to research. Dr. Marcia McNutt is
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The National Academy of Engineering was established in 1964 under the char-
ter of the National Academy of Sciences to bring the practices of engineering to
advising the nation. Members are elected by their peers for extraordinary contri-
butions to engineering. Dr. C. D. Mote, Jr., is president.

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established in 1970 under the charter of the National Academy of Sciences to
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is president.

The three Academies work together as the National Academies of Sciences,

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increase public understanding in matters of science, engineering, and medicine.

Learn more about the National Academies of Sciences, Engineering, and Medicine
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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Consensus Study Reports published by the National Academies of Sciences,

­Engineering, and Medicine document the evidence-based consensus on the
study’s statement of task by an authoring committee of experts. Reports typi-
cally include findings, conclusions, and recommendations based on information
­gathered by the committee and the committee’s deliberations. Each report has
been subjected to a rigorous and independent peer-review process and it repre-
sents the position of the National Academies on the statement of task.

Proceedings published by the National Academies of Sciences, Engineering, and

Medicine chronicle the presentations and discussions at a workshop, symposium,
or other event convened by the National Academies. The statements and opinions
contained in proceedings are those of the participants and are not endorsed by
other participants, the planning committee, or the National Academies.

For information about other products and activities of the National Academies,
please visit www.nationalacademies.org/about/whatwedo.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

COMMITTEE ON IMPROVING THE NEXT-GENERATION

EPA MULTI-SECTOR GENERAL PERMIT FOR
INDUSTRIAL STORMWATER DISCHARGES

ALLEN P. DAVIS, Chair, University of Maryland, College Park

ROGER T. BANNERMAN, Wisconsin Department of Natural Resources,
Madison (Retired)
SHIRLEY E. CLARK, Penn State Harrisburg
L. DONALD DUKE, Florida Gulf Coast University, Fort Myers
JANET S. KIELER, Denver International Airport, Colorado
JOHN D. STARK, Washington State University, Puyallup
MICHAEL K. STENSTROM, University of California, Los Angeles
XAVIER SWAMIKANNU, University of California, Los Angeles; California
Environmental Protection Agency, California Water Board, Los Angeles
Region (Retired)

National Academies of Sciences, Engineering, and Medicine Staff

STEPHANIE E. JOHNSON, Study Director, Water Science and Technology

Board
CARLY BRODY, Senior Program Assistant, Water Science and Technology
Board

NOTE: See Appendix G, Disclosure of Conflict of Interest.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

WATER SCIENCE AND TECHNOLOGY BOARD

CATHERINE L. KLING (NAS), Chair, Cornell University, Ithaca, NY

NEWSHA K. AJAMI, Stanford University, Palo Alto, CA
JONATHAN D. ARTHUR, Florida Geological Survey, Tallahassee
DAVID A. DZOMBAK (NAE), Carnegie Mellon University, Pittsburgh, PA
WENDY D. GRAHAM, University of Florida, Gainesville
MARK W. LeCHEVALLIER, Dr. Water Consulting, LLC, Morrison, CO
MARGARET A. PALMER, SESYNC—University of Maryland, Annapolis
DAVID L. SEDLAK (NAE), University of California, Berkeley
DAVID L. WEGNER, Woolpert Engineering, Tucson, AZ
P. KAY WHITLOCK, Christopher B. Burke Engineering, Ltd., Rosemont, IL

National Academies of Sciences, Engineering, and Medicine Staff

ELIZABETH EIDE, Director

LAURA J. EHLERS, Senior Program Officer
STEPHANIE E. JOHNSON, Senior Program Officer
M. JEANNE AQUILINO, Financial/Administrative Associate
COURTNEY R. DEVANE, Administrative Coordinator
BRENDAN R. McGOVERN, Research Assistant
CARLY BRODY, Senior Program Assistant

vi

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Acknowledgments

We would like to express our appreciation to the Jenelle Hill, EPA

following people who provided presentations or public Paul Hlavinka, Maryland Department of the
comment to the committee. Environment
Christopher Kloss, EPA
Fredric Andes, Barnes & Thornburg Matthew Lentz, California Stormwater Quality
William Ashton, Alaska Department of Association
Environmental Conservation Jeffrey Longsworth, Barnes & Thornburg
Jim Bachhuber, Brown and Caldwell James Maroncelli, Washington Department of
June Bergquist, Idaho Department of Environmental Ecology
Quality Melinda Pagliarello, Airports Council International–
Jim Bertolacini, Wisconsin Department of Natural North America
Resources Travis Porter, Washington Department of Ecology
Kevin Bromberg, Small Business Administration Bryan Rittenhouse, EPA
Seth Brown, Water Environment Federation and Edan Rotenberg, Super Law Group
National Municipal Stormwater Alliance Kenneth Schiff, Southern California Coastal Water
Patrick Burch, West Virginia Department of Research Project
Environmental Protection Brian Schweiss, Minnesota Pollution Control Agency
Margarita Chatterton, Rhode Island Department of Timothy Simpson, GSI Environmental
Environmental Management Richard Smith, Smith and Lowney
Brian D’Amico, Branch Chief, Environmental Brandon Steets, Geosyntec Consultants
Protection Agency (EPA) Office of Science and Eric Strecker, Geosyntec Consultants
Technology Rachel Urban, EPA
Sebastian Donner, West Virginia Department of Eric Van Genderen, International Zinc Association
Environmental Protection Jack Waggener, AECOM
Phillip Flanders, EPA David Wagger, Institute of Scrap Recycling
David Flores, Center for Progressive Reform Industries
Peter Ford, EPA Laurel Warddrip, CalEPA, California State Water
William W. Funderburk, Jr., Castellón & Funderburk Board
LLP Melissa Wenzel, Minnesota Pollution Control
Greg Gearheart, California Environmental Agency
Protection Agency (CalEPA), California State Ian Wren, San Francisco Baykeeper
Water Board

vii

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

viii ACKNOWLEDGMENTS

This Consensus Study Report was reviewed in Arthur W. Ray, City of Rockville
draft form by individuals chosen for their diverse per- Kenneth Schiff, Southern California Coastal Water
spectives and technical expertise. The purpose of this Research Project
independent review is to provide candid and critical Brandon Steets, Geosyntec Consultants
comments that will assist the National Academies of Eric Strecker, Geosyntec Consultants
Sciences, Engineering, and Medicine in making each Melissa Wenzel, Minnesota Pollution Control Agency
published report as sound as possible and to ensure that
it meets the institutional standards for quality, objectiv- Although the reviewers listed above provided many
ity, evidence, and responsiveness to the study charge. constructive comments and suggestions, they were not
The review comments and draft manuscript remain asked to endorse the conclusions or recommendations
confidential to protect the integrity of the deliberative of this report nor did they see the final draft before
process. its release. The review of this report was overseen by
We thank the following individuals for their review George Hornberger (NAE), Vanderbilt University,
of this report: and Michael Kavanaugh (NAE), Geosyntec Consul-
tants. They were responsible for making certain that
Peter deFur, Environmental Stewardship Concepts, an independent examination of this report was carried
LLC out in accordance with the standards of the National
Shawn Gibbs, Indiana University Academies and that all review comments were care-
Michael Hanemann, University of California, Berkeley fully considered. Responsibility for the final content
John Kosco, National Association of Home Builders rests entirely with the authoring committee and the
Gary Liberson, Gnarus Advisors, LLC National Academies.
Robert Pitt, University of Alabama

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Preface

Stormwater is dynamic and complex. Industrial stormwater monitoring in the MSGP. The committee
stormwater is only a subset of the stormwater universe, collected information from individuals and stakeholder
yet complexity is interwoven throughout its generation organizations representing various interests around the
and management due to the wide range of industrial United States and heard from several state industrial
classifications, the assortment of activities at specific stormwater permit regulatory agencies. Much has
industrial sites, the sizes of these industrial sites, and changed since the first MSGP with respect to under-
climate and weather variations. Regulation of industrial standing the science of stormwater and stormwater
stormwater through the Multi-Sector General Permit treatment, pollutant quantification, and toxicity. The
(MSGP) (EPA, 1995, 2000, 2008a, 2015d) provides committee considered these advancements and the
federal guidelines that attempt to balance protection sensitive balance of environmental protection with
of the environment without leading to excess burden business burden. In this report, the committee offers
on industry. Concerns related to industrial stormwater recommendations to address some of the challenges
and the MSGP were highlighted in a 2009 National of industrial stormwater, its discharge, and regulation.
Research Council (NRC, 2009) report on stormwater
in the United States.
In 2017, a committee was created by the National Allen P. Davis, Chair
Academies of Sciences, Engineering, and Medicine Committee on Improving the Next-Generation
through support by the Environmental Protection EPA Multi-Sector General Permit for Industrial
Agency to address several concerns related to the Stormwater Discharges

ix

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Contents

ACRONYMS AND ABBREVIATIONS xiii

SUMMARY 1

1 INTRODUCTION 9
The Clean Water Act and Industrial Stormwater Management, 9
Industrial Stormwater Monitoring in the MSGP, 11
Context for the Study, 18
Outline of the Report, 19

2 POLLUTANT MONITORING REQUIREMENTS AND

BENCHMARK THRESHOLDS 21
Assessment of Current MSGP Benchmark Monitoring, 21
Context of Recent MSGP Data, 22
Improving Pollutant Monitoring Requirements, 26
Adjusting Benchmark Threshold Levels, 31
Conclusions and Recommendations, 42

3 STORMWATER SAMPLING AND DATA COLLECTION 45

Challenges of Quantifying Stormwater Pollutant Discharge, 45
Recommended Improvements to Sampling and Analysis Protocols, 49
Tiered Approach to Monitoring, 53
Updating and Upgrading Current Methods of Data Management, 62
Conclusions and Recommendations, 64

4 CONSIDERATION OF RETENTION STANDARDS IN

THE MULTI-SECTOR GENERAL PERMIT 67
Stormwater Retention, 67
Retention Standards, 68
Merits of and Concerns About Retention for Industrial Stormwater, 69
Considerations for Retention at Industrial Sites, 72
Regulatory Context for Retention Standards, 77
Conclusions and Recommendations, 78
xi

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

xii CONTENTS

REFERENCES 81

APPENDIXES
A State Industrial Stormwater Permit Benchmark Monitoring
Comparison 89
B Lists of Pollutants from Which Industries Self-Identified the Need
for Monitoring in the 1992 Group Applications, Adapted from
EPA Form 2F, 1992 93
C Monitoring Parameters Required in Environmental Protection Agency
2015 Multi-Sector General Permit 97
D 2015 Multi-Sector General Permit (MSGP) Data Analysis 101
E Additional Data on Technical Achievability of Treatment
Stormwater Control Measures 137
F Biographical Sketches of Committee Members and Staff 149
G Disclosure of Conflict of Interest 153

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Acronyms and Abbreviations

AD activity description NAL numeric action level

AIM Additional Implementation Measure NEL numeric effluent limitation
NELAP National Environmental Laboratory
BAT best available technology Accreditation Program
BLM Biotic Ligand Model NetDMR Network Discharge Monitoring Report
BM benchmark NPDES National Pollutant Discharge
BMP best management practice Elimination System
BOD5 biochemical oxygen demand (5 day) NURP Nationwide Urban Runoff Program

CCL Contaminant Candidate List PAH polycyclic aromatic hydrocarbon

CEC cation exchange capacity PCB polychlorinated biphenyl
COD chemical oxygen demand
COV coefficient of variation QA/QC quality assurance and quality control
QISP Qualified Industrial Stormwater
DMR discharge monitoring report Practitioner
DOC dissolved organic carbon
SCM stormwater control measure
ELG effluent limitation guideline SIC standard industrial classification
EMC event mean concentration SMC Stormwater Monitoring Coalition
EPA Environmental Protection Agency SSC suspended sediment concentration
SWPPP stormwater pollution prevention plan
HDS hydrodynamic separator
TBEL technology-based effluent limit
IWTT Industrial Wastewater Treatment TDS total dissolved solids
Technology Database TMDL total maximum daily load
TOC total organic carbon
MS4 municipal separate storm sewer system TSS total suspended solids
MSGP Multi-Sector General Permit
WQBEL water quality-based effluent limit
NAICS North American Industrial Classification
System

xiii

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Summary

I
ndustrial stormwater is derived from precipitation tiveness at improving the quality of the nation’s waters.
and/or runoff that comes in contact with industrial That report recommended updates to outdated bench-
manufacturing, processing, storage, or material mark monitoring requirements and recommended the
overburden and then runs offsite and enters drainage use of more sophisticated sampling protocols. These
systems or receiving waters. In 1987, Congress sig- issues resurfaced in a recent settlement agreement made
nificantly expanded the National Pollutant Discharge between EPA, industries, and environmental groups
Elimination System (NPDES) program through regarding revisions to the nationwide MSGP for indus-
amendments to the Clean Water Act to include trial stormwater. As a result of this settlement agree-
industrial stormwater runoff conveyed through out- ment, EPA asked the National Academies of Sciences,
falls directly to receiving waters or indirectly through Engineering, and Medicine to convene a committee to
municipal separate storm sewer systems. This led to a study certain aspects of the industrial stormwater pro-
huge increase in the number of industrial facilities that gram, with an emphasis on monitoring requirements
needed to be permitted as point-source discharges. The and retention standards (see Box S-1). EPA will use the
Environmental Protection Agency (EPA) developed results of this study to inform its proposed revisions to
the Multi-Sector General Permit (MSGP) in 1995 to the 2015 MSGP, which are anticipated in 2020. The
provide permit coverage for the full range of industrial committee was not asked to analyze the financial costs
stormwater facilities, grouped by industrial activity. The of its recommendations; instead, EPA will assess the
2015 MSGP sets the requirements for industrial storm- costs of possible changes in its proposed revision of
water management and monitoring in areas where EPA the MSGP.
is the permitting authority, including most of Indian Although the 1995 MSGP was based on sound
country and some federally operated facilities, all U.S. scientific and public policy principles, the committee
territories, the District of Columbia, and four states found that many of the program elements have been
(Idaho, Massachusetts, New Hampshire, and New hampered by shortfalls in generating, considering, and
Mexico). The MSGP also serves as a model for states acting on new information. This has resulted in missed
with delegated permitting authority as they develop opportunities for refining the MSGP monitoring
their own industrial stormwater general permits. requirements in support of improved stormwater man-
The various industrial stormwater permitting agement. In this report, the committee recommends
requirements have come under scrutiny since the pro- updating MSGP benchmark monitoring requirements
gram’s inception. The 2009 National Research Council and thresholds using a periodic review process to incor-
report Urban Stormwater Management in the United porate the latest science and monitoring information
States stated that the industrial stormwater program has into each permit revision. Additionally, the committee
suffered from poor accountability and uncertain effec- recommends allowing more sophisticated monitor-

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

2 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

BOX S-1 Statement of Task

Three permit programs under the Clean Water Act are used to regulate discharges of stormwater to receiving
­ aters—one for municipalities, one for industrial facilities, and one for construction sites. Of these, industrial storm-
w
water is particularly challenging to control because of the wide range of industrial sectors that must be accounted
for, each of which produces a unique suite of contaminants in stormwater. The industrial stormwater permit program
includes a small number of individual facility permits as well as general permits that are issued to groups of industries
at state and federal levels. The current Multi-Sector General Permit (MSGP) for industrial stormwater covers more
than 2,000 facilities nationwide and is used as a framework for dozens of similar state programs.
The National Academies of Sciences, Engineering, and Medicine conducted a study to provide input to EPA as it
revises its MSGP for industrial stormwater. The National Academies’ committee was tasked to

1. Suggest improvements to the current MSGP benchmarking monitoring requirements. Areas to examine could
include
• Monitoring by additional sectors not currently subject to benchmark monitoring;
• Monitoring for additional industrial-activity-related pollutants;
• Adjusting the benchmark threshold levels;
• Adjusting the frequency of benchmark monitoring;
• Identifying those parameters that are the most important in indicating whether stormwater control measures
are operating at the best-available-technology or best-conventional-technology (BAT/BCT) level of control;
and
• New methodologies or technologies for industrial stormwater monitoring.

2. Evaluate the feasibility of numeric retention standards (such as volumetric control standards for a percent
storm size or standards based on percentage of imperviousness).
• Are data and appropriate statistical methods available for establishing such standards as both technology-
based and water quality-based numeric effluent limitations?
• Could such retention standards provide an effective and scientifically defensible approach for establishing
objective and transparent effluent limitations?
• What are the merits and faults of retention versus discharge standards, including any risks of groundwater
or surface water contamination from retained stormwater?

3. Identify the highest-priority industrial facilities/subsectors for consideration of additional discharge monitor-
ing. By “highest priority” EPA means those facilities/subsectors for which the development of numeric effluent
limitations or reasonably standardized stormwater control measures would be most scientifically defensible
(based on sampling data quality, data gaps and the likelihood of filling them, and other data quantity/quality
issues that may affect the calculation of numeric limitations).

ing methods, training, and support for enhanced data the terms of the permit and appropriately managing
analysis tools within the MSGP. The committee rec- stormwater on site to minimize discharges of harm-
ommends risk-based tiered monitoring requirements to ful stormwater pollutants to the local environment.
improve the quality of data from the largest, high-risk Under the MSGP, all industrial facilities are required
facilities, while moderating the burden on the lowest- to conduct quarterly site inspections performed by the
risk facilities. The major conclusions and recommenda- permittee, and approximately 55 percent of permittees
tions are summarized below. are required to conduct chemical-specific benchmark
monitoring through quarterly grab samples. If the
POLLUTANT MONITORING average of the four quarterly samples exceeds the EPA-
REQUIREMENTS AND established benchmark threshold, monitoring must be
BENCHMARK THRESHOLDS continued for another year. Sampling continues until
the facility’s data show 1 year in which the average of
The primary purpose of the MSGP monitoring the four quarterly samples meets the benchmark. A
program is to ensure that industries are complying with benchmark exceedance (based on an average of four

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

SUMMARY 3

samples) is not a permit violation, unless no corrective toxicological information. Where data gaps remain,
action is undertaken and exceedances persist. Chapter 2 additional s­ ector-specific data-gathering efforts should
includes recommendations to improve the benchmark be initiated.
monitoring requirements and thresholds to improve EPA should update the MSGP industrial-sector
industrial stormwater management. classifications so that requirements for monitoring
EPA should require industry-wide monitoring extend to nonindustrial facilities with activities simi-
under the MSGP for pH, total suspended solids lar to those currently covered under the MSGP. Many
(TSS), and chemical oxygen demand (COD) as basic facilities and activities generating pollutants of concern
indicators of the effectiveness of stormwater control in stormwater discharges are not included within the
measures (SCMs) employed on site. These parameters MSGP because the facilities themselves are not consid-
can serve as broad indicators of poor site management, ered to be industrial, even though the on-site activities
insufficient SCMs, or SCM failure, which can lead (and associated risks) are similar `to those of regulated
to high concentrations of these and other pollutants. facilities. These include school bus transportation
Industry-wide monitoring of pH, TSS, and COD facilities and fuel storage and fueling facilities. Some
would also provide a baseline understanding of indus- states have included these activities in their existing
trial stormwater management across all sectors. All per- industrial general permits. EPA should examine other
mitted facilities are currently required to conduct visual facilities with activities similar to regulated facilities
monitoring of quarterly stormwater samples, and these and add them to the MSGP so that pollutant risks from
additional analyses are relatively inexpensive, minimiz- these facilities can be appropriately reduced.
ing the additional monitoring cost burden. Replace- Benchmarks should be based on the latest toxicity
ment of COD with total organic carbon (TOC) should criteria designed to protect aquatic ecosystems from
be considered once EPA has adequate data to develop adverse impacts from short-term or intermittent expo-
a benchmark threshold level. sures, which to date have generally been acute criteria.
EPA should implement a process to periodically Aquatic life criteria are designed for protection against
review and update sector-specific benchmark moni- both short-term (acute) and long-term (chronic) effects
toring requirements that incorporates new scientific on both freshwater and saltwater species. Studies that
information. This process should consider updated form the basis of criteria development typically measure
industry fact sheets, published literature and industry acute end points following exposure of aquatic life to
data, advances in monitoring technology, and other consistent pollutant levels for short periods of time,
available information, so that the monitoring programs and measure chronic end points following exposure
adequately address the classes of pollutants used on site of aquatic life to consistent pollutant levels for longer
and their potential for environmental contamination. periods of time. Given the episodic nature of stormwater
The committee reviewed several sectors where data flow and the likelihood of instream dilution and attenu-
suggest that stormwater pollutants are common, but ation, aquatic life criteria based on short-term (acute)
little or no benchmark monitoring is required. In some or intermittent exposures are typically more appropriate
cases, this situation resulted from limitations in the for stormwater benchmark threshold levels than criteria
original process where industries self-determined what based on long-term (chronic) exposures. Where EPA
pollutants to monitor in their group applications, and identifies substantial chronic risks to aquatic ecosystems
those results were then analyzed to develop benchmark from intermittent exposures during criteria develop-
monitoring requirements. Additional information and ment, such as for contaminants that bioaccumulate, an
data gathering for polycyclic aromatic hydrocarbons equation should be provided to translate chronic criteria
(PAHs) could help EPA determine if benchmark for intermittent exposures. In this context, EPA should
monitoring is needed for sectors that have the potential
to release PAHs. Periodic monitoring reviews would • Develop acute aquatic life criteria for benchmarks
allow EPA to assess changing industry practices that where they do not currently exist, or where substan-
could affect monitoring needs, new analytical tech- tial chronic risks to aquatic ecosystems exist from
nology for pollutant quantification, as well as current repeated stormwater exposures, develop equations

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

4 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

to translate chronic criteria based on intermittent Limited data suggest that benchmark compliance is
exposures. more difficult at industrial sites for iron, aluminum,
• Revisit the application of three benchmarks (iron, copper, and soft-water conditions for lead and zinc;
arsenic, and selenium) that are currently based on inadequate data are available for other pollutants. To
chronic and, in some cases, outdated aquatic life improve understanding of industrial SCM performance
criteria. and technical achievability:
• Allow permittees with repeated benchmark exceed-
ances to use the latest aquatic life criteria for • Industries and industry groups should collect
­selenium and copper to evaluate water quality risk scientifically rigorous performance data for com-
on a site-specific basis and discontinue comparisons mon SCMs under typical stormwater conditions
to national benchmarks, as appropriate. The latest to expand the knowledge base and inform future
criteria for selenium and copper include equations decision making. An appropriate number of storms
for calculating toxicity criteria based on short-term should be monitored by employing proper quality
exposure, using additional water chemistry and/or assurance and quality control to ensure data reli-
flow data. ability, and design and maintenance information
• Based on little evidence of adverse effects to for the SCMs should be provided.
aquatic organisms at common levels, suspend or • EPA should encourage industries to collect these
remove the benchmarks for magnesium and iron; data and make them publicly available, such as
benchmarks for these metals can be reinstated if/ uploading to the International Stormwater Best
when acute aquatic life criteria are established or Management Practices database.
benchmarks are developed based on chronic effects • EPA should support maintenance of these data
from intermittent exposure. for industrial stormwater, just as they are currently
• Express all benchmarks in the units from which supporting the Industrial Wastewater Treatment
they are derived, to improve communication and Technology national database.
reduce reporting errors and provide guidance on
the expected level of precision in reported results. For benchmarks based on aquatic life criteria, the addi-
tional high-quality data collected can be used to assess
Additional monitoring data collection on the the feasibility of achieving the benchmarks with current
capacity of SCMs to reduce industrial storm­water technology and practices. For technology-based bench-
pollutants is recommended to inform periodic marks, additional data could inform future benchmark
reviews of the benchmark thresholds and identify revisions to reflect the state of practice, reducing total
sectors for which new national effluent limits could loads to the extent practicable.
help address treatment attainability. Publicly avail- Because of the paucity of rigorous industrial SCM
able stormwater data from industrial sites are currently performance data, the development of new numeric
insufficient to determine if there are specific conditions effluent limitations (NELs) is not recommended for
under which industries cannot meet the benchmarks any specific sector based on existing data, data gaps,
using conventional stormwater treatment systems (e.g., and the likelihood of filling them. Any new NEL that
sedimentation, filtration) or if other nontreatment is developed would require extensive new data collection.
SCMs could reduce concentrations on these sites. Several sectors can be identified in recent MSGP data
Based on limited available SCM performance data, with recurrent high-concentration discharges. However,
it appears that most standard treatment SCMs can the decision to develop new numeric effluent limitations
meet the benchmark in at least 50 percent of storm would need to be informed by thorough SCM perfor-
events for TSS and for many pollutants at lower inflow mance data that clearly document attainability issues
concentrations associated with municipal stormwater. by sector and include a large number of permittees that
Considering that benchmark exceedance is judged by cannot achieve the benchmarks under the increased
the average of four sample events, these results suggest oversight of the additional implementation measure
that technical achievability is not a major issue for TSS. (AIM) process, which is currently in planning.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

SUMMARY 5

STORMWATER SAMPLING exceeding the benchmark compared to first-flush grab

AND DATA COLLECTION sampling. Composite sampling is not appropriate for
pollutants for which the results may vary over time with
The current MSGP benchmark monitoring storage, such as those that transform or degrade rapidly
requirement focuses on low-cost, coarse indicators or interact with the atmosphere (e.g., pH).
of site problems, and the usefulness of the data can Quarterly stormwater event samples collected
frequently be hampered by its variability. Stormwater over 1 year are inadequate to characterize industrial
monitoring data display variability that originates from stormwater discharge or describe industrial SCM
many different sources, including the variability of performance over the permit term. Under the MSGP,
precipitation within and among storms and changes in if a permittee’s average of four consecutive quarterly
operations over the course of time. In Chapter 3, the samples meets the benchmark, a waiver is granted for
committee discusses and recommends improvements the remainder of the permit term. For permittees with
in sampling design and procedures, laboratory analysis average results that meet the benchmark, the MSGP
protocols, and data management to reduce error and should require a minimum of continued annual sam-
improve the reliability of monitoring results to support pling, to ensure appropriate stormwater management
improved stormwater management. throughout the remainder of the permit term. Extended
EPA should update and strengthen industrial sampling over the course of the permit would provide
stormwater monitoring, sampling, and analysis pro- greater assurance of continued effective stormwater
tocols and training to improve the quality of monitor- management and help identify adverse effects from
ing data. Specifically, EPA should modifications in facility operation and personnel over
time. Given the natural variability and the limitations
• Consider a training or certificate program in of grab samples, substantial uncertainty is associated
stormwater collection and monitoring to ensure with using the average of only four stormwater samples.
that required sampling and data collection are EPA should analyze industrial stormwater data and
representative of stormwater leaving the site to the sector-specific coefficients of variation to recommend
greatest extent possible. additional increases in sampling frequency, consistent
• Stay abreast of advancements in monitoring, sam- with EPA’s determination of an acceptable level of error
pling, and analysis technology that can provide for this indicator of SCM performance. Additional
more or better-quality information for similar or continued monitoring at a lower intensity throughout
reduced costs and consider these in future revisions the permit would also increase the overall sample size
of the MSGP. and thereby reduce the uncertainty in the monitoring
results.
EPA should allow and promote the use of com- State adoption of national laboratory accredita-
posite sampling for benchmark monitoring for all tion programs for the Clean Water Act with a focus
pollutants except those affected by storage time. on the stormwater matrix and inter­laboratory cali-
EPA’s disallowance of composite sampling and reliance bration efforts would improve data quality and reduce
on grab sampling in the interest of discrete characteriza- error. NPDES laboratory accreditation programs and
tion of the highest pollutant concentration is not war- stormwater inter­laboratory calibration efforts would
ranted based on the methods used to derive benchmark improve the comparability and reliability of monitor-
thresholds. Multiple composite sampling techniques ing data. To support these efforts, EPA should publish
are available that provide more consistent and reliable guidance and case studies on interlaboratory calibration
quantification of stormwater pollutant discharges com- specifically focused on the stormwater matrix, includ-
pared to a single grab sample. Composite samplers have ing the establishment of performance quantification
become common in stormwater monitoring as experi- levels for stormwater samples. These efforts would
ence with this approach has increased and costs have promote similar procedures at a national level to ensure
declined, and the event mean concentrations that result the comparability and reliability of test results reported
from composite sampling may reduce the likelihood of to permitting authorities.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

6 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

To improve stormwater data quality while bal- This tiered system would improve the overall quality
ancing the burden of monitoring, EPA should expand of monitoring data to inform future iterations of the
its tiered approach to monitoring within the MSGP, MSGP while balancing the overall burden to industry
based on facility risk, complexity, and past perfor- and permitting agencies.
mance. The committee proposes four categories: To improve the ability to analyze data nation-
ally and the efficiency and capability of oversight by
1. Inspection only. Low-risk facilities could opt for permitting agencies, EPA should enhance electronic
permit-term inspection by a certified inspector or data reporting and develop data management and
the permitting authority in lieu of monitoring. visualization tools. Electronic reporting has only been
Facilities could be classified as low risk based on required of permittees since 2016, and the data man-
facility size (e.g., less than 0.5 or 1 acre of indus- agement capabilities are still developing to make the
trial activity), recognizing that size may not fully most use of this information at the national and state
represent the risk profile, or more accurately based levels. Automated compliance reminders, improved
on a detailed assessment of the type and intensity checks on missing or unusual data, and data analysis
of industrial activities conducted on site, or a hybrid and visualization capabilities would improve the effec-
approach. tiveness of staff oversight and provide new opportuni-
2. Industry-wide monitoring only. All facilities ties to analyze trends. EPA should develop national
in sectors that do not merit additional pollutant visualization tools that can be used to easily examine
monitoring would conduct industry-wide moni­ data for patterns, trends, and correlations.
tor­ing for pH, TSS, and COD. These data would
provide broad, low-cost indicators of the effective- CONSIDERATION OF RETENTION
ness of stormwater control measures on site. STANDARDS IN THE MGSP
3. Benchmark monitoring. Sectors that merit
additional pollutant monitoring, based on the Stormwater retention for infiltration or beneficial
most recent data and industry literature review, use minimizes pollutant loads to receiving waters and
would conduct sector-specific benchmark moni- reduces damaging peak flows while potentially increas-
toring in addition to pH, TSS, and COD which ing water availability. Yet, infiltration of industrial
would be collected by all facilities with chemical stormwater, which can contain hazardous pollutants in
monitoring. toxic amounts, can pose serious risks to g­ round­water;
4. Enhanced monitoring. Facilities with repeated these risks must be managed to prevent ground-
benchmark exceedances or those characterized by water contamination. Chapter 4 discusses scientific
the permitting authority as large complex sites with and regulatory factors affecting the applicability of
high pollutant discharge potential would conduct stormwater retention standards for industrial storm-
more rigorous monitoring, in consultation with the water. Based on the potential environmental benefits,
permitting authority. These facilities could collect particularly in areas of water scarcity, the committee
volume-weighted composite samples at multiple encourages the use of industrial stormwater retention
outfalls if appropriate. Additional tools and moni- with infiltration or beneficial use under conditions
toring strategies could be used to assess the water where groundwater is protected.
quality impact to receiving waters from stormwater Rigorous permitting, (pre)treatment, and moni-
discharge, including wet weather mixing zones, toring requirements are needed along with careful
dissolved metal sampling, and site-specific inter- site characterization and designs to ensure ground-
pretation of water quality criteria, with additional water protection in industrial stormwater infiltra-
guidance from EPA. EPA should develop “non- tion systems. In lieu of other information on the
representative storm” criteria to exclude monitor- attenuation of contaminants in groundwater before
ing for events that would not be representative of they are transported to the site boundary, infiltrated
facility stormwater discharge. water should be required to meet primary drinking
water standards for inorganic chemicals and organic

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

SUMMARY 7

chemicals, and secondary standards for chloride and the corrective action process associated with episodic
total dissolved solids. Water quality should be moni- exceedances of benchmark thresholds during bypass
tored and evaluated in the infiltration device or at the situations. This could be done through a number of
base of the vadose zone. Many water quality treatment regulatory measures, including a mixing-zone allow-
options are available, ranging from natural removal ance, establishment of allowable frequencies of storm-
employing in situ soils to standard SCMs to advanced water discharge at levels above benchmark threshold,
treatment. Industries considering infiltration should development of water quality standard exceedance
evaluate whether potential stormwater contaminants allowances for extreme weather events, or establish-
from routinely occurring pollutants as well as accidents ment of separate water quality criteria for major wet
and spills are compatible with infiltration and what weather events. Finally, EPA could develop guidance
technologies are required to remove these contaminants and cases studies for demonstrating that exceeding the
prior to infiltration. Chemicals covered by the Safe benchmark during storms with precipitation amounts
Drinking Water Act and unregulated chemicals with greater than the design storm do not result in an
known human health risks at concentrations of concern exceedance of water quality standards.
should be evaluated. Meeting stringent water quality EPA should develop guidance for retention and
requirements may make infiltration cost prohibitive infiltration of industrial stormwater for protection of
at sites with contaminants that pose a high risk of groundwater. The guidance should include informa-
polluting groundwater. Other factors influencing the tion on applied water quality, treatment offered within
feasibility of a retention and infiltration system include the infiltration zone, monitoring requirements, natural
the land available, soil infiltration rate, soil chemistry, attenuation of pollutants, groundwater use designa-
and depth to groundwater. tions, and possible impacts of pollutant dilution or
Site-specific factors and water quality-based mobilization in the subsurface. Because of the potential
effluent limits render national retention standards for risks to groundwater, industrial stormwater infiltration
industrial stormwater infeasible within the existing is not recommended in states that lack the legal author-
regulatory framework of the MSGP. Retention with ity to manage and enforce groundwater quality.
infiltration or beneficial use is already allowed within
the MSGP as one of many possible SCMs. However, OVERARCHING MESSAGE
the suitability of retention with infiltration or beneficial
use is based on site-specific factors that cannot be gen- An overarching theme within the report’s recom-
eralized nationally into retention standards. Issues such mendations is that the MSGP should incorporate the
as the design storm size, stormwater quality, receiv- best available science in the MSGP process. Science
ing water quality goals, and site conditions must be continues to improve our understanding of the envi-
known to ensure performance reliability. Additionally, ronmental and human health impacts of industrial
although retention could be designed using site-specific stormwater. Technologies for water quality monitoring,
factors as a technology-based effluent limit, industrial stormwater treatment, and modeling are advancing at
stormwater must also comply with water quality-based rapid rates, and new data can inform understanding
effluent limits, which are typically concentration based. of the performance of stormwater control measures.
It is impractical to design stormwater retention to cap- New tools are being developed to improve toxicologi-
ture all potential rainfall events, and for storm events cal assessments and data management and visualiza-
that exceed the design standard, discharge or bypass tion. As electronic reporting of industrial stormwater
will occur that may exceed the benchmarks. monitoring data becomes fully implemented and
EPA should consider incentives to encourage integrated for all states, large amounts of valuable
industrial stormwater infiltration or capture and industrial stormwater data will be available for analysis,
use where appropriate. The most significant incen- evaluation, and identifying areas for improvement. In
tive would be assurance that installation of infiltration general, EPA has been slow to adopt new knowledge
in accordance with EPA guidance for determining into its MSGP permit revisions, but the MSGP should
the appropriate design storm provides relief from not be a static enterprise. Both permitted facilities and

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

8 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

the nation’s waters would be best served by a progres-

sive and continuously improving MSGP based on
analysis of new data and focused data-gathering efforts,
advances in industrial stormwater science and technol-
ogy, and structured learning to develop and evaluate
permit improvements.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Introduction

S
tormwater is rainfall or snowmelt runoff, which the Environmental Protection Agency’s (EPA’s) Storm-
can occur as sheet flow or flow in a conveyance water Program that covered all sectors of the program,
system or downstream waterway. The Clean including municipal, industrial, and construction. This
Water Act, which was developed “to restore and main- study builds on that report, with a focus on industrial
tain the chemical, physical, and biological integrity stormwater monitoring and management.
of the Nation’s waters” (33 U.S.C. § 1251) regulates
stormwater in municipal, construction, and indus- THE CLEAN WATER ACT AND INDUSTRIAL
trial settings under the National Pollutant Discharge STORMWATER MANAGEMENT
Elimination System (NPDES) (40 CFR § 122.3)
permit program. Industrial stormwater is derived from The Clean Water Act requires that effluent limits
precipitation and/or runoff that comes in contact with be established to meet state-determined water q ­ uality
industrial manufacturing, processing, storage, or mate- standards. State water quality standards include desig-
rial overburden and then runs off site and enters drain- nated uses, which identify the uses or goals of each water
age systems or streams. Industrial stormwater does not body or segment (such as aquatic life, water ­supply, and
include direct discharges of wastewater or process water recreation), and numeric or narrative criteria that will
from facilities or stormwater associated with activities protect or restore the designated use. Effluent limits
exempted from the NPDES program, such as certain must consider both the technological capability to con-
agricultural activities. trol or treat the pollutants (technology-based effluent
The NPDES was created to provide a regulatory limits or TBELs) and limits necessary to protect the
framework for the control and elimination of the dis- designated uses of the receiving water (water quality-
charge of pollutants to surface waters to restore and based effluent limits or WQBELs).
maintain the integrity of the nation’s waters. This pro- TBELs are applied through nationally developed
gram was initially focused on reducing point-source dis- effluent limitation guidelines (ELGs). National ELGs
charges of pollutants from industrial process wastewater are developed by EPA through a rigorous process to
and municipal sewage into receiving waters, which are determine the effluent limits that are achievable using
more easily regulated because they emanate from iden- the best available technology within the economic
tifiable locations on a relatively consistent basis. The means of the industry. The development process
added regulation of stormwater in the NPDES program includes studies of pollutant levels, industry surveys,
has been challenging. Stormwater is produced through- and a detailed analysis of technological controls, plus
out a developed landscape, and its production and economic considerations. ELGs are then applied
delivery are episodic. In 2009, the National Research nationally so that there is no economic advantage to
Council (NRC) released a comprehensive report on operating and discharging pollutants in one state over

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

10 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

another. ELGs may be specific to process wastewater limited to relatively large industrial sites with other
discharge or to stormwater discharge, or may be applied discharges of process wastewater. At that time, a large
to both. Where national ELGs have not been estab- number of industrial stormwater discharges had been
lished, a permit writer may develop numeric effluent deemed to be nonpoint sources or sources of diffuse
limits for categories of industries based on best profes- pollution and were unpermitted. In 1987, Congress
sional judgment. However, these limits must withstand significantly expanded the NPDES program through
intense industry and public scrutiny as well as be tech- amendments to the Clean Water Act to include
nically defensible in a court of law and, therefore, are industrial stormwater runoff conveyed through out-
more likely at individual sites with extensive data rather falls directly to receiving waters or indirectly through
than in national or statewide general permits. municipal separate storm sewer systems. Congress
WQBELs are established to meet the designated provided timelines for expanding industrial stormwater
use objectives of individual receiving waters, which are permit coverage and required EPA to report back with
identified, for the most part, by states. Water quality information regarding classes of industrial stormwater
criteria form the basis for WQBELs. A number of discharges that were not widely permitted, the nature
complexities refine the designated use criteria, such as and extent of pollutants in those discharges, and pro-
specific types of fish and macroinvertebrate populations cedures and methods specific to industrial stormwater
expected in the water body, the level of exposure to discharge control. Congress also clarified that permits
pollutants in drinking water over a lifetime and accept- authorizing discharges of stormwater associated with
able cancer risk, and the type and frequency of human industrial activity were required to meet all applicable
immersion expected in a recreational water body. The provisions of the established permitting program,
amount of pollution that a water body can assimilate including TBELs and WQBELs.1
and still support beneficial use goals is defined through The congressional expansion of industrial storm-
adoption of water quality criteria. Most often, the crite- water permitting meant a large increase in the number
ria are pollutant specific and numeric and are designed of industrial facilities that needed to be permitted as
around a low-flow dry weather condition, with the idea point-source discharges. In 1990, EPA promulgated
that this condition represents the highest pollutant these requirements, including details around the use
concentration in a water body. However, stormwater of general permits for administrative efficiency. The
flows will occur during quite different flow and load- general permit approach is an administratively efficient
ing conditions than those for which the criteria are and cost-effective alternative to the individual permit
typically established. Questions have been raised about application method. It reduces the administrative
the applicability and relevance of these criteria to wet burden on the permitting authority and on the permit
weather conditions, but separate criteria for wet weather applicant. Instead of each applicant having to charac-
allowances have not been developed and implemented terize representative samples of stormwater discharge,
for industrial stormwater discharges. WQBELs are EPA allowed industry groups to submit a group appli-
established when analyses determine that a discharge cation and characterize their wet weather discharges
causes or has the reasonable potential to cause or mea- based on monitoring data collected from a subset of
surably contribute to an instream excursion above water these facilities.
quality criteria. For discharges of process wastewater
from traditional sources, these WQBELs are typically Multi-Sector General Permit
numeric, and monitoring data are routinely used to
inform the analysis for compliance. EPA issued the first Multi-Sector General P ­ ermit
(MSGP) in 1995 as a 5-year permit. It was sub­sequently
Industrial Stormwater Permitting revised in 2000, 2008, and 2015, and the current MSGP
extends through 2020. The MSGP provides permit
Although industrial stormwater discharges were coverage through submittal of a “notice of intent,” self-
included in some individual NPDES permits in the
1  Water Quality Act of 1987, Pub. L. No. 100-4, 101 Stat. 7
1970s and 1980s, stormwater permitting was generally
(1987).

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

INTRODUCTION 11

certified implementation of a storm­water pollution 1990, EPA (EPA, 1990, p. 48002) outlined an esca-
prevention plan (SWPPP), and implementation of lating tiered implementation strategy to reduce the
stormwater control measures (SCMs) to reduce pol- discharge of industrial stormwater pollutants. The
lution levels in the discharge (see Box 1-1). The 2015 strategy included general permits that incorporated
MSGP provides permit coverage for industrial sectors basic pollution prevention strategies, site inspections,
listed in Box 1-2, grouped by general industrial activity and reporting (Tier 1); watershed permits (Tier 2);
descriptions and standard industrial classification (SIC) industry-specific permits (Tier 3) for sectors shown
codes.2 EPA includes a separate group AD for facilities to be significant sources of stormwater pollutants; and
not covered elsewhere, which may be designated by the individual permits (Tier 4) tailored to specific facilities
EPA administrator or a state with delegated permitting that are significant sources of stormwater pollutants.
authority. Industrial facilities with no industrial activ- The original implementation strategy has not been
ity exposed to rain, snow, snowmelt, and/or runoff can realized. Rather than move coverage to watershed per-
apply to be excluded from the permit coverage. mits, industry-specific permits, and individual permits,
Under the MSGP, TBELs are provided either EPA has continued to provide coverage under a single
through a limited number of ELGs or through a suite permit, the MSGP.
of narrative requirements, some of which are specified The MSGP sets the requirements for industrial
for particular sectors (discussed further in the next sec- stormwater management in areas where EPA is the per-
tion). WQBELs in the MSGP are narrative and require mitting authority, including most of Indian country,3
the discharge “to be controlled as necessary to meet some federally operated facilities, all U.S. territories,
applicable water quality standards.” EPA states that the District of Columbia, and four states (Idaho,
compliance with TBELs and other permit terms and ­Massachusetts, New Hampshire, and New Mexico).4
conditions are expected to result in compliance with As of September 2018, the MSGP covered 2,174
water quality criteria and standards. The ambiguity of facilities (R. Urban, EPA, personal communication,
such compliance expectations for industrial storm­water 2018). In most of the country, the MSGP serves as a
discharges raises questions of enforceability, public model for states with delegated permitting authority
involvement, and permittee liability. More specific to adopt their own industrial stormwater general per-
requirements have been developed locally in situations mits. Although some states do not venture beyond the
where industrial stormwater discharges flow to water requirements of the MSGP, others tailor their permit
bodies that do not meet established water quality cri- to address unique geographic conditions (see Appen-
teria and standards. These water bodies are considered dix A). For example, states may alter the stormwater
impaired and the impairment is addressed through sampling frequency, require monitoring of additional
development of a total maximum daily load (TMDL). water quality parameters, and/or specify use of certain
Development of a TMDL is a process that includes SCMs.
identification of the pollutant causing the impairment,
the sources of the pollutant, and controls needed to INDUSTRIAL STORMWATER
restore the water body to the point where it meets its MONITORING IN THE MSGP
designated use.
The original strategy for the MSGP envisioned Three types of monitoring are specified under the
a broad tool for control of industrial stormwater dis- MSGP, intended to promote sound stormwater man-
charges that, over time, would lead to improved control agement and provide indicators of compliance and the
measures, more specific numeric effluent limitations effectiveness of stormwater controls:
based on monitoring evidence, and reduced pollut-
3  Indian country is defined as “a) all land within the limits of
ant discharges to receiving waters (EPA, 1992b). In
any Indian reservation under the jurisdiction of the United States
Government … ; b) all dependent Indian communities within the
2  The North American Industrial Classification System (­NAICS) borders of the United States … ; and c) all Indian allotments” (18
has since supplanted the SIC codes for commercial activity in North U.S.C. § 1151).
America, and EPA provides a translation from SIC to NAICS on 4  Idaho recently received NPDES authorization and will begin

its website. issuing its own stormwater permits in 2021.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

12 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

BOX 1-1 Stormwater Control Measures for Industrial Stormwater Pollution Management

Stormwater control measures (SCMs) include structural and nonstructural practices designed to reduce a per-
mittee’s stormwater pollution discharges. Although SCMs vary by industry, they can broadly be grouped into the
following categories:

• Pollution prevention—efforts to use only materials that are nontoxic, nonhazardous, and nonpolluting;
• Good housekeeping—practices to prevent and contain spills and keep contaminants out of stormwater discharges
through orderly facilities;
• Minimizing exposure—efforts to move indoors or cover industrial activities and chemical storage;
• Managing runoff—diverting stormwater runoff from nonindustrial areas away from industrial areas;
• Erosion and sediment control, such as mulching or sodding; and
• Structural pollution treatment.

Nonstructural practices typically are selected first because they are lower cost to operate and maintain. One
exam­ple of a nonstructural practice is sweeping/vacuuming. Sweeping/vacuuming collects particulate matter that is
greater than a certain size range, depending on the efficiency of the sweeper, thus preventing it from being suspended
in stormwater during rain events. Sweeping also may be used on many sites that incorporate structural practices
because sweeping reduces the suspended solids concentration reaching the SCM during the storm and may reduce
the maintenance frequency of the SCM. Covering of stockpiles of materials also has been used by sites to reduce the
exposure of pollutant sources to stormwater runoff.
Structural treatment SCMs can be classified based on their removal mechanisms. The two most common processes
are sedimentation and filtration. Particulate matter itself is a pollutant and many pollutants of interest in industrial
stormwater are associated with particulate matter of various sizes, including many heavy metals and hydrophobic
organic compounds. Removal of these associated pollutants can occur concurrently with particulate matter removal
using sedimentation and/or filtration, depending on the particle sizes with which the pollutant is associated.
Larger particulate matter can be removed from stormwater via sedimentation or related density-driven processes.
For extended detention facilities, quiescent conditions are created in the stormwater flow path, such as in a pond or
wetland, allowing particles with settling velocities greater than the surface overflow rate to settle to the bottom of
the system, effectively removing them from the stormwater. For smaller detention facilities, such as manufactured
sedimentation devices, during storm events, rapid settling of large particles occurs during the storm and quiescent
conditions between storms provides additional removal. Periodic removal of accumulated sediment is required to
maintain performance and prevent scour of previously trapped materials.
Filtration is used to remove particulate matter that is too small to be removed effectively by sedimentation.
­Filtration involves allowing the water to pass through a porous medium. Particles are trapped and may attach to the
media. Filters, especially sand filters, historically have been used as a polishing technology. In stormwater treatment,
sediment forebays or sumps often precede a filtration system to reduce the solids load to the filter surface and ­reduce
the frequency of clogging. Once the system flow rate drops below a prespecified rate, the system is “clogged” and
the collected particles must be removed via physical removal of media and particles, or through a backwashing
process. Because of the power requirement for backwashing, it is rarely used in stormwater treatment and media
replacement becomes the preferred option. Industrial stormwater treatment mechanisms and treatment efficiencies
have been discussed in detail by Clark and Pitt (2012).
Removal of pollutants in industrial stormwater that pass through certain-sized laboratory filters (operationally
called “dissolved,” even though they may include both complexed and colloid-bound forms of certain pollutants) is
usually more difficult and complex than those associated with particulates. Generally, chemically reactive media, such
as peat, compost, activated carbon, biochar, zeolite, and surface-modified sands, are used to remove the dissolved
pollutants via adsorption or ion-exchange reactions. Similar to clogging with particulate matter, adsorption and
ion-exchange media have finite treatment lifetimes, because available surface sites on the media become s­ aturated.
Although some media can be regenerated, this rarely happens in practical applications and the media must be
­replaced. Other advanced treatment technologies are available to remove dissolved contaminants, including reverse
osmosis, but such systems are typically not used for stormwater due to their cost and complexity. Removal of dis-
solved stormwater pollutants has been reviewed by Clark and Pitt (2012) and LeFevre et al. (2015).
Biological transformations of some pollutants can occur in stormwater SCMs under specific conditions. Nitrogen
species and many organic compounds, especially hydrocarbons, can undergo aerobic or anaerobic biotransformations,
particularly when sufficient time is provided in the treatment system, usually between storm events.
These treatment devices can also be categorized as passive or active systems, based on their mode of operation,
including the use of electricity to operate the system and the use of chemicals to enhance treatment. Treatment
systems can be further divided into manufactured or nature-based treatment systems, such as ponds, vegetation,
and natural filtration media. These factors may affect long-term costs and environmental considerations.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

INTRODUCTION 13

1. Visual monitoring, where samples of runoff are

Types of Facilities Required
BOX 1-2 to Obtain Industrial collected and observed visually for certain water
Stormwater Permit Coverage quality characteristics (e.g., color, turbidity, and oil
sheen);
Sector A: Timber products facilities
Sector B: Paper and allied products 2. Benchmark monitoring, where stormwater samples
manufacturing facilities are collected and analyzed in a laboratory for
Sector C: Chemical and allied products
specific pollutants and compared to benchmark
manufacturing and refining
Sector D: Asphalt paving and roofing materials thresholds identified in the MSGP; and
manufacturers and lubricant manufacturers 3. ELG monitoring, where stormwater samples are
Sector E: Glass, clay, cement, concrete, and
gypsum product manufacturing facilities analyzed for specific pollutants that are compared
Sector F: Primary metals facilities for compliance with national ELGs (see also
Sector G: Metal mining (ore mining and
dressing) facilities
Table 1-1).
Sector H: Coal mines and coal-mining-related
facilities The monitoring requirements complement quarterly
Sector I: Oil and gas extraction facilities
Sector J: Mineral mining and processing facilities site inspections that must be performed and docu-
Sector K: Hazardous waste treatment, storage, mented by permittees.
and disposal facilities
Sector L: Landfills and land application sites
The primary purpose of the MSGP monitoring
Sector M: Automobile salvage yards program is to ensure that industries are complying with
Sector N: Scrap recycling and waste recycling the terms of the permit and appropriately managing
facilities
Sector O: Steam electric power generating stormwater on site to minimize harmful discharges of
facilities, including coal handling areas stormwater pollutants to the local environment. Moni-
Sector P: Motor freight transportation
facilities, passenger transportation facilities,
toring observations can signal shortfalls in stormwater
petroleum bulk oil stations and terminals, management, and exceedances can be cause for review
rail transportation facilities, and U.S. Postal and reconsideration of SCM selection and implemen-
Service transportation facilities
Sector Q: Water transportation facilities with tation. Monitoring results can also be used to quantify
vehicle maintenance shops and/or equipment improvement in stormwater quality on site based on
cleaning operations
Sector R: Ship and boat building or repair yards
implementation of stormwater control measures and
Sector S: Vehicle maintenance areas, equipment to identify pollutants not being successfully controlled.
cleaning areas, or deicing areas located at air At a program level, MSGP monitoring data should
transportation facilities
Sector T: Treatment works also provide an indication over time whether the quality
Sector U: Food and kindred products facilities of industrial stormwater across the country is improv-
Sector V: Textile mills, apparel, and other fabric
product manufacturing facilities
ing to meet the objectives of the MSGP (EPA, 2015d).
Sector W: Wood and metal furniture and fixture Additionally, MSGP monitoring would, ideally, inform
manufacturing facilities future decision making and updates to future general
Sector X: Printing and publishing facilities
Sector Y: Rubber, miscellaneous plastic permits, such as refinements in benchmark thresholds
products, and miscellaneous manufacturing over time based on the capabilities of treatment tech-
industries
Sector Z: Leather tanning and finishing facilities
nology. The various types of MSGP-required monitor-
Sector AA: Fabricated metal products ing are summarized in Table 1-1 and discussed in more
manufacturing facilities detail below.
Sector AB: Transportation equipment, industrial,
or commercial machinery manufacturing
facilities
Sector AC: Electronic and electrical equipment
Visual Monitoring
and components, photographic, and optical
goods manufacturing facilities The MSGP requires all permittees to conduct
quarterly visual assessment of stormwater samples
from each outfall. For events that result in a discharge,
samples must be taken within the first 30 minutes of
discharge (if feasible) resulting from a storm event that

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

14 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE 1-1
MSGP Monitoring Requirements

Tier Criteria Summary of Monitoring Reporting and Response

Visual All facilities under the Quarterly grab sample from each
•  Permittee summarizes samples in
• 
monitoring of MSGP. outfall (or representative outfall) annual report to EPA. If samples
stormwater taken within the first 30 minutes of are polluted, permittee must begin
discharge discharge. corrective action and document the
Observe for water quality
•  measures taken in the stormwater
characteristics: color, odor, clarity, pollution prevention plan.
floating solids, settled solids, foam, oil
sheen, and other obvious indicators
of pollution.
Benchmark Sectors in the original Quarterly grab sample from each
•  Report electronically on the Network
• 
monitoring permit development outfall (or representative outfall) Discharge Monitoring Report
process that were judged taken within the first 30 minutes of (NetDMR).
to have elevated pollutant discharge. If average of four monitoring
• 
concentrations that could Pollutants analyzed are determined
•  values exceeds benchmark, facility
be reduced with SCMs. by sector/subsector. must determine if modifications of
See list in Table 1-2. SCMs are necessary and continue
monitoring or seek waiver.
If average for four quarterly
• 
monitoring values is below the
benchmark, no further monitoring is
required during the permit term.
Numeric Sectors A, C, D, E, J, K, L, Grab samples collected once per
•  Report electronically on NetDMR.
• 
effluent O, S. year at each outfall containing the If exceedance, permittee is deemed
• 
limitations discharges for regulated activities. in violation of the MSGP. Permittee
(NELs) must take corrective actions and
monitoring conduct follow-up monitoring within
30 days or during the next runoff
event. When follow-up monitoring
is in exceedance, permittee must
continue to monitor quarterly until
discharge is in compliance.

occurs at least 72 hours following a previously measur- given the discretion to identify which facilities to
able event. The sample is then inspected visually for sample and for which pollutant. The sampling data
color, odor, floating or settled solids, suspended solids, requested included a mandatory list of pollutants (pH,
oil, sheen, and other indicators of stormwater pollution. oil and grease, biological oxygen demand, chemical
These results are documented by the permittee and oxygen demand, total suspended solids, total nitrogen,
summarized in an annual report to EPA. If evidence nitrite and nitrate, and total phosphorus), commonly
of stormwater pollution is observed, corrective actions referred to as the baseline sampling (EPA, 1992c).
are required (EPA, 2015d). Industry groups were asked to select other pollutants
to analyze for based on lists of pollutants that they
Benchmark Monitoring deemed to be representative of the industry sub­sector
activity (see Appendix B). The data collected were
EPA recognized the greater cost burden of ana- presumed to be representative of discharges without
lytical monitoring over visual monitoring and required the implementation of SCMs because, at the time,
analytical monitoring only of sectors that demonstrated those discharges were unpermitted. EPA compiled and
a potential to discharge pollutants at concentrations of analyzed the data by industry sector, and where indus-
concern. For the most part, EPA determined which tries were found to contain a wide range of industrial
industry sectors required benchmark monitoring using activities or potential pollutant sources, the industries
industry-supplied baseline data during a 1992 group were subdivided further and the data compiled on a
application process. The industry group leaders were subsector basis.

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

INTRODUCTION 15

Based on the group application data, EPA required stormwater discharge that was not adequately char-
benchmark monitoring for industrial sectors where acterized by the data generated through the group
pollutants were identified in stormwater at concen- application process. The different sectors with specific
trations of concern to receiving waters that could be required benchmark monitoring are listed in Table 1-2.
reduced through implementation of SCMs. EPA also The benchmark monitoring requirements in the 1995
required benchmark monitoring for a few industries MSGP have for the most part carried over to the cur-
that had a high potential for contamination from rent 2015 MSGP.

TABLE 1-2
Industrial Sectors and Subsectors and Their Benchmark Monitoring Requirements

Subsector Subsector Detail Benchmarking Monitoring Requirements

A1 General sawmills and planing mills Chemical oxygen demand (COD), total suspended solids
(TSS), zinc
A2 Wood preserving Arsenic, copper
A3 Log storage and handling TSS
A4 Hardwood and wood product facilities; sawmills COD, TSS
B1 Paperboard mills COD
C1 Agricultural chemicals Nitrate plus nitrite; lead, iron, zinc, phosphorus
C2 Industrial inorganic chemicals Aluminum, iron, nitrate plus nitrite
C3 Soaps, detergents, cosmetics, and perfumes Nitrate plus nitrite, zinc
C4 Plastics, synthetics, and resins Zinc
C5 Industrial organic chemicals, paints, lacquers, and None
pharmaceuticals
D1 Asphalt paving and roofing materials TSS
D2 Miscellaneous products of petroleum and coal None
E1 Clay product manufacturers Aluminum
E2 Concrete and gypsum product manufacturers TSS, iron
E3 Glass and stone products None
F1 Steelworks, blast furnaces, and rolling and finishing mills Aluminum, zinc
F2 Iron and steel foundries Aluminum, TSS, copper, iron, zinc
F3 Rolling, drawing, and extruding of nonferrous metals Copper, zinc
F4 Nonferrous foundries Copper, zinc
F5 Smelting and refining of nonferrous metals, miscella- None
neous primary metal products
G1 Active copper ore mining and dressing facilities TSS, nitrate plus nitrite, COD
G2 Active metal mining facilities TSS, turbidity, pH, hardness, antimony, arsenic, beryllium,
cadmium, copper, iron, lead, mercury, nickel, selenium,
silver, zinc
H Coal mines and related areas Aluminum, iron, TSS
I Oil and gas extraction facilities None
J1 Sand and gravel mining Nitrate plus nitrite, TSS
J2 Dimension and crushed stone and nonmetallic minerals TSS
J3 Clay, chemical, and fertilizer mineral mining None
K1 Hazardous waste treatment, storage, or disposal facilities Ammonia, magnesium, COD, arsenic, cadmium, cyanide,
lead, mercury, selenium, silver
L1 Landfills, land application sites, and open dumps TSS
L2 L1 except municipal solid waste landfill areas closed Iron
continued

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

16 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE 1-2
Continued

Subsector Subsector Detail Benchmarking Monitoring Requirements

M Automobile salvage yards TSS, aluminum, iron, lead
N1 Scrap recycling and waste recycling facilities COD, TSS, aluminum, copper, iron, lead, zinc
N2 Source separated recycling facilities None
O Steam electric generating facilities Iron
P Motor freight transportation facilities None
Q Water transportation facilities Aluminum, iron, lead, zinc
R Ship and boat building or repair yards None
S Airports Biochemical oxygen demand (5 day) (BOD5), COD,
­ammonia, pH
T Treatment works None
U1 Grain mill products TSS
U2 Fats and oils products BOD5, COD, nitrate plus nitrite, TSS
U3 Meat, dairy, and other food products and beverages None
V Textile mills, apparel, and other fabric products None
W Furniture and fixture manufacturing facilities None
X Printing and publishing facilities None
Y1 Rubber products manufacturing Zinc
Y2 Miscellaneous plastic products and manufacturing None
industries
Z Leather tanning and finishing facilities None
AA1 Fabricated metal products, except coating Aluminum, iron, zinc, nitrate plus nitrite
AA2 Fabricated metal coating and engraving None
AB Transportation equipment, industrial, or commercial None
machinery manufacturing facilities
AC Electronic and electrical equipment and components, None
photographic, and optical goods manufacturing facilities

The benchmarks were established as “the pol- national or state water quality criteria, using EPA
lutant concentrations above which EPA determined acute criteria where they exist and chronic criteria
represents a level of concern. The level of concern if no acute criteria exist. Aquatic life water quality
is a concentration at which a stormwater discharge criteria provide the basis for 15 of the 23 parameters
could potentially impair, or contribute to impairing in the 2015 MSGP for which benchmarks have been
water quality or affect human health from ingestion established.
of water or fish.” The benchmarks were also viewed Industries required to perform benchmark moni-
by EPA “as a level, that if below, a facility represents toring (see Table 1-2) must sample pollutants quarterly
little potential water quality concern” (EPA, 1995). in the first year of permit coverage. A benchmark
For the baseline sampling pollutants, EPA used a mix sample is collected as a grab sample within the first
of approaches to establish technology-based bench- 30 minutes of stormwater discharge after a rainfall
mark thresholds (see Table 1-3). For example, for total (if feasible) that results in an actual discharge from
suspended solids and nitrate plus nitrite, EPA derived the site and with an interceding dry period of at least
benchmarks from the median of the National Urban 72 hours. The reported results typically reflect pollutant
Runoff Program data. For other pollutants, EPA’s concentrations for an individual sample but can reflect
benchmark thresholds are largely based on published the average concentration for an outfall for all sampled

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

INTRODUCTION 17

TABLE 1-3
Sources of MSGP Benchmark Values

Pollutant Benchmark MSGP Source

pH 6.0–9.0 Secondary Treatment Regulations (40 CFR Part 133)
Biochemical oxygen 30 mg/L
demand (5 day) (BOD5)
Chemical oxygen demand 120 mg/L “Factor of 4 times BOD5 concentration—North Carolina benchmark”

Total suspended solids 100 mg/L National Urban Runoff Program (NURP) median concentration
Nitrate + nitrite nitrogen 0.68 mg/L
Ammoniaa 2.14 mg/L “Guidelines for Deriving Numerical National Water Quality Criteria for the Protection
of Aquatic Organisms and Their Uses” (EPA, 1985)
Total phosphorus 2.0 mg/L North Carolina stormwater benchmark (from North Carolina water quality standards)
Total magnesium 64 μg/L “Minimum Level (ML) based on highest Method Detection Limit (MDL) times a factor
of 3.18”
Turbidity 50 NTU “Combination of simplified variations on Stormwater Effects Handbook, Burton and
Pitt, 2001, and water quality standards in Idaho”
Total aluminum 750 μg/L Freshwater Acute Aquatic Life Criteria (EPA, 2006c)
Total antimony 640 μg/L Water Quality Criteria Human Health for Consumption of Organism (EPA, 2006b)
Total beryllium 130 μg/L Freshwater LOEL Acute Water Quality Criteria (EPA, 1980c)
Total cadmium FWb 2.1 μg/L Freshwater: Acute Aquatic Life Criteria (EPA, 2006c)
SW 40 μg/L Saltwater: Acute Aquatic Life Criteria (EPA, 2006c)
Total coppera FWb 14 μg/L
SW 4.8 μg/L
Cyanide FW 22 μg/L
SW 1 μg/L
Total leada FWb 82 μg/L
SW 210 μg/L
Total mercury FW 1.4 μg/L
SW 1.8 μg/L
Total nickel FWb 470 μg/L
SW 74 μg/L
Total silvera FWb 3.8 μg/L
SW 1.9 μg/L
Total zinc FWb 120 μg/L
SW 90 μg/L
Total iron 1,000 μg/L Freshwater Chronic Aquatic Life Criteria (EPA, 2006c)
Total arsenic FW 150 μg/L Freshwater: Chronic Aquatic Life Criteria (EPA, 2006c)
SW 69 μg/L Saltwater: Acute Aquatic Life Criteria (EPA, 2006c)
Total seleniuma FW 5 μg/L
SW 290 μg/L

NOTE: FW = freshwater; LOEL = lowest observed effect level; NTU = nephelometric turbidity unit; SW = saltwater.
 a “New criteria are currently under development, but values are based on existing criteria.”
 b “These pollutants are dependent on water hardness where discharged into freshwaters. The freshwater benchmark value listed is

based on a hardness of 100 mg/L. When a facility analyzes receiving water samples for hardness, the permittee must use the hardness
ranges provided in Table 1 in Appendix J of the 2015 MSGP and in the appropriate tables in Part 8 of the 2015 MSGP to determine ap-
plicable benchmark values for that facility. Benchmark values for discharges of these pollutants into saline waters are not dependent
on receiving water hardness and do not need to be adjusted.”
SOURCE: EPA, 2015c.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

18 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

separate runoff events that occurred during the quar- CONTEXT FOR THE STUDY
terly monitoring period. Facilities that are required to
conduct monitoring but have no stormwater discharge The various industrial stormwater permitting
during the reporting period are required to report “no requirements have come under scrutiny since the
discharge.” If the average of the four quarterly results program’s inception. It is widely recognized that the
exceeds any of the benchmarks, the monitoring must monitoring program suffers from a paucity of use-
be continued for another four quarters until the aver- ful data and from inconsistent sampling techniques.
age does not exceed the benchmark. Sampling results Benchmark monitoring has been variously described
exceeding benchmarks (based on an average of four as overly burdensome to industries and producing data
samples) is not a permit violation, unless no corrective that go unutilized. Some stakeholders question whether
action is undertaken and exceedances persist. Instead, benchmark exceedances serve as useful indicators of
an exceedance necessitates that the facility operator the effectiveness of implementation of stormwater
investigate stormwater control measures and make control measures or potential water quality problems.
necessary improvements. Any corrective action taken If problems are observed, others express concern about
must be documented as a modification to the facility’s a lack of enforcement mechanisms to ensure that the
SWPPP. If a facility determines that no further pol- issues are effectively addressed. State and local storm-
lutant reductions are technologically or economically water programs face a shortage of resources to review
feasible and benchmark exceedances continue to occur, monitoring data and conduct routine compliance
perhaps due to natural background conditions, run-on inspections. For these reasons, the NRC concluded
from adjacent properties, or other factors, permit- that “the stormwater program has suffered from poor
tees may apply for permission to reduce monitoring accountability and uncertain effectiveness at improving
frequency or eliminate it (also termed an “off-ramp”). the quality of the nation’s waters” (NRC, 2009).
Among dozens of recommendations for improv-
ing the stormwater program, the 2009 NRC report
Effluent Limitation Guidelines
recognized that many of the benchmark monitoring
EPA has established numeric ELGs for storm- requirements and effluent guidelines for certain indus-
water for 10 subcategories of industrial facilities (see trial subsectors were based on incomplete and outdated
also Table 1-1 and Appendix C);5 these subsectors information (NRC, 2009). The report recommended
are required to monitor at least once per year at each that “Industry monitor the quality of stormwater dis-
outfall containing the discharges subject to the ELG. charges from certain critical industrial sectors in a more
An exceedance of a numeric ELG in a single sample sophisticated manner, so that permitting authorities
is deemed a violation of the MSGP and subject to can better establish benchmarks and technology-based
enforcement action. If an exceedance of an ELG is effluent guidelines.” The report also noted the lack of
detected, it must be reported to EPA, and corrective a nationwide compilation and analysis of industrial
actions are required. After an exceedance, additional benchmark monitoring data, which could be used to
monitoring is required at least quarterly until the better understand typical stormwater concentrations
discharge is within compliance. The numeric effluent of pollutants from various industries. Additionally, the
limitations in ELGs tend to be substantially higher report recommended a risk-based approach for indus-
than benchmark thresholds (with the exception of total trial stormwater monitoring requirements so as to not
suspended solids). ELGs are based on extensive data unduly burden those industrial facilities with limited
collection on the performance and capacity of treat- exposure to runoff, while also not allowing high-risk
ment technology. sites to escape the more intensive monitoring that
would be necessary to ensure compliance with effluent
limitations.
5  ELGs have been established for specific constituents in storm-
These issues resurfaced in a recent settlement
water for cement manufacturing, petroleum refining, steam electric agreement made between EPA, industries, and environ­
power generation, timber products processing, coal mining, hard mental groups regarding revisions to the nationwide
rock mining, mineral mining and processing, and airports.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

INTRODUCTION 19

MSGP for industrial stormwater. The agreement numeric effluent limitations or reasonably stan-
requires the parties to suspend all legal actions against dardized stormwater control measures would be
EPA regarding the revisions to the MSGP until the most scientifically defensible (based on sampling
National Academies of Sciences, Engineering, and data quality, data gaps and the likelihood of filling
Medicine have conducted a study on certain aspects of them, and other data quantity/quality issues that
the industrial stormwater program. In particular, the may affect the calculation of numeric limitations).
agreement asked the National Academies’ committee to:
EPA will use the results of this study to inform
1. Suggest improvements to the current MSGP its proposed revisions to the 2015 MSGP, which are
benchmarking monitoring requirements. Areas to anticipated in 2020. The committee was not asked
examine could include to analyze the financial costs of its recommendations;
• Monitoring by additional sectors not currently instead, EPA will assess the costs of possible changes
subject to benchmark monitoring; in its proposed revision of the MSGP.
• Monitoring for additional industrial-activity- EPA’s proposed revisions of the 2015 MSGP will
related pollutants; also address other provisions of the legal settlement
• Adjusting the benchmark threshold levels; that will increase the importance of the benchmark
•  A djusting the frequency of benchmark thresholds. The settlement required that EPA develop
monitoring; requirements for “Additional Implementation Mea-
• Identifying those parameters that are the most sures” (AIMs) “substantially similar” to those detailed
important in indicating whether stormwater in Box 1-3. AIM would set specific actions that must
control measures are operating at the best- be taken upon different levels of exceedance of the
available-technology or best-conventional-­ benchmarks or repeated exceedances. The specifics of
technology level of control; and the AIM tiers and the consequences of exceedances
• New methodologies or technologies for indus- have not been finalized, but repeated exceedances of
trial stormwater monitoring. annual averages or large repeated exceedances could
require additional structural stormwater control mea-
2. Evaluate the feasibility of numeric retention stan- sures if feasible. If exceedances continue, an individual
dards (such as volumetric control standards for a permit may be required. These requirements would
percent storm size or standards based on percentage provide stronger consequences to benchmark exceed-
of imperviousness). ances, thus increasing the significance of the bench-
• Are data and appropriate statistical methods mark thresholds.
available for establishing such standards as The committee’s report and its conclusions and
both technology-based and water quality-based recommendations are based on a review of relevant
numeric effluent limitations? technical literature, briefings, and discussions at its five
• Could such retention standards provide an effec- in-person meetings and three web conferences, and the
tive and scientifically defensible approach for experience and knowledge of the committee members
establishing objective and transparent effluent in their fields of expertise. The committee received
limitations? briefings from a range of experts, including federal,
• What are the merits and faults of retention state, and local government officials; practitioners;
versus discharge standards, including any risks industry representatives; environmental organizations;
of groundwater or surface-water contamination and academics (see the Acknowledgments).
from retained stormwater?
OUTLINE OF THE REPORT
3. Identify the highest-priority industrial facilities/
subsectors for consideration of additional discharge Following this Introduction, the Statement of Task
monitoring. By “highest priority” EPA means those is addressed in three subsequent chapters of this report.
facilities/subsectors for which the development of In Chapter 2, the committee discusses benchmark

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

20 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

BOX 1-3 Additional Implementation Measure (AIM) Requirements

The following tiers were outlined in the settlement agreement, and EPA is required to propose for public comment
for the next MSGP a substantially similar approach:

Tier 1: If (A) an annual average for a parameter is over the benchmark threshold; or (B) a single sampling event re-
sult for a parameter is over 4× the benchmark threshold, then the operator must immediately review the selection,
design, installation, and implementation of its control measures to determine if modifications are necessary to meet
the benchmark threshold for that parameter. If any modifications are necessary, the operator must implement those
modifications….

Tier 2: If (A) two consecutive annual averages for a parameter are each over the benchmark threshold; or (B) two
sampling event results for a parameter within a two-year period are over 4× the benchmark threshold; or (C) a single
sampling event for a parameter is over 8× the benchmark threshold (unless the operator immediately documents in
its SWPPP that the single event was an aberration, how any measures taken within 14 days of such event will prevent
a reoccurrence, and takes a sample during the next qualifying rain event…), then the operator must implement all
feasible control measures in the relevant sector-specific fact sheet….

Tier 3: If (A) three consecutive annual averages for a parameter are each over the benchmark threshold; or (B) three
sampling event results for a parameter within a three-year period are each over 4× the benchmark threshold; or
(C) two sampling events for a parameter within a three-year period are each over 8× the benchmark threshold; or
(D) four consecutive samples for a parameter are over the benchmark threshold and their average is more than 2× the
benchmark threshold, then the operator must install structural source controls (permanent controls such as permanent
cover, berms, and secondary containment), and/or treatment controls (e.g., sand filters, hydrodynamic separators, oil-
water separators, retention ponds, and infiltration structures) within 30 days.... In addition, the operator does not have
to install structural source controls or treatment controls if it adequately demonstrates to EPA within 30 days of the
Tier 3 trigger occurrence that its discharge does not result in any exceedance of water quality standards.... The dem-
onstration to EPA, which will be made publicly available, must include the following minimum elements in order to be
considered for approval by EPA: (1) the water quality standards applicable to the receiving water; (2) the flow rate of
the stormwater discharge; (3) the instream flow rates of the receiving water immediately upstream and downstream
of the discharge point; (4) the ambient concentration of the parameters) of concern in the receiving water immediately
upstream and downstream of the discharge point demonstrated by full storm composite sampling; (5) the concentra-
tion of the parameters) of concern in the stormwater discharge demonstrated by full-storm, flow-weighted composite
sampling; (6) any relevant dilution factors applicable to the discharge; and (7) the hardness of the receiving water....
If a facility continues to exceed the benchmark threshold for the same parameter even after installation of structural
source controls or treatment controls, EPA may require the operator to apply for an individual permit.

SOURCE: Waterkeeper Alliance v. U.S. EPA, 2016.

monitoring requirements and benchmark thresholds. stormwater management while reducing burden for
Chapter 3 identifies opportunities for improving small, low-risk facilities. In Chapter 4, the commit-
industrial stormwater MSGP monitoring, including tee evaluates the merits and concerns associated with
evaluations of sampling methods, laboratory analysis, retention standards for industrial stormwater under the
and data management. The committee recommends a MSGP framework.
new tiered approach to monitoring to provide improved

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Pollutant Monitoring Requirements

and Benchmark Thresholds

T
his committee was charged with recommending (EPA, 1995, p. 50804). Many of the program elements,
potential improvements to the current Multi-­ however, have been hampered by shortfalls in gener-
Sector General Permit (MSGP) monitoring ating, considering, and acting on new information.
requirements, including monitoring by additional This has resulted in missed opportunities for refining
­sectors not currently subject to benchmark monitor- the MSGP monitoring requirements in support of
ing, monitoring for additional industrial-activity- improved stormwater management. Some of these key
related pollutants, and adjusting benchmark threshold program elements are summarized in Table 2-1.
levels. As discussed in Chapter 1, the Environmental The pollutant monitoring requirements of the
Protection Agency (EPA) is currently developing MSGP are particularly dated and have not been sub-
Additional Implementation Measure (AIM) require- stantially updated over time. Many industrial sectors
ments in response to a legal settlement that would have never collected and reported data for any of the
provide actionable consequences for large or repeated conventional and nonconventional pollutants, toxic
benchmark exceedances. These changes place greater pollutants, and hazardous substances listed in Appen-
emphasis on ensuring that the MSGP uses appropriate dix B. With the group application process, industrial
benchmarks. In this chapter, the committee provides sectors were directed to sample for specific pollutants
a broad assessment of the current MSGP benchmark based on their own determination of whether they had
monitoring process and summarizes the most recent knowledge or reason to believe a pollutant may be pres-
MSGP monitoring data. Then, the chapter describes ent in their stormwater discharges.
ways to improve pollutant monitoring requirements, Consequently, some industrial groups submitted
including industrial activities not currently covered by more information than others, causing monitoring data
the MSGP, industry-wide benchmarks, and sector- submittal discrepancies among some sectors. The result
specific monitoring requirements. The committee also is a disparity in the relative monitoring burden across
discusses the adequacy of current benchmark threshold sectors. This disparity is shown in Table 2-2 for five
levels, considering recent information on toxicity and example sectors. Sectors M and N1 have benchmark
treatment attainability. monitoring requirements and Sectors I, P, and R have
no benchmark monitoring requirements. Sectors I
ASSESSMENT OF CURRENT MSGP and R self-determined through the group application
BENCHMARK MONITORING process that no sector-specific pollutants needed to be
tested in their discharges. In contrast, Sectors M, N1,
The original 1995 MSGP monitoring scheme and P determined that pollutant testing was, in fact,
was based on program elements embedded in sound necessary, with Sector N1 making that determination
administrative, scientific, and public policy principles for the highest number of pollutants.

21

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

22 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE 2-1
Evolution of Key MSGP Program Elements Affecting Monitoring Requirements

Factors Affecting Monitoring

Requirements 1995 MSGP Approach Current Assessment
Pollutant Characterization
Gross characterization of Baseline sampling was required, Has not been repeated since 1992
basic chemical parameters in and results were statistically
stormwater discharges characterized
Characterization of specific Industries self-determined which Gaps in industry sampling have not been addressed
pollutants in discharges by pollutants (beyond the baseline
sector/subsector sampling pollutants) needed
to be analyzed; industry data,
where provided, were statistically
characterized
Evaluation of pollution Activities indicated in permit Industry fact sheets have not been updated since 2006.
potential by sector/subsector applications were summarized along Fact sheets and other sector-specific information have
based on activities, sources, with likely sources of stormwater not been used to update MSGP monitoring requirements
and pollutants contamination and pollutants that
may be present
Sampling and Analysis
Availability of sampling Sampling methods were limited to Sampling methods have not been updated to include
methods and protocols grab more broadly available and representative methods
Availability of sufficiently The sensitivity of analytical methods Improvements in analytical capabilities were incorporated
sensitive analytical test informed choice of benchmarks and into the 2008 MSGP for some metals, but it remains
methods monitoring requirements unclear whether or how updated analytical methodology
is considered for other benchmark requirements
Pollutant Reductions
Capacity of stormwater Options for controlling pollutants The 1995 evaluation was limited and has not been
control measures (SCMs) to and the capacities of SCMs were comprehensively updated
reduce pollutant loads evaluated when setting benchmarks

Literature reviews generating new information e­lectronically by permittees under the 2015 MSGP
regarding pollutants with industrial activity and their through February 13, 2018. Tables 2-3 and 2-4 sum-
presence in the environment (EPA, 1995, 2006a; marize the discharge monitoring results. For each
O’Donnell, 2005) have not been systematically or com- pollutant-sector combination, the graphical results
prehensively used to update the MSGP. This reveals are color coded based on the percentage of individual
missed opportunities to characterize and likely reduce reported results with concentrations above the bench-
pollution in industrial stormwater discharges. mark (see Table 2-3) or eight times the benchmark (see
Table 2-4). Tables 2-3 and 2-4 do not indicate MSGP
CONTEXT OF RECENT MSGP DATA benchmark exceedances, which are determined based
on the average of four quarterly samples, and trigger
A review of recent MSGP monitoring results is review of the stormwater pollution prevention plan
instructive to evaluate the current state of MSGP and 1 year of additional monitoring. However, they
benchmark monitoring compliance and provide impor- provide insight into the sectors and pollutants with
tant context for the committee’s findings. More than frequent elevated discharge concentrations. Eight
17,000 reported results were evaluated from MSGP times the benchmark was selected as indicative of a
permitted facilities in the four states where EPA has major elevated discharge concentration, as suggested
primacy for the regulations (Idaho, ­Massachusetts, for the AIM process (see Box 1-3). In order for a
New Hampshire, and New Mexico), the District data set to be included in Tables 2-3 and 2-4, each
of Columbia, U.S. territories, Indian country, and pollutant considered had to have a minimum of eight
some federal facilities. The data were submitted reported results for that subsector. The committee rec-

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 23

TABLE 2-2
Benchmark Pollutant Evaluation for Five Sample Sectors

Motor Freight, Rail,

Passenger, and
Oil and Gas U.S. Postal Service Ship and Boat
Extraction and Automobile Scrap and Waste Transportation Building or Repair
Refining Salvage Yards Recycling Facilities Facilities Yards
(Sector I) (Sector M) (Sector N1) (Sector P) (Sector R)
Monitoring Data None Aluminum, iron, Lead, zinc, Lead, zinc None
Supplied in 1992 lead cadmium, copper,
Industry Group zinc, chromium,
Application iron, nickel,
(beyond arsenic, aluminum,
baseline polychlorinated
parameters) biphenyls (PCBs)
1995 MSGP Total suspended TSS, oil and Hydraulic fluids, Fuel, oil, heavy Paint solids, heavy
Common solids (TSS), total grease, ethylene oils, fuels, metals, chlorinated metals, suspended
Pollutants Listed dissolved solids, glycol, heavy grease and solvents, acid/ solids, spent
oil and grease, metals, sulfuric other lubricants, alkaline wastes, abrasives, solvents,
chemical oxygen acid, petroleum accumulated ethylene glycol, dust, oil, ethylene
demand (COD), hydrocarbons, particulate matter, arsenic, organics, glycol, acid/alkaline
chlorides, barium, chlorinated chemical additives, paint, dust, wastes, detergents,
naphthalene, solvents, acid/ PCBs, antifreeze, sediment, fuel, BOD, bacteria
phenanthrene, alkaline wastes, acid, mercury, lead, detergents, salts,
benzene, lead, arsenic, organics, heavy metals phosphorus,
arsenic, fluoride, detergents, sodium chloride,
pH, acetone, phosphorus, BOD
toluene, ethanol, salts, bacteria,
xylenes, antimony biochemical
oxygen demand
(BOD)
2015 MSGP None TSS, aluminum, TSS, COD, None None
Benchmark iron, lead aluminum, copper,
Monitoring iron, lead, zinc
Requirements

ognizes that for stormwater data analysis more storm for aluminum and iron. In Sector A2 (wood preserv-
event results (18 to 24) are preferable, considering the ing), more than half of the samples exceeded the
inherent variability of precipitation events. However, benchmarks for chemical oxygen demand (COD),
based on the limited available data on industrial sites, copper, and TSS, and 81 percent of the samples
a lower threshold of eight reported results was used. exceeded eight times the benchmark for copper. Sec-
Additional pollutant-specific tables and graphs, a tor F4 (nonferrous foundries) reported frequent high
description of the data set, specific details on the com- levels of zinc and copper, with 30 and 50 percent of
mittee’s analysis, and known limitations of the data set the samples, respectively, above eight times the bench-
are provided in Appendix D. mark. In Sector N1 (scrap recycling), more than 50
When evaluating the results by sector, several sec- percent of the samples are above the benchmarks for
tors emerge that have a large percentage of samples copper, iron, and zinc, while more than 10 percent of
with concentrations above the benchmark threshold samples exceed eight times the benchmarks for alumi-
for more than one pollutant, and even some with a num, copper, iron, and zinc.
large percentage of samples with concentrations above Additionally, meeting benchmarks proved more
eight times the benchmarks. For example, in Sector H difficult for some pollutants than others. No sector was
(coal mines and coal-mining-related facilities), more able to meet the magnesium benchmark in more than
than half of the samples exceed eight times the bench- 50 percent of the samples. Copper, zinc, and iron also
mark for total suspended solids (TSS) and 95 percent showed large percentages of samples above the bench-
of the samples exceeded eight times the benchmarks marks from most sectors.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

24 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE 2-3
NetDMR 2015 MSGP Data According to the Percentage of Results Above Benchmarks
NO2+
Sector Al NH3 As BOD5 Cd COD Cu CN Fe Pb Mg Hg pH TP Se Ag TSS Turb Zn
NO3
A1: Sawmills
A2: Wood
A3: Log storage
A4: Hardwoods
B1: Paperboard
B2: Pulp mills
C1: Agricultural
C2: Industrial inorganics
C3: Cleaning, cosmetics
C4: Plastics
C5: Medicinals
D1: Asphalt
E2: Concrete
E3: Glass
F1: Steel works
F2: Iron/steel foundries
F3: Nonferrous metals
F4: Nonferrous foundries
G1: Copper ore
G2: Other ores
H: Coal mines
J1: Construction sand
J2: Stone
J3: Clay mineral mining
K: Hazardous waste
L1: Landfills
L2: Landfills, not MSW
M: Automobile salvage
N1: Scrap recycling
O1: Steam electric
P: Transportation, postal
Q: Water transportation
R: Ship and boat building
S: Air transportation
T: Sewage treatment
U1: Grain mill products
U3: Meat, dairy, tobacco
Y1: Rubber
Y2: Misc. plastics
AA1: Fabricated metals
AA2: Fabr. metal coating
AB: Machinery
AC: Electronics

Insufficient data <10% above

No data (<8 results) ­benchmark (BM) 10–25% above BM 26–50% above BM >50% above BM
 

NOTE: MSW = municipal solid waste.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 25

TABLE 2-4
Categorization of NetDMR Data Based on the Percentage of Results Above Eight Times the Benchmark
NO2+
Sector Al NH3 As BOD5 Cd COD Cu CN Fe Pb Mg Hg TP Se Ag TSS Turb Zn
NO3
A1: Sawmills
A2: Wood 81% 13%
A3: Log storage
A4: Hardwoods
B1: Paperboard
B2: Pulp mills
C1: Agricultural 13% 25%
C2: Industrial inorganics
C3: Cleaning, cosmetics
C4: Plastics 16%
C5: Medicinals 50%
D1: Asphalt
E2: Concrete 17%
E3: Glass
F1: Steel works
F2: Iron/steel foundries
F3: Nonferrous metals 14% 12%
F4: Nonferrous foundries 50% 30%
G1: Copper ore
G2: Other ores
H: Coal mines 95% 95% 55%
J1: Construction sand
J2: Stone 11%
J3: Clay mineral mining
K: Hazardous waste 83%
L1: Landfills
L2: Landfills, not MSW 17%
M: Automobile salvage
N1: Scrap recycling 13% 26% 18% 13%
O1: Steam electric
P: Transportation, postal
Q: Water transportation 12% 61% 12%
R: Ship and boat building 81%
S: Air transportation 16%
T: Sewage treatment 10%
U1: Grain mill products
U3: Meat, dairy, tobacco 13%
Y1: Rubber 23%
Y2: Misc. plastics
AA1: Fabricated metals 46%
AA2: Fabr. metal coating
AB: Machinery
AC: Electronics

Insufficient data 10–25% above 8×

No data (<8 results) 0% above 8× BM 1–9% above 8× BM BM >25% above 8× BM

NOTE: MSW = municipal solid waste.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

26 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE 2-5
Percent Benchmark Exceedances in 2008 MSGP NetDMR Data Based on Annual Averages as Reported in EPA (2012)
NO2+
Al NH3 As BOD5 Cd COD Cu CN Fe Pb Mg Hg TP Se Ag TSS Zn
NO3
A 33 100 16 17
B 0
C 19 48 0 17 0 100
D 14
E 71 19
F 71 50 0 72
J 25 8
K 0 0 0 0 0 0 100 0 0 0
L 91 38
M 25 52 22 9
N 53 43 75 88 50 37 67
O 67
Q 40 33 0 100
U 0 0 20 20
Y 0
AA 36 65 26 74

<10% above BM as 10–25% above BM as 26–50% above BM as >50% above BM as

No data reported ­annual average annual average annual average ­annual average

Table 2-5 provides a graphical representation of the a large portion of facilities have results above the
results of an analysis of electronically submitted bench- benchmarks under both the 2008 and 2015 MSGP.
mark monitoring data from the 2008 MSGP (EPA, The remainder of this chapter discusses ways MSGP
2012). Table 2-5 is presented parallel to Table 2-3. The pollutant monitoring requirements could be improved
main exception is that Table 2-5 reflects the percent- to enhance industrial stormwater management within
ages of annual averages (based on four quarterly results) the program.
that exceeded the benchmark, rather than individual
results; thus, the color coding by percentage exceedance IMPROVING POLLUTANT
is more stringent than that in Table 2-3. Additionally, MONITORING REQUIREMENTS
the monitoring data were not separated into subsectors.
The EPA (2012) analysis was based on far fewer data In this section, the committee reviews the bench-
compared to the above analyses, because electronic mark monitoring requirements within the MSGP. The
submission of the MSGP data was not mandated until committee identifies industrial activities not currently
2016. Several parameters have data from only one per- covered by the MSGP, discusses the value of industry-
mittee and, in some cases, at only one outfall; therefore, wide benchmark monitoring, and analyzes sector-
the data are too limited to assess any trend between specific pollutant monitoring requirements.
2008 and 2015. Nonetheless, several of the same issues
from EPA (2012) are apparent when reviewing the Industrial Activities Not Covered by the MSGP
recent data in Table 2-3. Again, pollutants frequently
above the benchmark include magnesium, copper, iron, Industrial facilities in the MSGP are classified
and zinc. Without subsector breakdown, comparisons within sectors based on the products they generate using
among sectors are more problematic. the legacy standard industrial classification (SIC) code.
Tables 2-3 through 2-5 highlight the ongoing The MSGP was intended to cover discharges associated
challenges faced by several industrial sectors for which with industrial activity—not just discharges from facili-

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 27

ties whose main purpose has been defined as industrial works garages) operated by federal, state, or municipal
(EPA, 1995b). SIC codes are not ideal for characteriz- governments.
ing the industrial activities that occur at a site with the EPA should identify the industrial, commercial,
potential risk of stormwater pollution. Some facilities and retail activities currently excluded by the MSGP’s
like gas stations and school bus transportation facilities SIC-based approach that have stormwater pollution
are not included in the MSGP, because they are not potential comparable to industrial facilities currently
considered to be industrial facilities, even though the regulated under the MSGP. In their upcoming revisions
environmental risks associated with their outdoor activi- to the MSGP, EPA should consider ways to include
ties may be similar to or greater than other facilities that these facilities under the MSGP, with monitoring
the MSGP covers. requirements equivalent to like facilities. This would
The MSGP should extend coverage for facilities, facilitate improved stormwater management and char-
including commercial ones, that are not explicitly acterization of discharges at these facilities.
defined as “industrial” under the National Pollutant
Discharge Elimination System stormwater regulation Industry-wide Benchmark Measurements for
SIC structure if they conduct on-site activities that All Sectors
are equivalent to industrial activity covered under the
MSGP. These facilities should be subject to the same A primary goal of the MSGP benchmark moni-
monitoring requirements as those industries with like toring requirements is to indicate the performance
on-site activities. These facilities include of structural and nonstructural SCMs for ensuring
the quality of stormwater leaving industrial sites.
• Timber lots, The committee recommends a suite of water quality
• Fuel storage and on-site fueling, parameters for benchmark monitoring by all industrial
• Vehicle maintenance (e.g., school bus transporta- sites that must do stormwater sampling, including
tion facilities), those that currently only do visual monitoring. Such
• Facilities with numerous parked diesel vehicles, industry-wide monitoring would provide indicators
• Outdoor materials storage that poses stormwater of problems for a wide range of sites and a baseline
contamination threats (e.g., liquid tanks with understanding of industrial stormwater risk for all
operational valves or in poor condition, solids such sectors. Industry-wide monitoring would also provide
as salt or wood chips that are exposed to storm­ stormwater quality information that could be com-
water), and pared across all industries regardless of sector, and
• Outdoor handling of materials (e.g., filling liquid would help address some of the monitoring disparities
tank trucks, conveyors handling solids in particu- that resulted from the group application process. Such
late form). monitoring has been recommended by other reviews
of the MSGP (O’Donnell, 2005), and several states
Some states have done this. Maryland, for example, currently use some degree of industry-wide monitoring
describes Department of Public Works highway main- (see Appendix A). The committee recommends three
tenance facilities and school bus facilities as specific industry-wide parameters:
types of facilities designated for coverage under the
Sector AD of the general permit (MDEP, 2014). • pH detects excess acidic or alkaline substances in
Connecticut’s general permit includes several activi- the water, and pH excursions indicate corrosive
ties that have been added to the definition of “storm- (acidic or basic) and/or toxic concerns. Stormwater
water associated with industrial activity” (CT DEEP, discharges that are excessively polluted may not
2018). This includes small-scale composting facilities; exhibit problems with respect to pH. However,
road salt and deicing material storage facilities; wood pH excursions that are highly acidic or highly
processing facilities not otherwise covered, including alkaline and do not fall into the benchmark range
mulching, chipping, and mulch coloring facilities; and (6.0–9.0) can be indicative of a major polluting
vehicle service and storage facilities (including public event or process failure and can be impactful to

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

28 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

receiving waters. Unexpected pH values also can measure of oxygen demand). It is a conventional
indicate that a stormwater treatment system is water quality parameter with established indus-
not operating properly. For example, a limestone- trial stormwater benchmarks. In addition to the
based treatment system will typically raise pH, measure of oxygen demand, high COD can also be
so a low effluent pH may indicate system failure. indicative of oils and hydrocarbon pollution (Han
pH is simple and low cost to measure and is cur- et al., 2006a) and, as with TSS, can be an indicator
rently required as an industry-wide benchmark of overall site cleanliness. Increases in COD could
by California, Connecticut, and Washington (see also indicate problems with the treatment SCM
Appendix A). effectiveness, including the need for maintenance.
• Total Suspended Solids (TSS) is a measure of The committee recognized that total organic
suspended particulate matter in a water sample. ­carbon (TOC), which generally provides the same
­Particulate matter can result from erosion of indus- information of interest as COD, would be a better
trial soils, deposited particulate matter on the drain- measure of organic pollution in water for several
age area, erosion/corrosion of materials present on reasons. TOC analysis is simple, standardized,
the site, and general overall site cleanliness. TSS and easier to automate than COD. TOC analysis
also provides information about possible high con- uses fewer toxic chemicals and can produce results
centrations of numerous other pollutants that will much more sensitive, precise, and accurate than
partition onto particulate matter, including phos- COD. However, TOC does not have an EPA-
phorus, many heavy metals, and many hydrophobic established benchmark or history of data as COD
organic chemicals. Stormwater TSS concentrations does. TOC may also be less effective in measuring
in receiving waters are highly correlated with the colloidal/particulate organic matter. Once an EPA
concentrations of metals such as copper, lead, and benchmark is developed for TOC, EPA should
zinc, known to cause freshwater and marine bio- consider the overall advantages and disadvantages
toxicity (Schiff and Tiefenthaler, 2011). Several of conversion to TOC monitoring. While both
treatment and nonstructural SCMs are available COD and TOC are gross measures of organic
to control TSS (Clark and Pitt, 2012), and TSS pollution, they are not specific enough or sensi-
can provide information about their performance tive enough to detect possible excursions of toxic
or the need for additional SCMs at a site (Avila et pollutants (e.g., polycyclic aromatic hydrocarbons
al., 2008). TSS is a standardized, well-established [PAHs]) at moderate/low concentrations. COD is
test. Suspended sediment concentration (SSC; currently required as an industry-wide benchmark
an approved method under 40 CFR § 136.3 for by Connecticut (see Appendix A).
filterable residue) was considered as an alternative
surrogate for TSS. SSC is generally judged to be All three parameters are direct measures of water
a more accurate measure of particulate matter in quality and are appropriate choices for industry-
stormwater because it will capture all sediment, wide sampling because all three can be indicators
not just suspended matter. However, use of SSC of broader water quality problems and the presence
complicates the monitoring process by requiring of other pollutants. In addition, these industry-wide
an independent sample for this parameter only. water quality parameters can provide indications of
Turbidity measurements have also been suggested SCM absence, neglect, or failure, which can lead to
as an indicator for suspended solids. However, TSS high concentrations of potential pollutants. There are
provides a better basis for comparisons against his- well-established standardized analytical procedures for
torical data, which are more commonly reported all three recommended industry-wide parameters, and
as TSS. TSS is currently required as an industry- analytical determinations are expected to be relatively
wide benchmark by California, Connecticut, and inexpensive (less than $100 for all three). Considering
­Minnesota (see Appendix A). that all permittees must collect quarterly storm event
• Chemical Oxygen Demand (COD) is a surrogate samples for visual monitoring, the additional cost
measure of organic pollutants in water (through ­burden of these analyses is expected to be small.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 29

Review of Pollutant Monitoring Requirements by for those parameters. Among the limited monitoring
Sector data reported for Sector M from the 2015 MSGP
(11 samples), the benchmark for copper was exceeded
For the most part, the monitoring requirements in 82 percent of the time compared to 63 percent for
the MSGP were based on the best information avail- Sector N1 (see Appendix D).
able at the time they were derived. However, based on
information gained since the MSGP was developed,
Sectors Not Subject to Benchmark Monitoring
changes for a number of sectors are merited. Some
­sectors are not required to conduct benchmark moni- Of the industrial sectors listed in Table 1-2, 10
toring. Other sectors are required to monitor for only sectors (including all their subsectors) have no bench-
a very limited number of pollutants (see Table 1-1) and mark monitoring requirements in the MSGP. Other
some sectors are not required to monitor for the sub- sectors have at least some subsectors required to
stances that could potentially be important p­ ollutants conduct benchmark monitoring, representing varying
that may be discharged in stormwater from their sites. proportions of the sector facilities. According to EPA,
This section reviews the monitoring requirements of 45 percent of all facilities permitted under the MSGP
the MSGP and discusses areas that the committee are not required to conduct benchmark monitoring
recommends to be updated based on the current under- (R. Urban, EPA, personal communication, 2018). In
standing of risk and pollutant occurrence. this section, the committee examines in more detail
three of the sectors where no benchmark monitoring is
Inconsistent Monitoring Requirements for Similar Sectors currently required (see also Table 2-2). These analyses
with Similar Industrial Activities highlight the need for updated evaluations of p­ ollutant
potential and opportunities for pollutant reduction
Analysis of the sector-specific benchmark monitor- through implementation of additional SCMs.
ing requirements shows inconsistencies across sectors
that have comparable industrial activities, highlight- Oil and Gas. Sector I includes oil and gas exploration,
ing shortfalls in the current MSGP. For example, production, processing or treatment operations, and
Sectors M (automobile salvage yards) and N1 (scrap transmission facilities. A number of chemicals are used
recycling and waste recycling facilities) have similar at these operations that could contribute to stormwater
activities on site but different monitoring requirements pollution, including diesel fuel, oil, solvents, drilling
(see Table 2-2). Both sectors include material handling fluid, acids, and chemical additives (EPA, 2006c).
and storage, material processing and dismantling, Ammonia, lead, nickel, nitrate, and zinc have been
including ferrous and nonferrous metals, equipment detected at these sites in stormwater in greater than
maintenance and cleaning, reclaiming and recycling 10 percent of the reported data (O’Donnell, 2005). No
liquid wastes such as used oils and antifreeze, and monitoring data on Sector I have been submitted as
other operations that occur at industrial facilities often part of the 2015 MSGP (see Appendix D). Spills and
exposed to stormwater. Monitoring requirements for leaks can also lead to petroleum hydrocarbon contami-
Sector M are TSS, total aluminum, total iron, and total nants in stormwater, including PAHs, which have been
lead, whereas Sector N1 is required to monitor for these shown to be highly toxic to aquatic life (Incardona et
parameters and also total copper, total zinc, and COD. al., 2011; Abdel-Shafy and Mansour, 2016; McIntyre et
As discussed earlier in this chapter, Sector M does not al., 2016). Chemical-specific monitoring is appropriate
have benchmark monitoring requirements for total for this sector to ensure that stormwater is appropriately
copper and total zinc, at least in part, because they did managed.
not self-determine through the 1992 group application
process that monitoring for these two p ­ ollutants was Motor Freight and Transportation Facilities. Sector P
necessary. As such EPA did not have data to evaluate includes motor freight and passenger transportation
pollutant potential when developing the 1995 MSGP facilities, petroleum bulk oil stations and terminals,
and has not, to date, required Sector M to sample rail transportation facilities, and post office facilities.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

30 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Activities on these sites include vehicle and equipment the same pollutants as water transportation Sector Q
fluid changes, mechanical repairs, parts cleaning, fuel- because they have common industrial activities. In
ing, and vehicle storage. Chemicals used on site include the 1992 group application process, Sector Q self-­
solvents, diesel fuel and gasoline, hydraulic fluids, determined that aluminum, iron, lead, and zinc needed
antifreeze, and transmission fluids (EPA, 2006b; see to be tested in their discharge, and EPA applied bench-
Table 2-2). Benchmark monitoring for lead and mer- mark monitoring for those four pollutants to Sector Q
cury in addition to pH, TSS, and COD were recom- in the MSGP. The MSGP monitoring requirements for
mended by O’Donnell (2005) because of the frequency Sector R are likely insufficient due to shortfalls in the
of occurrence in Toxic Release Inventory stormwater original 1992 group application process.
data, but the 2015 MSGP does not include any bench-
mark monitoring requirements for this sector. Although Need for Periodic Monitoring Reviews
benchmark monitoring is not required nationally, some
Sector P monitoring data have been reported in EPA’s These examples show that monitoring require-
Network Discharge Monitoring Report (NetDMR). ments within the MSGP are not consistently applied.
Greater than 25 percent of results had concentrations Additionally, updates to the benchmark monitoring
above the benchmarks for aluminum, copper, and iron. requirements have not been made over time in spite
As with Sector I, petroleum hydrocarbon leaks and of data and several analyses showing that specific
spills could lead to harmful stormwater discharges of contaminants are commonly detected or likely to
PAHs. The activities in Sector P and risk of stormwater occur in stormwater at these facilities (Harcum et al.,
pollution suggest that chemical-specific monitoring 2005; O’Donnell, 2005; EPA, 2012; see also Appen-
within the MSGP would be appropriate. dix D). Sector-specific monitoring requirements for
all sectors should be rigorously reviewed to assess
Ship and Boat Building. Sector R covers ship and whether the monitoring requirements are appropriate
boat building or repair yards, which includes activities to ensure control of stormwater pollution and deter-
such as fluid changes, mechanical repairs, parts clean- mine whether benchmark monitoring requirements
ing, refinishing, paint removal, painting, and fueling. should be adjusted.
Chemicals used on site include solvents, oil, fuel, anti- The committee recommends the following specific
freeze, and acid and alkaline wastes. As discussed earlier steps be taken to periodically review the MSGP moni-
in this chapter, Sector R self-determined through the toring requirements and update them as appropriate
1992 group application process that no sector-specific based on new information:
pollutants needed to be tested in their discharges,
which was a significant reason for the lack of bench- • Prior to each permit renewal, EPA should conduct
mark monitoring in the 1995 MSGP. This determina- a literature review and update its industry fact
tion has carried over into the 2015 MSGP. O’Donnell sheets, which describe potential pollutants from
(2005) suggested that chromium, copper, lead, nickel, common industry activities, pollutant sources, and
and zinc be considered for future monitoring for Sector practices that could reduce pollutant discharge on
R and noted that Toxic Release Inventory stormwater site.1 Changes in industry practice over time may
data were limited. Greater than 300 Sector R monitor- introduce new contaminants and render other
ing data points have been submitted to the NetDMR contaminant monitoring of limited value.
under the 2015 MSGP. Greater than 25 percent of • EPA should continue the process conducted
reported results were above the benchmark for alumi- by Tetra Tech in advance of the 2008 MSGP
num, copper, and iron (see Appendix D). (O’Donnell, 2005) where sector-specific data
Rhode Island recently added benchmark monitor- from the previous MSGP as well as Toxic Release
ing for aluminum, iron, lead, and zinc for Sector R Inventory and Toxic Substances Control Act data
in their 2013 MSGP (RI DEM, 2013). The Rhode are assessed to determine whether the chemical
Island Department of Environmental Management
1  See https://www.epa.gov/npdes/industrial-stormwater-fact-
determined that Sector R has the potential to generate
sheet-series.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 31

monitoring requirements are adequate to detect Updated Toxicity Information

stormwater management concerns.
• State industrial stormwater permits should be Advancements in the understanding of aquatic
reviewed for advancements in sector-specific moni- toxicology suggest that some benchmark threshold
toring that would be appropriate for the national levels may require adjustment to reflect the latest sci-
permit (e.g., Bulkley et al., 2009). entific information in meeting the MSGP’s intended
• New understanding of pollutant effects in the envi- water quality protection goals. This section reviews the
ronment and advances in monitoring technology application of water quality criteria toward the bench-
should be evaluated. For example, PAHs were not mark and the need to update benchmarks to reflect the
previously monitored as part of the MSGP process, latest aquatic life criteria. Additionally, the committee
but aquatic impacts of PAHs are now better under- discusses new benchmarks to better characterize storm-
stood and analytical technologies have advanced water risks, unnecessary benchmarks, and benchmark
significantly since the 1992 group application. units used in documentation and communication.
Scientific advances that identify cost-effective
monitoring surrogates should also be considered. Application of Water Quality Criteria

This level of analysis should be adequate to substantiate Many of the current benchmark thresholds were
the addition of benchmark monitoring requirements derived from aquatic life criteria (see Table 1-3). EPA
for specific sectors. That said, where EPA finds that recommends that aquatic life criteria be derived for
the sector review is not substantial enough to with- protection against toxicity from both acute (short-
stand the scrutiny of adding benchmark monitoring term) and chronic (longer-term) exposure, when pos-
requirements, as was the case after EPA proposed to sible (EPA, 1985). Given the intermittent nature of
add benchmark monitoring for several sectors in the stormwater exposures and the likelihood of dilution
2006 draft MSGP, the committee suggests an alter- and attenuation within watersheds, organisms will be
native process. Where data are lacking to inform the exposed to chemicals from stormwater discharges over
analysis, additional sector-specific monitoring data short time frames. For stormwater benchmarks based
should be collected to provide the information neces- on aquatic life criteria, the committee recommends the
sary to quantify whether stormwater pollutants are use of criteria designed to protect against short-term or
present at levels of concern, using a process similar intermittent exposures when they exist, which, to date,
to that used in 1992 for the group applications. The have generally been acute criteria.
committee recommends that additional monitoring be Most benchmarks in the 2015 MSGP are set
performed over a 1-year period, at the same outfalls according to acute criteria (see Table 1-3); however,
for which industry-wide monitoring is conducted, but chronic criteria are used in three cases—selenium,
without the application of benchmark threshold. These arsenic, and iron—each for different reasons. Chronic
data would then inform future revisions of the MSGP criteria are established to protect aquatic life against
monitoring requirements. mortality and impacts to growth and reproduction after
longer-term exposure.
ADJUSTING BENCHMARK
Selenium. EPA originally considered establishing
THRESHOLD LEVELS
the selenium benchmark at a value equal to the acute
Benchmark threshold levels were established dur- freshwater criteria (20 µg/L; EPA, 1987), but suffi-
ing the early iterations of the MSGP, based on several ciently sensitive test methods were lacking at that time.
different criteria and employing several simplifying Thus, in the MSGP, EPA originally set the selenium
assumptions. In this section, the committee reviews benchmark at 238.5 µg/L based on the value that could
the latest information on toxicity and technical achiev- be accurately and precisely quantified (EPA, 1995,
ability, relative to the current benchmark levels, and p. 50825). In the development of the 2008 MSGP, EPA
discusses the implications to the benchmark levels. updated benchmark thresholds for which more sensi-

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

32 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

tive analytical methods were available. For selenium, it develops a criterion based on intermittent exposure,
EPA stated in 2008 that they based the benchmark EPA should adopt the acute aquatic life criterion
threshold on chronic criteria (5 µg/L) because at the (340 µg/L) for the arsenic benchmark.
time of development of the 2008 MSGP, no acute
criterion was in effect (EPA, 2008a). Iron. EPA based the iron benchmark threshold on the
The selenium benchmark based on chronic aquatic chronic criterion (1,000 µg/L) given the lack of an acute
life criteria is now outdated. In 2016, EPA released criterion, and that decision has remained over the itera-
updated ambient aquatic life criteria for selenium, with tions of the MSGP. No acute aquatic life criterion for
new chronic freshwater criteria reduced to 1.5 and iron has been developed since the MSGP was originally
3.1 µg/L for still or flowing waters, respectively (EPA, established.
2016b). However, no concentration-based acute criteria The committee found very few studies on the
were derived. The updated selenium criteria are unique acute effects of iron on aquatic organisms, and these
in that they were derived to specifically account for the ­studies suggest lethal effects occur well above the
bioaccumulative properties of selenium and reproduc- current benchmark over longer time periods. For
tive effects on fish species and included a translation example, an iron concentration of 6,700 µg/L over 48
of the chronic criteria for short-term or intermittent hours caused acute immobilization in 50 percent of
exposure, in lieu of an acute criterion. The translation the population (EC50) in Daphnia magna (Okamoto
of the chronic criteria must be calculated based on the et al., 2014). An iron concentration of 2,000 µg/L in
background base-flow concentration of selenium in humus-free water was lethal to 23 percent of one-
the receiving water and the length of exposure. This summer-old grayling fish after being exposed for 72
complicates the monitoring requirements for selenium hours (­Vuorinen et al., 1998). Zahedi et al. (2014)
given the additional data required to translate chronic determined that 122,000 µg/L iron is lethal to 50
criteria to intermittent conditions. Although such percent of a population (LC50) of kutum fish over
intensive data collection would not be desirable for 96 hours. The science on which the criterion is based
most permittees, enhanced sampling and analysis for is dated and limited (EPA, 1976). The committee
facilities with repeated benchmark exceedances would suggests that EPA reevaluate the aquatic toxicology
allow EPA to determine if their discharge is causing literature for acute toxicity studies of iron and develop
adverse effects under the site-specific conditions (see a benchmark for iron based on acute toxicity. Because
also Enhanced Monitoring, discussed in Chapter 3). iron has relatively low toxicity and bioaccumulation
of iron does not pose a substantial hazard to higher
Arsenic. Even though an acute criterion of 360 µg/L trophic levels (Cadmus et al., 2018), it is unlikely that
arsenic had been developed (EPA, 1986), the MSGP a criterion based on intermittent exposure would be
benchmark was originally set at 164.8 µg/L based on necessary. Given the basis of the iron criterion and the
the analytical detection limit (EPA, 1995, p. 50825). difficulty many facilities have in meeting the bench-
At the time of the 2008 MSGP review and update mark (see Tables 2-3 through 2-5 and Appendix D),
for more sensitive detection methods, the updated EPA should suspend the benchmark for iron until an
acute criterion (340 µg/L) was more than two times acute criterion is developed unless EPA can articulate a
the previous value. EPA decided not to substantially specific rationale for protecting against chronic effects
weaken the benchmark based on concerns about near- of iron from intermittent events.
coastal freshwater discharges flowing quickly into
sensitive saline waters, which had a saltwater acute Updating Benchmarks to Match Aquatic Life Criteria
aquatic c­ riterion of 69 µg/L (EPA, 2008b). Therefore,
the benchmark was adjusted to the chronic criterion Other aquatic life criteria are currently under revi-
of 150 µg/L. Unless EPA can justify specific unique sion or have recently been revised. For example, revised
concerns for arsenic discharge from freshwater in acute aquatic life criteria for cadmium have been devel-
near-coastal settings that do not apply to all other oped (EPA, 2016a) and will need to be incorporated
benchmarks with lower saltwater benchmarks or until into the next MSGP revisions (see Table 2-6).

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 33

TABLE 2-6 
Outdated Benchmarks or Inconsistencies with Aquatic Life Criteria

2015 MSGP Benchmark Source Current Aquatic Life Criteria Source

Cadmium 2.1 µg/L EPA, 2001 1.8 µg/L Acute; EPA, 2016a
Copper 14 µg/L EPA, 1980a BLM EPA, 2007
Selenium 5 µg/L EPA, 1987 1.5 µg/L (lentic), 0.0031 µg/L (lotic) Chronic; EPA, 2016b
Intermittent equation

NOTE: See https://www.epa.gov/wqc/national-recommended-water-quality-criteria-aquatic-life-criteria-table.

EPA has adopted or is considering more complex the next version of the MSGP should reflect this
approaches to defining aquatic life criteria for some change, if the new aluminum criteria are finalized.
constituents, which could have implications for the
MSGP benchmark monitoring requirements. For Developing New Benchmarks to Better Characterize
­copper, the most recent aquatic life criteria (EPA, 2007) Stormwater Risks
do not provide a single-concentration acute criterion,
but instead provide an equation or model that is used PAHs have been shown to be extremely toxic
to calculate acute criteria with additional site-specific to fish and aquatic invertebrates and are known to
data. The biotic ligand model for copper, which takes bioaccumulate (Incardona et al., 2011; McIntyre et
into account the fact that the bioavailability and hence al., 2016). PAHs are expected at industrial sites with
toxicity of certain metals is affected by water chemistry, petroleum hydrocarbon exposure. However, no bench-
uses 10 input parameters for toxicity determination.2 mark has been set for PAHs for any of the industrial
Given the extra sampling burden, the 2015 MSGP sectors. Analytical methods for determination of PAHs
did not recommend using the biotic ligand model for are standardized and readily available (EPA, 2015b).
copper benchmark monitoring, which is reasonable It may appear that COD can be used as a surrogate
for a national permit. Nevertheless, in Chapter 3, the for PAHs, but PAHs can be toxic at concentrations
committee discusses giving permittees the option to orders of magnitude lower than the COD benchmark
monitor for additional components and to use the (120 mg/L). Canadian water quality guideline values
biotic ligand model and updated acute criteria if they for PAHs for the protection of aquatic life range from
routinely exceed the benchmark. 0.012 µg/L (anthracene) to 5.8 µg/L (acenaphthene)
Draft 2017 aquatic life criteria for aluminum simi- (Canadian CME, 1999). Currently, EPA has no rec-
larly involve the measurement of multiple parameters ommended aquatic life criteria for individual or total
to determine the acute criteria based on bioavailability. PAHs. EPA evaluated the need for ambient water qual-
The new approach to determine aluminum toxicity uses ity criteria for PAHs in 1980 and noted at the time that
a multiple linear regression method, considering total the data regarding aquatic life toxicity were extremely
hardness, pH, and dissolved organic carbon (DOC) limited (EPA, 1980b). Information gathering and/or
(DeForest et al., 2018). The 2015 MSGP freshwater preliminary monitoring of PAHs from some sectors
aluminum benchmark is set at 750 µg/L (EPA, 1988), would be valuable; such data could be correlated with
but the 2017 draft update recommends increasing the COD concentrations to help EPA determine if COD
acute criteria to 1,400 µg/L (based on pH = 7, hard- is an adequate surrogate for PAH concentrations and
ness = 100 mg/L, and DOC = 1 mg/L; EPA, 2017). impacts or if additional PAH monitoring is needed for
Considering the minimal additional analysis required, sectors that have the potential to release PAHs.

2  pH, dissolved organic carbon, calcium, magnesium, sodium,

sulfate, potassium, chloride, alkalinity, and temperature.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

34 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Unnecessary Benchmark: Magnesium due to incorrect units. Consistent expectations for

reporting would also reduce error in the data set.
Magnesium is a natural component of surface water EPA should also explain the uncertainty and
and groundwater and does not appear to be toxic to a rounding inherent in the expression of criteria upon
­majority of aquatic organisms at concentrations likely which benchmark values are derived and provide
to be encountered in most waters, with reported LC50 guidance on the level of precision expected in reported
values ranging from 780 to more than 20,000 mg/L results. Specifically, EPA should describe how many
(van Dam et al., 2010). No EPA aquatic life criterion significant figures should be included and when sample
is provided for magnesium. Nevertheless, total magne- results should be rounded. This ensures that the correc-
sium is listed in the MSGP as a benchmark monitoring tive actions triggered by the exceedance of a benchmark
requirement for Sector K (hazardous waste treatment, have a threshold of significance and a relationship to
storage, or disposal facilities), with a threshold concen- the scientific basis of the value.
tration of 0.064 mg/L as a benchmark. Data submitted
under the 2015 MSGP show that all samples reported
for Sector K exceeded the benchmark, and 83 percent of Assessing Technical Achievability
the samples exceeded eight times the benchmark. It is Pollution prevention and nonstructural SCMs are
unclear why magnesium is a required benchmark for this generally the preferred method to address industrial
sector, given the lack of toxicity at concentrations likely stormwater discharges. Using nonhazardous m ­ aterials,
to be observed in industrial stormwater discharges. general site cleanliness, and creating no-exposure
Therefore, the committee recommends that magnesium ­scenarios will greatly minimize pollutant discharge
be removed as a benchmark monitoring requirement. concentrations and masses. However, for some sites,
structural SCMs including treatment systems will
Units be necessary to meet MSGP benchmark concentra-
tions. For technology-based benchmarks, an important
Benchmark threshold levels should be expressed consideration is whether the benchmarks continue to
in the same units as the values from which they ­represent the capability of technologies (when combined
are derived. For example, benchmark thresholds for with appropriate pollution prevention and nonstructural
­parameters such as TSS, total nitrogen, and total SCMs) for pollution control. For water quality-based
phosphorus should be expressed in milligrams per benchmarks, the feasibility of achieving these thresholds
liter and benchmark threshold for metals should be with current technology and appropriate site manage-
expressed in micrograms per liter. Units of measure- ment must be understood. This section examines the
ment are a foundation of science used to communicate capacity of treatment SCMs to meet industrial storm-
the magnitude of a quantity. The adoption of common water benchmarks (see Table 1-2) considering available
units of measurement allows scientists to consistently (albeit limited) data. For this analysis, the committee
communicate findings in context and in a manner that examined stormwater treatment data from two sources:
eases comparative analysis. In the case of the MSGP,
expression of benchmark thresholds for metals in 1. A study of treatment SCM performance at several
micrograms per liter would promote understanding industrial sites in the United States by Clark and
of the potential consequence of an exceedance relative Pitt (in press) and
to the scientific basis from which the benchmark was 2. The International Stormwater Best Management
derived, for example, acute toxicity to aquatic life. This Practices (BMP) Database, which includes mostly
change in expression would also eliminate the need nonindustrial sites.3
to report sampling results using leading zeros, which
can create confusion and the opportunity for reporting All the data reported represent influent and effluent
error. In the committee’s analysis of the 2015 MSGP concentrations at various treatment SCMs, and the
monitoring data (see Appendix D), there were numer-
ous reported values that appeared to be reporting errors
3  See http://www.bmpdatabase.org.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 35

results are organized by pollutant. In general, it is Given the limitations of the data sets, the com-
expected that pollutant behavior in a treatment device mittee used a simple comparison of the data presented
is independent of the industrial sector and, instead, is a in box plots to assess the capability of the treatment
function of influent concentration and other chemical systems to meet the benchmarks. The boxes of the box
characteristics (e.g., association with solids, complex- plots highlight the 25th, 50th, and 75th percentiles of
ation with inorganic and organic ligands, pH). the pollutant concentrations, while the whiskers repre-
The Clark and Pitt (in press) data were collected sent the 10th and 90th percentiles. The committee then
at industrial sites, including Sectors M, N, R, S, and examined the percentage of the effluent samples that
AB (see details in Appendix E), although separation by met the benchmark, by treatment types, categorizing
industry types is not analyzed here. The data from each the performance by the percent of the effluent con-
site are reported separately, labeled by the type of treat- centrations that met the relevant benchmark according
ment SCM. The study included three broad categories to the components of the box-and-whisker plot (<10,
of treatment SCMs: (1) sedimentation systems (hydro- 10–25, 25–50, 50–75, 75–90, and >90 percent). Under
dynamic separator systems, ponds, and wetlands); the MSGP, the results of four quarterly samples are
(2) filtration/adsorption systems; and (3) treatment averaged for evaluation against the benchmark thresh-
trains that included two or more serial SCMs. old; thus, meeting the benchmark in at least 50 percent
The International Stormwater BMP Database of events provides a reasonable likelihood of the average
includes data on SCM treatment performance from a also meeting the benchmark. However, the occurrence
wide range of studies that meet specific quality control of even one very high concentration can lead to an
criteria. Data were analyzed for pollutants with MSGP average above the benchmark. The data are plotted on
benchmarks and focused on five treatment SCMs con- a linear scale (in some cases with split axis) because of
sidered relevant to industrial stormwater: dry detention the need to clearly visualize performance at concentra-
ponds, wet retention ponds, wetlands, media filters, and tions near the benchmark level(s).
bioretention. The BMP Database contains many more The analysis considers seven common industrial
sites than the Clark and Pitt (in press) study, and data stormwater pollutants for which adequate data are
for each SCM selected likely represent multiple sites. available: TSS, total aluminum, total copper, total iron,
Additionally, the BMP Database includes multiple land total lead, total zinc, and chemical oxygen demand. For
uses, including primarily municipal sites. Because of copper, lead, and zinc, the benchmark is based on the
this, the stormwater concentrations tend to be lower receiving water hardness; therefore, two benchmarks
than the Clark and Pitt (in press) industrial data. were used for analysis—one for a softer water (60 mg/L
Detailed design, sizing, and operational/maintenance hardness) and one for a harder water (200 mg/L hard-
information was not available for any of these sites, so it ness). All data reported are from composite samples,
cannot be assumed that they are or are not appropriately typically volume weighted. When compared to the
designed, sized, or maintained. early-storm grab sample of benchmark monitoring,
The Clark and Pitt (in press) and International the flow-weighted composite generally would be either
Stormwater BMP Database data were analyzed in the equal or lower in concentration.
same manner. Data analysis was performed to answer In this analysis, the data are treated as independent
the following question: For treatment systems that dem- events, an assumption underlying a box-plot analysis.
onstrated statistically significant removal of a pollutant, Temporal trends were not analyzed and no conclusions
were the treatment systems able to reduce influent can be drawn regarding rolling averages meeting the
concentrations that exceeded the MSGP benchmarks benchmark.
(see Table 1-2) to effluent concentrations that met
the benchmark? Therefore, the analysis only included Results
­influent/effluent data pairs where the influent exceeded
the benchmark threshold. As with the 2015 MSGP data To highlight examples of the results of this analysis,
analysis, for a data set to be included, each site consid- the SCM treatment performance for two pollutants,
ered had to have a minimum of eight storm events. TSS and total iron, are discussed in this section along

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

36 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

with the overall findings of the analysis of all pollut- of storm events in which the flow-weighted effluent
ants. The remaining pollutant-specific data plots are concentration for that treatment met the benchmark.
presented in Appendix E. For example, yellow, red, and magenta shades indicate
treatments where 25 to 50 percent, 50 to 90 percent,
Total Suspended Solids (TSS). For the treatment and >90 percent, respectively, of the effluent event mean
SCMs at industrial sites, neither of the two sedimenta- concentrations were above the benchmarks. Green
tion systems met the benchmark for at least 50 percent shades represent treatments for which at least 75 per-
of the monitored events; the media filter and both cent of the effluent event mean concentrations met the
treatment train systems met the benchmark for at least benchmark (darker shades reflect better performance).
75 percent of the storm events (see Figure 2-1). Data Gray shading represents sites where sufficient data
from the International Stormwater BMP Database, pairs (a minimum of eight storm events in which the
which represent slightly lower concentrations, showed influent exceeded the benchmark) for that treatment
that all systems were able to meet the benchmark for and pollutant were not available or the removals were
at least 50 percent of the monitored storm events; dry not statistically significant—the committee’s criteria for
ponds, media filters, and bioretention systems were able inclusion. Table 2-7 shows that for industrial sites, less
to meet the benchmark for at least 75 percent or more than a third of the treatment and pollutant types met
of the monitored events (see Figure 2-2). These data the inclusion analysis criteria, limiting the data avail-
suggest that several treatment SCMs are available and able for drawing conclusions.
can be operated in a manner that provides sufficient There are several important limitations/caveats to
treatment to meet the TSS benchmark for at least these summaries. Overall, the industrial site-level data
50 percent of the monitored events. are limited to a relatively small number of storm events.
Data included in this analysis used a low threshold of
Total Iron. The available data for total iron show a dif- inclusion, only eight events. Additionally, both data sets
ferent story. At the Clark and Pitt (in press) industrial lack sufficient site-level information to make definitive
stormwater monitoring sites, none of the four systems assessments of the capacity of any these treatment types
was able to meet the benchmark concentration for to meet the benchmarks in other locations. In many
50 percent, or even 25 percent, of the monitored storm cases, specific design information about the systems is
events (see Figure 2-3). Two treatment systems (reten- not known. For the Clark and Pitt (in press) individual
tion ponds and media filters) from the International site evaluation, many site owners noted that their filter
Stormwater BMP Database were able to meet the total media were proprietary mixes developed by a vendor
iron benchmark concentrations for at least 50 percent and optimized for their site pollutants. In the Inter-
of the monitored storm events, but the average influent national Stormwater BMP Database, all media filters
concentrations were substantially lower in this data set are placed into a single category, even though the per-
(see Figure 2-4). Although the number of industrial formance of filtration media is known to vary based on
sites, treatment types, and storm events were quite the composition of the media (Clark, 2000; Johnson et
limited for the Clark and Pitt (in press) study, the al., 2003). Although some of the sedimentation device
data suggest that industrial sites with high influent sizes could be determined, the size of the drainage area
concentrations may have difficulty attaining the total could not, and, therefore, the appropriateness of device
iron benchmark, although more data would be needed sizing is unknown. Incorporation of specific features,
with more information about the nature of the SCMs such as energy dissipaters that would prevent scour
to definitively reach this conclusion. of captured sediment, is not known. Finally, as noted
previously, the data reflect composite samples (typically
Summary of Treatment Systems. Table 2-7 synthe- flow-weighted composites), which are often lower in
sizes the treatment performance results from Figures concentration than first-flush grab samples, as required
2-1 to 2-4 and E-1 to E-13 (see Appendix E) for all by benchmark sampling. Thus, treatment technologies
of the treatments and pollutants analyzed. The per- and sites shown to meet the benchmark with composite
formance is color coded according to the percentage samples may not necessarily meet it consistently with

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 37

FIGURE 2-1 Total suspended solids (TSS) influent versus effluent concentrations at industrial sites.
NOTE: BM = benchmark; HDS = hydrodynamic separator; MF = media filter; n = number of storm events sampled; TT = treatment train.
SOURCE: Clark and Pitt, in press.

FIGURE 2-2 International Stormwater BMP Database comparison of influent and effluent concentrations for total suspended solids (TSS).
NOTE: BM = benchmark; BR = bioretention; DP = dry detention ponds; MF = media filters; n = number of storm events sampled; RP =
wet retention ponds; WB = wetlands.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

38 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

FIGURE 2-3 Total iron influent versus effluent concentrations comparison at industrial sites.
NOTE: BM = benchmark; DP = dry detention pond; HDS = hydrodynamic separator; n= number of storm events sampled; TT = treatment train.
SOURCE: Clark and Pitt, in press.

FIGURE 2-4 International Stormwater BMP Database comparison of influent and effluent concentrations for total iron.
NOTE: BM = benchmark; BR = bioretention; DP = dry detention ponds; MF = media filters; n = number of storm events sampled; RP =
wet retention ponds; WB = wetlands.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 39

TABLE 2-7
Comparison of Treatment Performance, Shown as Percentage of Storm Events with Event Mean Concentrations Above the Benchmark,
with Sample Size Noted

Total Copper Total Lead Total Zinc

Total
TSS Total Soft Hard Iron Soft Hard Soft Hard COD
(100 Al (750 (9 (28.5 (1,000 (45 (213 (80 (230 (120
System mg/L) µg/L) µg/L) µg/L) µg/L) µg/L) µg/L) µg/L) µg/L) mg/L)
INDUSTRIAL SITE-SPECIFIC EVALUATIONS
Hydrodyn. Separator 1 11 12 12 12 12 12
Hydrodyn. Separator 2
Hydrodyn. Separator 3 10 8
Hydrodyn. Separator 4 13
Dry Detention Pond 1 10 10 11 10 10 10
Dry Detention Pond 2 14 16 16 13 15 9 14 11
Wetlands
Media Filter 1
Media Filter 2
Media Filter 3 10 18
Media Filter 4 13 13
Media Filter 5 8 9 12 9 12
Media Filter 6 9 16
Treatment Train 1 11 15 14
Treatment Train 2 10 14 13 16 9 16 16 9
INTERNATIONAL STORMWATER BMP DATABASE EVALUATIONS (includes many sites)
Dry Detention Ponds 149 146 65 51 114 53 14
Wet Retention Ponds 306 8 336 110 144 105 31 223 55 11
Wetlands 75 47 55 8
Media Filters 115 225 43 44 18 252 54
Bioretention 104 127 39 10 100 33 27

<10% above BM 10–25% above BM 25–50% above BM 50–90% above BM >90% above BM
     

NOTE: Numbers of influent/effluent sample pairs displayed in each cell. Gray cells indicate that the system was not included in the
­analysis because it did not meet the criteria for inclusion. The removal either was not statistically significant or the data set did not include
at least eight storm events for that treatment/pollutant where the inflow exceeded the benchmark.

first-flush grab samples. Where composite samples SCMs met the benchmarks for at least 50 percent of
consistently fail to meet the benchmark, the same storm events for TSS, aluminum, copper (soft water),
would be expected when first-flush grab samples are lead (hard water), and zinc (hard water). In contrast,
used, although not necessarily from discharge of stor- no systems/sites analyzed were able to meet the bench-
age SCMs. mark for at least 25 percent of storm events for iron or
Despite the limitations of the data sets, some at least 50 percent of events for lead (soft water).
general findings emerge. In the site-level industrial The International Stormwater BMP Database pro-
evaluation (Clark and Pitt, in press), at least one treat- vides a larger data set, but it includes many non­industrial
ment SCM was capable of meeting benchmarks for at sites, and on average it has much lower pollutant influ-
least 50 percent of storm events for TSS, aluminum, ent concentrations than the site-level industrial data.
copper, zinc, and COD. Multiple sites and treatment Under conditions of lower influent concentrations that

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

40 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

might be found more commonly in municipal settings, to meet the benchmarks as often for other parameters
the International Stormwater BMP Database data sug- (e.g., iron, copper, zinc). Even though some guidance
gest that treatment SCMs are available that are effective documents, such as California Water Boards (2018),
in reducing concentrations below freshwater benchmark state that TSS removal can be used as a predictor of par-
threshold levels in at least 50 percent of the storm events ticulate metals removal, these data suggest that attain-
where the influent concentration exceeds the bench- ing the benchmark for TSS at industrial sites is not a
mark for TSS, copper, iron, lead, zinc, and COD. Data sufficient surrogate for meeting the metals benchmark.
for aluminum were extremely limited (only eight storms Overall, the committee’s evaluation of technical
for one treatment type). achievability is hampered by the acute lack of SCM
The committee cannot say definitively that lower performance data for industrial stormwater. Table 2-4
influent concentrations led to more successful treat- highlights the paucity of industrial stormwater data
ment performance for these particular sites because available with which to evaluate the attainability of
of the limited information on design and operation of benchmarks. None of these data are sufficient to
the SCMs. With median inflow iron concentrations determine that certain benchmarks cannot be achieved
ranging from 1,500 to 3,500 µg/L, two of the three with existing treatment technology combined with
treatment SCMs in the BMP Database met the iron appropriate site management and pollution preven-
benchmark for at least half of the storm events, while tion strategies. It does appear, however, that some type
none of the four industrial sites/treatments (with of treatment train approach, where an initial SCM
median inflow concentrations of 8,500 to 19,000 handles part of the pollutant load followed by a second
µg/L) could meet the benchmark for at least 10 per- “polishing” treatment, has the potential to meet many
cent of events. Similarly, at the industrial sites where of the benchmarks for more than 50 percent of storm
the median inflow zinc concentrations ranged from events. The initial treatment may be an SCM that spe-
500 to 900 µg/L, only one of the treatments met the cifically targets high-particulate-matter loads or some
lower soft-water benchmark. In contrast, in the BMP nonstructural SCM that can reduce input pollutant
Database, where median inflow concentrations ranged concentrations.
from 100 to 400 µg/L, four out of five treatments met Although this analysis focuses on treatment and
the soft-water benchmark for at least half of the storms. the 2015 MSGP monitoring data are based on bench-
Although some dependence on influent concentration mark monitoring discharge concentrations, some com-
is found generally in SCM performance, SCM treat- monalities are noted. Again, copper, iron, and zinc are
ment is not linear with influent concentration (Clark, the pollutants that have benchmark concentrations that
2000). Some SCMs will discharge pollutant concentra- are the most difficult to meet.
tions near a treatment value determined by their design This analysis clearly highlights the critical need for
characteristics, independent of influent concentrations more data to assess the achievability of many bench-
up to the design storm size (most storm events rarely marks. Specifically, more data would be particularly
approach the design storm size). valuable regarding the treatment performance for iron,
The analyses also indicate that all SCMs will not but would also be useful for aluminum, copper, lead,
provide equal performance. Dry detention ponds and and zinc, which in the 2015 MSGP monitoring data
hydrodynamic separators generally performed poorly show results that commonly exceed the benchmarks
compared to other treatment types in both the indus- across multiple sectors (see Tables 2-3 and 2-4 and
trial site evaluation and using the BMP Database. Appendix D).
Much of the poor performance likely is attributable
to scour of previously captured sediment (Avila and Priorities for Additional Monitoring
Pitt, 2009). Media filters, treatment trains, wet deten-
tion ponds, and bioretention were among the better- The Statement of Task asked the committee to
performing SCMs.
Three of the industrial sites met the benchmark for Identify the highest priority industrial facilities/
TSS for at least 50 percent of storm events, but failed subsectors for consideration of additional discharge

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 41

monitoring. By “highest priority” EPA means those at the same time. The ELG process includes a com-
facilities/subsectors for which the development of prehensive consideration of economic factors specifi-
numeric effluent limitations or reasonably stan- cally related to treatment technology performance, and
dardized stormwater control measures would be
most scientifically defensible (based on sampling
extensive opportunity for public input from planning
data quality, data gaps and the likelihood of filling to final promulgation.
them, and other data quantity/quality issues that Based on the paucity of industrial SCM perfor-
may affect the calculation of numeric limitations). mance data available at this time, no specific sectors are
recommended for development of new numeric efflu-
As discussed in Chapter 1, national effluent limita-
ent limits solely based on existing data, data gaps, and
tion guidelines (ELGs) are used to set enforceable
the current likelihood of filling them. Any new NEL
technology-based effluent limits. ELGs are developed
that is developed would require extensive collection of
based on performance of specified technologies and
new data. Instead, NELs are appropriate for sectors and
can be numeric or narrative. In the absence of ELGs,
pollutants that cannot be effectively controlled within
technology-based effluent limits can also be applied by
the MSGP and proposed AIM process (see Box 1-3)
best professional judgment on the technical capabilities
or for which there are documented benchmark attain-
of achieving effluent limits (EPA, 2010).
ability issues, considering implementation of reason-
All technology-based numeric effluent limita-
able structural and nonstructural SCMs.
tions (NELs) that currently apply in the MSGP were
In the committee’s review of the 2015 MSGP
developed through the ELG process in the 1970s
monitoring data, a few sectors stand out as having
and 1980s (see list in Appendix B). Additional NELs
a large percentage of samples with high discharges
for industrial stormwater could be developed based
(eight times the benchmark levels), including Sectors
on the performance of structural and nonstructural
H (coal mines and coal-mining-related facilities), A2
SCMs. Developing new NELs based on the capabili-
(wood preserving), F4 (nonferrous foundries), Q (water
ties of treatment technology and other on-site storm­
transportation), and R (ship and boat building or
water management practices would require significant
repairing yards) (see Table 2-4). A few of these sectors
amounts of rigorous monitoring data. For this reason,
reflect only a small number of sites. The AIM process,
the ELG process has several important advantages
which is under development, is intended to provide
over the MSGP process for development of NELs for
structured mechanisms to improve compliance under
industrial stormwater. First, although both the MSGP
the MSGP. Thus, it is premature to judge whether
and ELG processes can consider publicly available data
AIM will be effective to reduce these high stormwater
on the performance of treatment technology, the ELG
pollutant discharges. Those sectors that consistently fail
process includes the capability to generate additional
to meet the benchmark under the most intense scrutiny
performance data through targeted sampling, ques-
within the AIM process may be appropriate candidates
tionnaires, and other means. Key aspects of monitor-
for the development of ELGs, although individual per-
ing SCM performance include study design, sample
mits may also be a more efficient pathway for sectors
type and locations, data validation and reporting, and
with relatively few facilities.
performance analysis (Geosyntec Consultants and
Where benchmark attainability is questionable,
Wright Water Engineers, Inc., 2009). These elements
industries and industry groups should collect detailed
of performance monitoring go beyond the capability of
performance data for common SCMs under typical
what can be prescribed in a national general permit
stormwater conditions to expand the knowledge base
and reported through discharge monitoring reports
and potentially identify future sectors and pollutants
and annual reports in a useful manner. The ELG pro-
where numeric effluent limits may be appropriate.
cess also affords a more focused analysis of treatment
Such data should be collected using appropriate quality
technology performance, because it analyzes treat-
assurance and quality control (QA/QC) practices for
ment technology performance on a waste stream by
stormwater monitoring and include information on
waste stream basis, for specific sectors and subsectors.
SCM design, sizing, maintenance during monitoring,
In contrast the objective of the MSGP is to update
and on-site characteristics, such as watershed area, land
permit requirements for many sectors and subsectors

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

42 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

cover, and anticipated pollutants. These monitoring minimizing the additional monitoring cost burden.
data should be made available via a mechanism similar Replacement of COD with TOC should be considered
to (or directly employing) the International Storm- once EPA has adequate data to develop a benchmark
water BMP Database. The open nature of the BMP threshold level.
Database is an opportunity for a wide range of study EPA should implement a process to periodically
authors and reviewers to submit performance data with review and update sector-specific benchmark moni-
quality assurance reviews. Relatedly, in December 2017, toring requirements that incorporate new scientific
EPA released its Industrial Wastewater Treatment information. This process should consider updated
Technology Database (IWTT) as a publicly accessible industry fact sheets, published literature and industry
web application.4 EPA now conducts routine literature data, advances in monitoring technology, and other
reviews to identify performance data that could be available information so that the monitoring programs
included in the IWTT, and considers performance data adequately address the classes of pollutants used on site
in the IWTT in its ELG process (EPA, 2018). Similar and their potential for environmental contamination.
EPA efforts for industrial stormwater would strengthen The committee reviewed several sectors where data
the value of the BMP Database. suggest that stormwater pollutants are common, but
For water quality-based criteria, rigorous treat- little or no benchmark monitoring is required. In some
ment performance data are necessary to determine if cases, this situation resulted from limitations in the
there are benchmarks that are not attainable based on original process where industries self-determined what
current technology and best practices for site man- pollutants to monitor in their group applications, and
agement and pollution prevention. These data could those results were then analyzed to develop benchmark
provide scientific support for the development of new monitoring requirements. Additional information and
numeric effluent limits via the ELG process to reflect data gathering for PAHs could help EPA determine if
treatment attainability. For benchmarks based on treat- benchmark monitoring is needed for sectors that have
ment technology, such as TSS, the data could indicate the potential to release PAHs. Periodic monitoring
whether current benchmarks represent appropriate reviews would allow EPA to assess changing indus-
performance targets or, in fact, should be lower, based try practices that could affect monitoring needs, new
on improvements in the state of practice of structural analytical technology for pollutant quantification, as
and nonstructural SCMs. well as current toxicological information. Where data
gaps remain, additional sector-specific data-gathering
CONCLUSIONS AND efforts should be initiated.
RECOMMENDATIONS EPA should update the MSGP industrial sector
classifications so that requirements for monitor-
EPA should require industry-wide monitoring ing extend to nonindustrial facilities with activities
under the MSGP for pH, TSS, and COD as basic similar to those currently covered under the MSGP.
indicators of the effectiveness of stormwater control Many facilities and activities generating pollutants of
measures employed on site. These parameters can concern in stormwater discharges are not included
serve as broad indicators of poor site management, within the MSGP because the facilities themselves are
insufficient SCMs, or SCM failure, which can lead not considered to be industrial, even though the on-site
to high concentrations of these and other pollutants. activities (and associated risks) are similar to those of
Industry-wide monitoring of pH, TSS, and COD regulated facilities. These include school bus transpor-
would also provide a baseline understanding of indus- tation facilities and fuel storage and fueling facilities,
trial stormwater management across all sectors. All such as gas stations. Some states have included these
permitted facilities are currently required to conduct activities in their existing industrial general permits.
visual monitoring of quarterly stormwater samples, EPA should examine other facilities with activities
and these additional analyses are relatively inexpensive, similar to regulated facilities and add them to the
MSGP so that pollutant risks from these facilities can
4  See https://www.epa.gov/eg/industrial-wastewater-treatment-
be appropriately reduced.
technology-database-iwtt.

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

POLLUTANT MONITORING REQUIREMENTS AND BENCHMARK THRESHOLDS 43

Benchmarks should be based on the latest toxicity benchmarks for these metals can be reinstated if/
criteria designed to protect aquatic ecosystems from when acute aquatic life criteria are established or
adverse impacts from short-term or intermittent benchmarks are developed based on chronic effects
exposures, which to date have generally been acute from intermittent exposure.
criteria. Aquatic life criteria are designed for protec- • Express all benchmarks in the units from which
tion against both short-term (acute) and long-term they are derived, to improve communication and
(chronic) effects on both freshwater and saltwater spe- reduce reporting errors and provide guidance on
cies. Studies that form the basis of criteria development the expected level of precision in reported results.
typically measure acute end points following exposure
of aquatic life to consistent pollutant levels for short Additional monitoring data collection on the
periods of time, and measure chronic end points follow- capacity of SCMs to reduce industrial storm­water
ing exposure of aquatic life to consistent pollutant levels pollutants is recommended to inform periodic
for longer periods of time. Given the episodic nature of reviews of the benchmark thresholds and identify
stormwater flow and the likelihood of instream dilution sectors for which new national effluent limits could
and attenuation, aquatic life criteria based on short- help address treatment attainability. Publicly avail-
term (acute) or intermittent exposures are typically able stormwater data from industrial sites are cur-
more appropriate for stormwater benchmark thresh- rently insufficient to determine if there are specific
old levels than criteria based on long-term (chronic) conditions under which industries cannot meet the
exposures. Where EPA identifies substantial chronic benchmarks using conventional stormwater treatment
risks to aquatic ecosystems from intermittent exposures systems (e.g., sedimentation, filtration) or if other non­
during criteria development, such as for contaminants treatment SCMs could reduce concentrations on these
that bioaccumulate, an equation should be provided to sites. Based on limited available SCM performance
translate chronic criteria for intermittent exposures. In data, it appears that most standard treatment SCMs
this context, EPA should can meet the benchmark in least 50 percent of storm
events for TSS and for many pollutants at lower inflow
• Develop acute aquatic life criteria for benchmarks concentrations associated with municipal stormwater.
where they do not currently exist, or develop Considering that benchmark exceedance is judged by
­equations to translate chronic criteria into bench- the average of four sample events, these results suggest
marks based on intermittent exposures where that technical achievability is not a major issue for TSS.
substantial chronic risks to aquatic ecosystems exist Limited data suggest that benchmark compliance is
from repeated short-term stormwater exposures. more difficult at industrial sites for iron, aluminum,
Revisit the application of three benchmarks (iron, copper, and soft-water conditions for lead and zinc;
arsenic, and selenium) that are currently based on inadequate data are available for other pollutants. To
chronic and, in some cases, outdated aquatic life improve understanding of industrial SCM performance
criteria. and technical achievability:
• Allow permittees with repeated benchmark exceed-
ances to use the latest aquatic life criteria for • Industries and industry groups should collect
­selenium and copper to evaluate water quality risk scientifically rigorous performance data for com-
on a site-specific basis and discontinue comparisons mon SCMs under typical stormwater condi-
to national benchmarks, as appropriate. The latest tions to expand the knowledge base and inform
criteria for selenium and copper include equations future decision making. An appropriate number of
for calculating toxicity criteria based on short-term storms should be monitored by employing proper
exposure, using additional water chemistry and/or QA/QC to ensure data reliability, and design and
flow data. maintenance information for the SCMs should be
• Based on little evidence of adverse effects to provided.
aquatic organisms at common levels, suspend or • EPA should encourage industries to collect these
remove the benchmarks for magnesium and iron; data and make them publicly available, such as

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

44 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

uploading to the International Stormwater BMP Because of the paucity of rigorous industrial
Database. SCM performance data, the development of new
• EPA should support maintenance of these data for NELs is not recommended for any specific sector
industrial stormwater, as they are currently sup- based on existing data, data gaps, and the likelihood
porting the IWTT national database. of filling them. Any new NEL that is developed would
require extensive new data collection. Several sectors
For benchmarks based on aquatic life criteria, the can be identified in recent MSGP data with recurrent
additional high-quality data collected can be used to high-concentration discharges. However, the decision
assess the feasibility of achieving the benchmarks with to develop new numeric effluent limits would need
current technology and practices. For technology- to be informed by thorough SCM performance data
based benchmarks, additional data could inform future that clearly document attainability issues by sector
benchmark revisions to reflect the state of practice, and include a large number of permittees that cannot
reducing total loads to the extent practicable. achieve the benchmarks under the increased oversight
of the AIM process, which is currently in planning.

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Stormwater Sampling and Data Collection

S
ampling is required in the Multi-Sector General sample point at different times. Additionally, different
Permit (MSGP) because it provides information ­pollutants mobilize after different periods in contact
on the quality of stormwater leaving an industrial with flowing water.
site and on the performance of stormwater control mea- Generally, stormwater pollutant concentrations
sures (SCMs) in reducing pollutant burden. However, will follow a first-flush pattern, with the highest con-
stormwater sampling is complicated by the dynamic centrations occurring early in the storm. During the
characteristics of stormwater flow, the diffuse nature start of a storm, the rainfall is washing the drainage
of many stormwater flows, and the myriad potential area at its most polluted state. As the duration of the
­pollutants and pollutant characteristics that may exist storm con­tinues, the concentrations of pollutants gen-
on an industrial property. This chapter d ­ iscusses the erally fall (e.g., Sansalone and Cristina, 2004; Han et
many challenges in quantifying pollutant discharges al., 2006c). The difference between concentrations in
and includes recommendations to enhance the reli- first-flush runoff and later runoff can be an order of
ability and consistency of stormwater monitoring, magnitude or more for some pollutants. This is not
laboratory analysis, and data management to improve always the case, however, because changing rainfall
industrial stormwater management under the MSGP. intensity during a storm can provide energy mid-storm
that may scour the drainage areas and produce high
CHALLENGES OF QUANTIFYING concentrations after the first flush. Concentrations in
STORMWATER POLLTUANT DISCHARGE the effluent of treatment SCMs typically do not vary
as much as in untreated stormwater. Treatment SCMs
Quantifying stormwater pollutant concentrations, generally will reduce concentration and buffer high-
loads, and subsequent environmental impacts is a chal- and low-concentration excursions. For SCMs that have
lenge due to the variability in activities taking place stormwater storage, sampling of initial discharge at the
on the land, storm occurrences, stormwater flows, outfall may consist mostly of (treated) water that has
and ­pollutant concentrations (Breault and Granato, been stored from the previous storm. Different ­sampling
2000; Bent et al., 2001). Variations in water quality approaches, therefore, can lead to different results.
­parameters can occur within a single storm, between
storms, seasonally, and annually. Stormwater composi- Effect of Sampling Methodology
tion shows great temporal variation, especially in the
early stages of runoff, for many reasons. Storms of The volume-weighted (or flow-weighted) pollutant
different intensity (rainfall energy) mobilize and trans- concentration, also called the event mean concentra­tion
port pollutants at different times after runoff begins. (EMC), provides the most consistent and comprehen-
Runoff from different parts of a facility reaches the sive assessment of stormwater pollutant discharges and

45

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

46 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

loads. Pollutant loads are important for understanding centration due to the effect of the first flush (unless the
longer-term water-body impairments and toxicity con- discharge is coming out of a treatment SCM or other
cerns. The EMC is defined as the total pollutant mass device that stores water from the prior storm). Thus,
discharged in the stormwater divided by the total runoff current MSGP monitoring provides a low-cost, coarse
volume for the storm event, as measured at a specific indicator of the effectiveness of nonstructural and
outfall or measurement point: structural SCMs, and potential stormwater discharge
pollution concerns. Carefully collected and analyzed
T
Total Pollutant Mass �0 d C Q dt grab samples, as part of benchmark monitoring, have
EMC = = Td value in this regard.
Total Stormwater Volume �0 Q dt
Sampling the first runoff could add further consis-
where C is the pollutant concentration, Q is the flow tency and comparability to the grab sample data set and
rate, t is time, and Td is the storm duration. Determina- reduce monitoring variability. Inexpensive passive first-
tions of pollutant mass load or the EMC requires com- flush samplers are currently available that automatically
prehensive understanding of the flows and concentra- capture the initial runoff from a storm. These samplers
tions occurring over the entire storm event, which can hold approximately 1 liter and are placed in the field
be measurement intensive. A pollutant concentration before a storm event. They will fill with the first-flush
measured at a single time during a stormwater event runoff flow. A float (plastic ball) or other mechanism
cannot be considered to be representative of the EMC. blocks the collector input once the vessel is full. Com-
Different types of sampling schemes can be used to mercial first-flush samplers appear to provide useful,
quantify stormwater pollutant discharges, ranging from reproducible information on runoff water quality
simple grab samples to volume-weighted automatic (Landsman and Davis, 2018). However, these samplers
composite sampling that supports the calculation of the collect the first flow reaching the collection vessel. This
EMC. Stormwater sampling can be resource intensive, flow could be highly contaminated if it is the first wash
and sampling plan decisions need to balance the ben- of the drainage area, or, conversely, it could be the first
efits of the information obtained against the costs and flow from stored water in an SCM and be relatively
labor requirements. unpolluted. The use of first-flush samplers may elimi-
nate some of the variation associated with direct human
collection of samples, such as inconsistent placement of
Grab Sampling the sample bottle in the stormwater stream and vari-
A grab sample will always be a snapshot of a r­ apidly able time of collection. This type of sampling can also
changing situation. Trying to infer an EMC from a reduce the burden of sampling of remote sites.
grab sample is not scientifically justifiable. However, An additional problem of grab sampling is lack of
the more controlled and consistent the collection of the mixing of solids and the associated pollutants in the
grab sample(s), the more valuable and comparable water column. Grab sampling often consists of insert-
the information becomes. Comparing grab samples ing a bottle into the flow at the end of an outfall, and
that come from stormwater collected at different parts it is important to realize that the location of sampling
of the respective storm hydrographs will not have within the stormwater flow can introduce variability,
meaning. However, if samples are collected at the same particularly when sampling runoff has not been treated
(or near-same) sampling time during each storm, grab in a structural SCM to remove particulates. As a result
samples can be more reliable measures of stormwater of poor mixing, sampling near the bottom of the pipe
pollution, subject to the limitations described about can result in higher total suspended solids (TSS) con-
differences in rainfall energy in separate storms. centrations than samples collected at the water surface.
The current MSGP requires benchmark grab To address this, bedload samplers have been developed
­sampling to occur within 30 minutes of the start of and tested that can be installed in stormwater pipes to
runoff at the discharge point (see Table 1-1). The con- capture the solid material that will not be collected in
centrations in a sample taken in the first 30 minutes of traditional grab sampling or even by automatic samplers
a storm are likely to be higher than the event mean con- (Burton and Pitt, 2002).

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 47

Composite Sampling the samplers must be carefully set up and maintained

before runoff begins. Additionally, automated samplers
Composite sampling involves taking multiple can be set to collect samples during a storm that does
samples and combining them to obtain a measure of not occur during normal working hours. Finally, the
the overall stormwater condition. Several different EMCs resulting from composite sampling are likely
techniques of sample compositing are possible, depend- to be lower than pollutant concentrations in a grab
ing on the number of samples collected, if they are sample collected during the first flush and therefore
weighted by time or stormwater volume, and if they may reduce the likelihood of an exceedance that could
are collected manually or automatically. Generally, the trigger additional compliance requirements for the
greater the number of samples used in the compositing, discharger. Collection of composite volume-weighted
the closer the resulting sample is to the EMC of the samples over multiple storm events can provide a com-
pollutants in the stormwater (Ma et al., 2009). prehensive understanding of annual pollutant discharge
loads from a site and/or SCM performance over storms
Manual Composite Sampling. A reasonably complete of different size.
picture of pollutant concentrations in stormwater can Nonetheless, automated sampling may not be
be attained by collecting a composite sample over some appropriate for some water quality parameters. For
period of time, although not necessarily the entire example, samples for bacteria (not an MSGP ­parameter)
discharge from a storm. Taking multiple grab samples are usually not collected with an automatic sampler
during a storm event, especially at equally timed inter- because of holding time and contamination concerns.
vals, and combining them into a single sample for Most automatic samplers cannot sample bedload mate-
analysis will provide a closer approximation of the true rial (e.g., larger solids that are moving in the flow but
volume-weighted average concentrations of pollutants near the bottom of the pipe or channel and below the
in the runoff than single grab samples (Burton and Pitt, sampler intake tube). Therefore, automated sampling
2002; Ma et al., 2009). Extended composite sampling may miss sediment and associated pollutants that are
also would likely reduce the likelihood of exceeding a traveling very near the pipe bottom. Because they settle
benchmark compared to first-flush sampling because of quickly, bedload-sized particles are rarely seen in the
the expected high first-flush concentration. However, effluent of structural SCMs. Also, the MSGP uses TSS
this will come at the expense of increased sampling time as the measure of particulate matter, which does not
and complexity. generally include bedload material. The goal, whether in
grab or composite sampling, is to find a location where
Automated Composite Sampling. The most compre- the flow and solids are well mixed (Fischer et al., 1979;
hensive approach for assessing pollutant discharges is Saunders et al., 1983).
to install automated composite sampling equipment, Composite volume-weighted sampling can be
connected to stormwater flow measurement devices resource intensive, with initial costs of equipment and
(calibrated flumes or weirs), both of which are widely installation of the flume/weir, and operational costs
available. Composite volume-weighted sampling can with the time and expertise needed to maintain the
be employed to reliably quantify stormwater EMCs and sampling site, program the samplers, and check equip-
pollutant loads for most pollutants. Protocols for full- ment calibrations. For a facility, unless required, this
storm monitoring are now well established for volume- level of accuracy may not merit the additional costs,
weighted composite sampling and quality assurance depending on the goals of the sampling scheme.
and quality control procedures1 (Water Quality Pro-
gram, 2011). Volume-weighted automated composite
sampling has multiple advantages. It can reliably collect Other Sources of Error and Variability
sample aliquots after passage of specific stormwater Additional choices regarding sampling and analy-
volumes to obtain pollutant EMCs. Automated sam- sis, such as sampling design, equipment, analytical
pling also reduces the labor costs of sampling, although protocols, and operator skill, will affect the results.
1 
Example sources of sampling and analysis error capable
See www.bmpdatabase.org/monitoring-guidance.html.

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

48 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE 3-1
Example Sources of Sampling Error and Variability in MSGP Stormwater Monitoring Results

Sources of Error Ways to Reduce Error

Sampling Outfall sampled not representative of all sources
•  Review of outfall selection
• 
design Sample not collected at outfall or samples collected
•  Training of sampling personnel
• 
at different outfalls Clear identification of sampling point
• 
Sampling Incorrect type of sampling container
•  Use correct type of sampling container
• 
procedure Contaminated sample container
•  Proper cleaning/rinsing of container
• 
Improper sampling procedure for intended purpose
•  Training of sampling personnel
• 
Improper method of compositing multiple grab
• 
samples
Laboratory Interlaboratory variability
•  Establish National Pollutant Discharge Elimination
• 
analysis System (NPDES) laboratory accreditation programs
with a focus on stormwater matrices
Encourage interlaboratory calibration efforts
• 
Data Incorrect units reported
•  Units clearly specified
• 
management Incorrect data input
•  Electronic reporting with database flags for
• 
­benchmark exceedances and unusual data

of causing a difference between true water quality at the pipes, a single grab sample may not reflect the true
desired measuring point and water quality measure- volume-averaged or depth-averaged concentration at
ments are highlighted in Table 3-1. that sampling time. Collecting samples at different
Sampling variability can result from inconsistencies flow depths (depth-integrated sampling) can provide
in selecting the monitoring point. Because of the dif- concentrations more representative of the true values
fuse nature of stormwater, isolating a specific discharge (Selbig et al., 2012). Finally, collecting from defin-
point may be challenging. Because of the highly variable able channels is much more repeatable than sheet flow
conditions at industrial facilities, stormwater sampling sampling. Sheet flow can be difficult to monitor and
points can be difficult to define. Facilities regulated multiple samples may have to be collected at different
under the current MSGP that have multiple discharges points spatially in order to generate a clear picture of
may collect their stormwater samples at one discharge the pollutant concentrations.
point and list it as being representative of all discharge Variation in sample processing and analysis pro-
points. This is a valid procedure if the activities taking tocols can also affect results. The time prior to sample
place on each drainage area are adequately similar. If the processing in the field or laboratory has been noted as
land uses are not similar, such an approach can provide an important factor affecting solids and metals mea-
an inaccurate assessment of the pollutant discharges surements due to the creation and dissolution of flocs
from the site. Where industrial activity is not equally in the stormwater over time (Furumai et al., 2002;
distributed across various discharge points, the MSGP ­Kayhanian et al., 2005; Li et al., 2005). The a­ nalytical
requires that multiple sampling points be included. method chosen also can be a factor in variability of
Complex facilities with a large footprint will need results (Gray et al., 2000; Clark and Siu, 2008). Varia-
to sample multiple discharge points to represent the tion in technique and skill among analysts within a
myriad activities taking place at the facility. single laboratory also has an effect (Clark and Pitt,
Once the outfalls are selected, locating an appropri- 2008). In addition to the sources of variability dis-
ate point in the flow path also is required, as discussed cussed above, other factors that affect sample results
in the previous section on the effects of sampling could include sample contamination and improper data
approaches. The type of equipment used and its instal- handling. In all stormwater monitoring situations, it is
lation location also will impact the results (Winterstein important to minimize the error and understand and
and Stefan, 1983; Graczyk et al., 2000; Cristina et manage the variability.
al., 2002; Clark et al., 2009). In discharges from large

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 49

RECOMMENDED IMPROVEMENTS TO (EPA, 1984, p. 37006). Composite sampling over a

SAMPLING AND ANALYSIS PROTOCOLS period up to 24 hours also better aligns with the time
period used to express water quality-based effluent
Several opportunities exist to reduce error in limits in permits. Although EPA recommends that
industrial stormwater monitoring. In this section, the acute aquatic life criteria be expressed as maximum
committee recommends improvements to the MSGP 1-hour average concentrations (EPA, 1985), effluent
sampling requirements and training for sampling and limits in NPDES permits that are derived from acute
laboratory personnel. The committee also discusses aquatic life criteria are expressed as daily average limits
novel technologies that in the years ahead may offer (see 40 CFR § 122.45(d)). EPA explains in its Techni-
additional efficiencies and improved accuracy for cal Support Document for Water Quality-Based Toxics
MSGP monitoring. Control (EPA, 1991) that the 1-hour averaging period
was derived primarily from data on response time for
Sampling Type ammonia, a fast-acting toxicant. EPA (1991) states that
there are scientifically justifiable alternative averaging
The committee recommends that the Environ- periods and that, in practice, 1-day periods are the
mental Protection Agency (EPA) allow and promote shortest periods for which wasteload allocation mod-
the use of composite sampling for benchmark moni- elers and enforcement personnel have adequate data.
toring for all pollutants except those that transform or Multiple types of composite sampling techniques
degrade rapidly or are especially time sensitive (e.g., could be used, including more simplistic manual com-
pH). As discussed previously, composite sampling posite methods that characterize the first few hours
provides more consistent and reliable data and resolves of discharge and more complex automated methods
or reduces the problem of intrastorm variability in pol- that composite samples over a discharge period up to
lutant concentrations. For sites with treatment SCMs 24 hours. EPA originally required both grab and com-
that store water from the previous storm, composite posite sampling in the 1992 baseline permits and as
sampling would provide more accurate stormwater dis- part of the 1992 group application process. At that
charge results. The committee recognizes that the cost time EPA stated that it was necessary to provide infor-
and complexity of managing composite samplers may mation on the first-flush concentration as well as the
be more than some permittees want to bear, and there- average concentration of pollutants discharged during
fore, composite sampling should not be required. How- an event (EPA, 1992a, p. 41296). In issuing the 1995
ever, the advantages of composite sampling (including MSGP, EPA allowed the use of grab sampling for the
the increased likelihood of meeting the benchmark vast majority of permit-required sampling, presumably
with an EMC as compared to a grab) may encourage as a burden reduction (EPA, 1995, p. 50828). In issu-
the investment in higher-quality data collection. ing the 2008 MSGP, EPA discontinued all remaining
The current MSGP requirement for grab samples composite sampling and essentially dis­allowed the use of
during the first 30 minutes to capture the first flush is composite sampling in favor of grab sampling to charac-
inconsistent with the methods used to derive bench- terize the high pollutant concentration that would occur
mark thresholds. Technology-based MSGP benchmark during a first-flush effect (EPA, 2008b). The limita-
thresholds are derived from Nationwide Urban Runoff tions of grab samples, including the high variability, are
Program (NURP) values and secondary treatment now well understood. Composite sampling technology,
requirements. NURP values are calculated from event which is widely available and relatively inexpensive,
mean concentrations in stormwater discharges that could significantly improve the consistency and reli-
were characterized using composite sampling tech- ability of stormwater data and should be encouraged.
niques (EPA, 1983). Secondary treatment requirements
are derived from measured performance of publicly Monitoring Frequency
owned treatment works over longer time periods, and
the values that form a basis for MSGP benchmark The current MSGP allows benchmark monitoring
thresholds are based on 30-day averaging periods to be stopped for that permit term if the mean concen-

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

50 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

tration of the four previous quarterly measurements is Therefore, for a TSS benchmark of 100 mg/L, any
below the benchmark threshold. Given the pronounced quarterly average concentration from 0 to 225 mg/L
variability in the flow and quality of stormwater and the is statistically indistinguishable from the benchmark.
potential for major changes in site management over COV greater than 1.0 would produce a larger range.
time, four quarterly samples are insufficient to assess Reducing this range to a scientifically preferred value,
the adequacy of stormwater management at a facility such as 20 percent (80 to 120 mg/L TSS) would require
over the course of a permit term of 5 years. 150 samples at 1.0 COV.
The first concern relates to the number of storm Obviously, this level of sampling is unrealistic. Col-
event samples that is sufficient to determine a bench- lection of more samples increases the confidence that a
mark exceedance, given the inherent variability in site is complying with the requirements by reducing the
stormwater runoff. Stormwater pollutant concentra- acceptable error. Ultimately the decision on the num-
tions will vary with antecedent dry conditions, storm- ber of samples to require is based on what amount of
water flow rates, industrial activity on the site, and error is acceptable, relative to the cost of the increased
many other factors. monitoring. Technology verification for SCMs used in
The number of samples required to be statistically municipal stormwater requires monitoring of a mini-
confident that the sample mean is less than a spe- mum of 12 storm events (composite sampling) over a
cific value, such as a benchmark, is dependent on the range of storm intensities (with other constraints on
acceptable error and the coefficient of variation (COV, type of storm, etc. [Water Quality Program, 2011]).
standard deviation of the samples divided by the mean) In addition to the drawbacks of a limited sampling
(see Burton and Pitt, 2002; Ott and Longnecker, 2015). population, the MSGP sampling waiver after four
Figure 3-1 displays the acceptable difference between samples poses additional temporal concerns, because a
the stormwater discharge sample mean and the respec- facility would then not be required to monitor storm-
tive benchmark based on the number of samples and water for up to 4 years (or more) until the next permit
COV of the data set (α = 0.05; power = 0.80).2 In an term begins. Various modifications to the facility,
analysis of the National Stormwater Quality Database,3 changes in activities at the facility, and turnover in site
Burton and Pitt (2002) determined that coefficients of personnel could take place during the period of moni-
variation for stormwater runoff across multiple sites toring relief, all of which could impact the stormwater
when appropriately categorized by land use, region, discharge characteristics. Structural SCM performance
and sometimes seasons were 0.5 to 1.0 (measured using can also degrade over time, if not maintained, usually
composite sampling). Use of grab samples and other due to clogging in a media filter or sediment buildup
sources of error and variability (see Table 3-1) will that reduces the treatment volume and increases scour
increase the COV. in sedimentation devices. Sustained monitoring can
For example, with a COV of 1.0 (optimistic for help ensure that permittees continue to implement and
grab samples), with collection of only four samples, maintain SCMs consistently during the entire permit
the acceptable error is 125 percent, as the difference period. More frequent continual sampling allows a
between the measured mean value and the benchmark. consistent representation of stormwater discharge as
operations and personnel change over the duration of
2  The Type 1 error rate (α) is the risk of a false positive, where a permit term. Additional sampling throughout the
something is assumed to be true when it is actually false. According permit term also helps reduce the uncertainty associ-
to Burton and Pitt (2002), “an example would be concluding that
ated with natural variability among storms and wet
a tested water was adversely contaminated, when it actually was
clean. The most common value of α is 0.05 (accepting a 5 percent versus dry years.
risk of having a Type 1 error).” Power is 1 – β. Type 2 error rate Some states have acted to increase the frequency
(β) is the risk of “a false negative, or assuming something is false of chemical monitoring beyond that specified in the
when it is actually true. An example would be concluding that a
tested water was clean when it actually was contaminated. If this
MSGP (see Appendix A). Washington allows monitor-
was an effluent, it would therefore be an illegal discharge with the ing relief only after having eight consecutive quarterly
possible imposition of severe penalties from the regulatory agency” samples with concentrations less than the benchmark.
(Burton and Pitt, 2002). California allows a reduction in sampling frequency to
3  See http://www.bmpdatabase.org/nsqd.html.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 51

FIGURE 3-1 Number of samples necessary to detect a statistically significant difference between the sample means and the benchmark,
given the acceptable relative error (the percent difference between the sample mean and the benchmark that is statistically indistin-
guishable from the benchmark) and the coefficient of variation at α = 0.05 and power of 80 percent.
NOTE: Graph is approximate because it assumes a normal distribution of samples.
SOURCE: Developed based on Burton and Pitt, 2002, p. 231.

once per 6 months after four consecutive samples with frequencies (such as 2 years of quarterly monitoring,
concentrations less than the benchmarks. or twice annual monitoring for those with monitoring
The MSGP should include a minimum of annual relief ) in terms of reductions in acceptable error.
sampling for those that qualify for monitoring relief
to ensure that appropriate stormwater management Role of Training for Sampling and Laboratory
continues throughout the permit term and to provide Personnel
additional data to indicate the effectiveness of SCMs.
Furthermore, EPA should also analyze COVs for Data and field experience show substantial differ-
­sector- and site-specific industrial stormwater data to ences in the reliability of samples collected by facility
evaluate the benefits of additional increases in sampling personnel as compared to trained (watershed agency)

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

52 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

personnel (K. Schiff, Southern California Coastal perform specific types of testing and measurements.
Water Research Project, personal communication, While the Safe Drinking Water Act contains labora-
2018). This difference is attributed to the fact that tory certification requirements, other major federal
agency personnel are trained in water, wastewater, and environmental statutes including the Clean Water Act
stormwater sampling procedures and pollutant trans- do not contain similar requirements. Some states have
port concepts and have experience with multiple storm- independently established certification programs for
water situations. In contrast, industrial facility staff may additional environmental media, including waste­water,
not be trained in stormwater concepts or procedures. solid and hazardous wastes, and air samples, and require
Inconsistent sampling at a given facility across multiple the use of certified laboratories through their own
storms may result from using untrained personnel statutes and regulations. The National Environmental
or by employing different personnel who implement Laboratory Accreditation Program (NELAP) exists to
procedures differently. The committee recommends promote technical competence of environmental labo-
training and guidance, including the possibility of a ratories and develops nationally recognized standards
training/certificate program in stormwater collection for accreditation.5 For stormwater, EPA encourages the
and monitoring, to reduce the variation in sampling use of laboratories certified by agencies accredited by
design and sample collection. NELAP (EPA, 2009a). Minnesota is an example of a
Water quality analysis of stormwater samples is state that has a Clean Water Act laboratory accredita-
most often performed by private contract laboratories, tion program and requires use of an accredited labora-
but may also be done in house, particularly for facilities tory in its MSGP (MPCA, 2015). Because storm-
of large corporations. Some variation in measured pol- water is distinct from wastewater, it is important to
lutant concentrations can routinely be expected due to understand to what extent the laboratory certification
variability in laboratory methods, individual behaviors, program includes evaluation of technical competence
reporting levels, and degree of quality control, which with the stormwater matrix.
affect the accuracy and precision of measurements Periodic interlaboratory calibration programs
(Clark and Pitt, 2008). For example, for TSS, three ­represent another approach that has been used to
methods are approved in 40 CFR § 163.3,4 which promote the comparability of testing results among
yield different results for known concentrations of laboratories. These programs bring standardization
storm­water solids even when a well-trained analyst is to analytical procedures for stormwater samples. Such
performing the analysis, with results varying by up to programs facilitate communication among laboratory
25 percent (Gray et al., 2000; Clark and Siu, 2008). personnel, help set performance-based criteria to mea-
There was added variability when different well-trained sure success, and utilize a locally derived stormwater
analysts measured TSS using one of the methods. matrix while improving comparability and reliability
Much of this variability was attributed to the methods of stormwater sampling results used to determine
employed to obtain the aliquot used in the analytical compliance with water quality benchmarks. After
method and the difficulty in capturing larger, heavier inter­laboratory co­efficients of variation were noted
particles in the subsampling of the initial sample. in California that were greater than 40 percent for
The industrial stormwater matrix poses particular some stormwater pollutants, the Southern California
challenges in analysis because many of the pollutants Stormwater Monitoring Coalition (SMC) developed
of concerns (e.g., metals, organics) are likely to sorb to guidelines and protocols to ensure comparability of
solids. For some organics, the difficulty of extracting the laboratory results for stormwater samples (Gossett and
pollutant into an aqueous or solvent phase for measure- Schiff, 2010). The SMC also instituted inter­laboratory
ment can result in matrix interferences that reduce the calibration exercises6 that have reduced variability
accuracy of the analytical method (EPA, 1986).
Laboratory certification programs evaluate and
certify the technical competence of laboratories to 5  See https://cfpub.epa.gov/si/si_public_record_report.

cfm?dirEntryId=56216.
4  Standard Methods SM 2540D-2011, USGS I-3765, and 6  See http://socalsmc.org/smc-regional-stormwater-­monitoring-

ASTM D5709-18. comparison-and-evaluation.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 53

(K. Schiff, SCCWRP, personal communication, 2018), perature sensors. Data can be collected in the field in
but such regional efforts are rare. real time over wireless networks. Reliable field turbidity
To reduce analytical variability and improve the sensors now available could open up the possibility of
utility of monitoring results, the committee recom- using turbidity as a surrogate for TSS in future permit
mends that EPA encourage state adoption of national terms. For construction site erosion, turbidity, instead
laboratory accreditation programs for the Clean Water of TSS, is used to document performance of erosion
Act with a focus on the stormwater matrix and on control practices in several states, such as Vermont
reducing the variabilities associated with stormwater (State of Vermont, 2008).
pollutants that have been noted above. EPA should Field sensors that accurately and reliably measure
initiate and encourage interlaboratory calibration other water quality parameters are currently available,
efforts, including the establishment of performance although they are somewhat costly. A wider range of
quantification levels developed from samples with a sensors at lower cost is anticipated to be available in
stormwater matrix. EPA should publish guidance and the near future. Newly developed sensors may be able
case studies on interlaboratory calibration, with specific to directly measure concentrations of water quality
focus on known challenges to stormwater analysis (e.g., parameters of interest, or may measure useful surrogates
solids capture, matrix interference). These efforts would of water quality. The overall result will be more reliable
promote the comparability and reliability of test results monitoring of industrial stormwater discharges. The
reported to permitting authorities. use of real-time control of SCM performance using
water level/storage information and weather predic-
New Methodologies or Technologies for Industrial tions is also possible (Kerkez et al., 2016).
Stormwater Monitoring Future revisions of the MSGP monitoring require-
ments should consider advances in sensor technology
In this section, a few examples are offered of and reductions in costs since the previous permit
potential improvements in monitoring technology release. By considering the latest technology, EPA can
for industrial stormwater discharge; some of these are take advantage of opportunities to improve the value
available today, and others may be reasonably expected of the MSGP monitoring program, providing more or
in a not-too-distant future. Monitoring technology better-quality information for similar or reduced costs.
is considered here in the broadest sense and includes
hardware, software, sensors, sampling techniques and TIERED APPROACH TO MONITORING
timing, mobile technologies, and apps.
Visual monitoring information could be addressed The current MSGP monitoring approach could be
with the future development of mobile apps that may substantially improved to provide more useful informa-
be useful in identifying stormwater clarity, sheens, or tion on the quality of stormwater discharges and their
other visual water quality indicators via still imaging impact on receiving waters while balancing the net
or video. Drone imaging may be useful in visual storm­ burden to industry and limited agency resources. In
water discharge monitoring and delineation of drainage this section, a tiered approach, with different levels of
areas and covered/exposed areas. inspection or monitoring according to risk, complex-
Sensors and real-time control are ubiquitous in ity, and past performance of stormwater management,
process control, water quality measurements, and is recommended. This approach is expected to provide
documenting water quality in drinking water and better overall protection of the environment and public
wastewater treatment facilities of all sizes. Stormwater health.
applications are obviously complicated by the episodic
and dynamic nature of stormwater flows and quality. Proposed Categories of Monitoring
Advances in sensors can lead to improved monitor-
ing of stormwater discharges and SCM performance. The committee envisions a framework (see
Field-employable flow/moisture sensors are available Table 3-2) where the most complex, high-risk facili-
now, as are turbidity, pH, dissolved oxygen, and tem- ties or those with recurring exceedances would be

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

54 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE 3-2
Table of Criteria and Implications for Proposed Monitoring Tiers

Tier Criteria Summary of Monitoring Difference from Current MSGP

Category 1: The type, extent, and intensity
•  Visual only. Facility inspections New allowance for low-risk sites
Inspection Only of industrial activities conducted by the permitting authority or a to replace chemical monitoring
on site meet specified criteria for certified inspector are used as an with inspection, conducted by
a low-risk site, and alternative to chemical monitoring, permitting authority or certified
The facility has been subject
•  with inspection report submitted inspector, at least once per permit
to one or more inspections by to permitting authority. term.
the permitting authority or a The permitting authority can deny
certified inspector during the the chemical monitoring waiver
previous permit term. in cases where inspection results
demonstrate SCM implementation
is inadequate.
Category 2: Sector/subsectors for which the Visual and industry-wide (pH, TSS, Adds monitoring for pH, TSS, and
Industry-Wide permitting authority determines, COD) monitoring in addition to COD for all facilities subject to the
Monitoring based on recent data and industry routine facility inspections. MSGP, except for low-risk facilities
literature review, that sector- in Category 1.
specific benchmark monitoring is EPA decision not to require sector-
not warranted and facilities do not specific benchmark monitoring
qualify for Category 1. should be based on updated,
periodic data and literature reviews.
Category 3: Sectors/subsectors for which the Visual, industry-wide (pH, TSS, Similar to current MSGP for sectors
Benchmark permitting authority determines, COD), and sector-specific with benchmark monitoring,
Monitoring based on most recent data and benchmark monitoring in addition except chemical monitoring
industry literature review, that to routine facility inspections. requirements are based on
potential pollutant levels warrant updated data and industry
benchmark monitoring and literature review. Includes industry-
corrective actions and SCMs are wide pH, TSS, and COD monitoring
reasonably available for additional for all sectors.
pollutant reduction.
Category 4: Either Potentially includes composite New category for high-risk
Enhanced Benchmark monitoring results
•  sampling, possibly at multiple complex sites or facilities with
Monitoring have triggered the Additional outfalls, to supplement or in lieu of repeat exceedances.
Implementation Measure (AIM) benchmark monitoring.
Tier 3 process, or
The permitting authority has
• 
determined that more robust
monitoring is warranted
(e.g., TMDL development or
implementation; large, complex
sites; inspection identifies major
concerns).

NOTE: In all categories, no exposure exemptions may be granted. Additional monitoring may be required for facilities with effluent
limitation guidelines (see Appendix B).

required to conduct more-sophisticated monitor- water quality monitoring with rigorous inspections
ing to assess their impact to receiving waters and and reporting. Details of the monitoring categories
target future improvements to stormwater control proposed by the committee are provided below.
measures, consistent with the recommendations of
the National Research Council (NRC, 2009). Those Inspection Only (Category 1)
facilities required to conduct benchmark monitoring
would continue to do so, but a much larger number of The committee recommends that EPA define and
facilities that currently conduct only visual monitoring create a category for facilities that could rely on inspec-
would also monitor for TSS, pH, and chemical oxygen tion by a permitting authority or certified inspector as
demand (COD) as part of industry-wide monitoring a complete alternative to chemical discharge moni-
(see Chapter 2). Facilities with low risk and low likeli- toring. Providing an option for inspection in lieu of
hood of substantial pollutant discharges could replace monitoring can reduce the burden on small, low-risk

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 55

facilities while improving stormwater management.

This recommenda­tion is distinct from how the current BOX 3-1 Inspections with the MSGP
MSGP operates. Facilities would be allowed to rely
on inspections as an alternative to chemical discharge
The value of facility inspections to evaluate
monitoring if they exhibit a low risk of contributing to site conditions and pollutant discharge potential
water quality problems via stormwater discharge. This was recognized early in the implementation of
industrial stormwater permitting as an important
determination would be facility based rather than sec-
supplement to chemical monitoring. The permit-
tor based and would be verified by a certified inspector. ting regulations were modified in 1992 specifically
Given the limited capacity for permitting authorities to to allow for the use of facility self-inspections as
a monitoring tool (EPA, 1992a). The 1995 MSGP
perform compliance inspections, the committee envi- incorporated the use of facility self-inspections
sions that certified inspectors would be contracted by as an alternative to or to supplement discharge
monitoring.
many of the permittees to conduct the inspection. The The current MSGP has a continued strong reli-
certified inspector could be an employee of a municipal ance on routine facility inspections conducted by
separate storm sewer system (MS4), a private third- the permittee. Inspections must be conducted
at least quarterly, and at least once per calendar
party company, or a parent corporation, as long as the year the inspection must be conducted during a
inspector is not directly involved in the day-to-day period when a stormwater discharge is occurring.
Inspections must be conducted by qualified per-
operation or oversight of the facility being inspected. sonnel, who are those who are “knowledgeable in
The value of facility inspections to evaluate site the principles and practices of industrial storm­
conditions and pollutant discharge potential was recog- water controls and pollution prevention,” and who
possess “the education and ability to assess con-
nized early in the implementation of industrial storm- ditions at the industrial facility that could impact
water permitting (see Box 3-1). In-person education stormwater quality, and the education and ability
to assess the effectiveness of stormwater controls
was highlighted by several state permitting agencies in selected and installed to meet the requirements
meetings with the committee as an effective means to of the permit” (2015 MSGP).
improve stormwater management and permit compli- Compliance inspections are visits by a permit-
ting authority or EPA to officially assess compli-
ance at small industrial sites. Small businesses may ance with environmental regulations and require-
also have difficulty collecting reliable monitoring data, ments (EPA, 2004). EPA’s most recent published
goal for inspection rates for industrial stormwater
because they may have limited financial resources or states that 10 percent of permitted facilities be
limited staff to develop monitoring expertise. Many of inspected each year (EPA, 2014). In meetings with
those businesses, and some larger businesses, operate this committee, several state permitting agencies
noted that their ability to conduct stormwater
small facilities that qualify as low risk. For those facili- compliance inspections at industrial facilities
ties, both the permittee and the permitting authority is significantly resource limited. Additionally,
inconsistent training of personnel who conduct
could potentially be better served with a rigorous inspections can hinder the effectiveness of the
inspection instead of monitoring. A site inspection by inspections.
a certified inspector would increase the reliability of the
results, and the business may welcome the exemption
from chemical monitoring.
The committee recognizes the difficulty in defin-
ing the characteristics of a low-risk facility. For a site that pose a pollution risk to water quality, assuming that
to be considered at low risk of impacting water quality, the facility is not part of larger network of integrated
it should have a low likelihood of discharging toxic operations at multiple facilities. Additionally, a site that
substances in toxic amounts, generally have a small area has little area of exposed industrial activities reason-
of exposed industrial activity, and be well managed. A ably may be expected to discharge lower volumes of
simple approach would be to base low pollutant risk industrial runoff (and corresponding lower mass load of
on the amount of surface area that contains industrial pollutants) for a given size of precipitation event com-
activities exposed to stormwater. Many very small pared to facilities with larger industrial operations. A
facilities (e.g., less than 0.5 to 1 acre of industrial activi- 1-acre threshold is used to distinguish the relative risk
ties) have relatively few activities exposed to stormwater of water quality impacts associated with discharges of

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

56 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

stormwater associated with construction activity (EPA, presented in Table 3-3. Conformance to the criteria
1999). However, several states established a smaller would be verified by an inspection.
area threshold for exempting certain erosion controls, Also important in the determination of a low-risk
especially in high-value watersheds. facility is certification that the site is well managed. To
A criterion of facility area, although simple to assess this the committee recommends that the facility
implement, nonetheless is not a robust indicator of inspection be conducted at least once per permit term
risk. A small industrial facility may or may not store and include the elements of a stormwater compliance
hazardous materials, handle materials in large volume, inspection, such as
or rely on outdoor equipment or operations that release
stormwater pollutants during operation. Small facili- • Reviewing the permit and the stormwater pollu-
ties in some industry categories have the potential to tion prevention plan (SWPPP) and determining
generate substantial amounts of pollutants capable whether the SWPPP meets the requirements set
of causing harm in receiving waters. In urban areas, forth in the permit;
clusters of small facilities in aggregate may generate • Reviewing records, including self-inspection
substantial discharges. At the same time, many mid- reports, to verify that the permittee is complying
sized industrial facilities conduct limited activities with the permit and the SWPPP;
exposed to stormwater, for which effective management • Walking the site and verifying that the SWPPP
strategies are relatively easy to implement and maintain. is accurate and that the SCMs are in place and
Industrial stormwater discharges from these mid-sized functioning; and
facilities would be expected to produce much lower • Identifying actions that need to be taken to effec-
pollutant mass loadings compared to smaller facilities tively manage stormwater pollution.
with more active operations. Research has documented
substantial variation among facilities of a given sector In addition, inspections can provide opportunities to
in the type, extent, and intensity of industrial activities educate facility operators on the most effective steps
they conduct that may be expected to govern a facility’s to improve stormwater management.
risk of discharging pollutants (Swamikannu et al., 2000; A publicly accessible report filed with the permit-
Cross and Duke, 2008). Because industrial facilities are ting authority would document the findings of the
so highly variable, classifying facility risk is most accu- inspection and any specific concerns and recommenda-
rately based on a characterization of the intensities and tions for additional SCMs. Current facility conditions
types of industrial activities conducted at each facility. would be compared to previous conditions documented
Specific criteria could be developed that charac- in prior inspection reports. If the inspection indicates
terize the presence or absence of activities considered substantial concerns, recurrent problems that have
likely to generate stormwater pollutants that could remained unaddressed, or a lapse in inspections, the
cause water quality problems. The criteria could be permitting authority or inspector could recommend
similar to those developed for no exposure exemptions, the facility be placed in another category that would
which describe in narrative form all activities that, if include required chemical monitoring. Local entities
present, would preclude a facility from qualifying for such as MS4 permittees, agencies responsible for total
the exemption. The criteria envisioned here are admit- maximum daily load (TMDL) implementation, or
tedly more complex, because they are intended to assess those responsible for other watershed protection pro-
the expected magnitude or scale of pollutants from grams could also petition the permitting authority to
activities rather than simply the presence or absence of exclude from Category 1 industry types or individual
an activity. However, the process of establishing criteria facilities that are found to be potentially discharg-
is the same, and EPA can rely on its experience and the ing pollutants that are causing or contributing to the
experience of the states to define activities that may impairment of receiving waters.
reasonably be expected to discharge toxic pollutants Because inspection would serve as an alternative to
in toxic amounts during routine operation. Examples chemical benchmark monitoring, an inspector certifica-
of possible criteria for low pollutant discharge risk are tion program (see Box 3-2) is recommended to promote

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 57

TABLE 3-3
Example Criteria for Determining Low-Risk Facilities (Category 1)

Activity Conditions That Will Attain “Low Risk”

Outdoor temporary storage Intent: Low volume of water contacts surfaces where residuals may accumulate.
of “factory floor wastes” Possible criteria: Containers covered. No process chemicals or hazardous substances. Residuals
such as lumber, containers, that may fall to surfaces removed, and surfaces cleaned, in at most 5 days, with verified operating
and debris procedure in place.
Outdoor storage of waste, Intent: Should be routinely maintained, unusable items removed, and kept to minimal space, with no
scrap, and equipment be- items stored long term. Stored on impermeable hard surface.
lieved potentially usable in Possible criteria: Storage area no larger than 100 m2. No materials that contain or have exposed
future patches of lubricants, fuels, or process liquids. Routinely inspected to remove wastes, with verified
operating procedure.
Outdoor materials handling Intent: Handling infrequent, materials well packaged, with detailed spill prevention and response
or transport of packaged procedures in place.
materials or drums of liquids Possible criteria: Handling limited to 1 hour of operations daily (weekly average). Verified operating
or particles procedure includes inspection after each handling operation to identify, remove, or clean up spills,
leaks, and debris.
Vehicles or equipment used Intent: Vehicles well maintained so fuels and lubricants do not leak.
outdoors or in plant yard Possible criteria: Vehicle maintenance, fueling, and cleaning conducted indoors. Vehicles used less
(small trucks, forklifts, hand than 1 hour per day, weekly average. Vehicles do not operate outdoors during precipitation, or else
trucks, etc.) vehicles are routinely cleaned indoors to keep free of pollutants that may accumulate on vehicle
surfaces.
Material handling/­loading Intent: Limited in number and in frequency of usage.
­areas, loading docks or Possible criteria: Materials handled in packaged, boxed, or drum form—no handling of materials
doors in powder, liquid, or slurry form, and no hazardous or toxic materials. No more than three loading
docks, with no more than five loadings/unloadings each per week. Verified operating procedures
for inspection and cleaning.
Vehicle maintenance Intent: Vehicle maintenance limited to nonpolluting activities.
Possible criteria: No washing of vehicles with accumulated surface residuals except indoors or in
areas with separate drains to process wastewater. Vehicle fueling prohibited in locations exposed
to stormwater. Lubricant and liquids work only in small amounts (e.g., one oil change volume) with
proper trays and spill avoidance/response procedures and on hard surface. Verified operating
procedures include inspection and cleaning of these areas.

NOTE: These criteria are intended to lead to a determination that the type, intensity, and extent of industrial activities are unlikely to
generate discharges of pollutants of a kind and a quantity that may cause or contribute to water quality problems in receiving waters.
The intent is to create a category of facilities that do not meet the rigorous criteria of “no exposure” but encompass facilities with activi-
ties that are small but nonzero in spatial extent, frequency, intensity, and/or presence of residuals. These are committee suggestions, but
EPA should develop concrete and implementable criteria conditions.

BOX 3-2 California’s Industrial Stormwater Practitioner Certification

The California Industrial Stormwater General Permit issued in 2014 establishes requirements for industrial storm­
water permittees to have a Qualified Industrial Stormwater Practitioner evaluate and certify the adequacy of corrective
actions at industrial facilities when basic numeric action levels or EPA Sector Specific Benchmarks are exceeded (CA
NPDES Permit No. CAS000001; Order No. 20014-0057-DWQ). Qualified Industrial Stormwater Practitioners complete
a permitting-authority-sponsored or -approved training course and register themselves in the state’s electronic data-
base. They are authorized to evaluate SCM implementation and pollutant sources; prepare technical reports, action
plans, and extension requests when exceedances persist; and evaluate permit coverage eligibility for new facilities
to discharge to impaired waters. If judged to be noncompetent, they can have their certifying eligibility revoked.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

58 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

confidence in the thoroughness and reliability of results. recent data and literature to reflect recent knowledge of
The certified inspector would evaluate the facility’s pollutant toxicity, sector risks, and stormwater manage-
SCMs and conformance to the criteria for this low- ment capabilities, as discussed in Chapter 2. All facili-
risk category. An inspector certification program would ties with benchmark monitoring should also conduct
provide a means to certify and track the credentials of industry-wide monitoring for pH, TSS, and COD, in
the inspector, promote inspector accountability, and addition to the currently required visual monitoring of
help inspectors stay current with the latest develop- stormwater discharge and routine site inspections by
ments, skills, and technologies available to promote facility staff.
MSGP permit compliance.
Enhanced Monitoring (Category 4)
Industry-wide Monitoring (Category 2)
A fourth category of enhanced monitoring is
The committee recommends that the MSGP envisioned for industrial facilities with the highest risk
continue to have a category of facilities that are not for discharging pollutants that may adversely impact
subject to sector-specific benchmark monitoring based surface waters. This designation should be based on
on a determination that they do not have the poten- past repeated exceedances of benchmarks (e.g., AIM
tial to discharge sector-specific pollutants at a level of Tier 3; see Box 1-3), severe concerns raised upon site
concern. However, the committee recommends that all inspections of Category 1 facilities, or recommenda-
facilities without sector-specific benchmark monitor- tions of the permitting authority for sites that are large
ing conduct industry-wide monitoring for pH, TSS, and complex with high pollutant discharge potential
and COD, as discussed in Chapter 2, in addition to or where TMDL development and implementation
the currently required visual monitoring of stormwater merits additional monitoring. The largest facilities will
discharge and routine site inspections by facility staff. typically produce the greatest volume of runoff, leading
As also discussed in Chapter 2, the committee recom- to high risk from high pollutant mass loads. Complex
mends that EPA conduct data and literature reviews sites could include those with multiple outfalls and
prior to the next permit renewal and, as a part of each varying land uses throughout the industrial site or high-
permit renewal, determine whether benchmark moni- risk chemicals used in exposed areas.
toring should be added for some industries currently Monitoring plans would be developed as appro-
exempted from benchmark monitoring, and whether priate for the site and the site issues that need to be
the pollutant-specific benchmark monitoring require- addressed. For sites with repeated exceedances, facili-
ments for each sector should be revised. ties may need to monitor at multiple outfalls and to
implement volume-weighted composite monitoring to
Benchmark Monitoring (Category 3) calculate stormwater discharge event mean concentra-
tions to help to determine whether they are causing
Chemical-specific benchmark monitoring in the or contributing to violations of water quality criteria.
current MSGP applies to 55 percent of industrial per- For the largest and most complex sites, facilities would
mittees7 (R. Marcus, EPA, personal communication, be expected to develop and implement a sampling
2018). These permittees are classified within sectors for program that is spatially and temporally representative
which it has been determined that potential pollutant of stormwater discharges from all parts of the facility
levels warrant such monitoring and SCMs are reason- where industrial activities are conducted. This informa-
ably available for additional pollutant reduction (see tion may be needed to determine whether and where
Table 1-1). The committee recommends ongoing use additional stormwater control measures are warranted,
of this category. Nevertheless, the specific benchmark including moving exposed industrial activities under
monitoring requirements should be updated based on cover or enhanced treatment. Should monitoring and
subsequent actions be implemented that bring the site
7 Data applicable to the states and territories permitted by the into compliance, the permitting authority could evalu-
federal MSGP and does not include data from states with delegated ate whether the facility can return to Category 1, 2, or 3.
regulatory authority.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 59

For facilities that use event mean concentrations to water pollutant discharge. The committee encourages
determine compliance, some consideration is needed EPA to add both enhanced and reduced levels of
regarding extreme storms. EPA should establish a monitoring to the existing program. The elimination
“nonrepresentative storm” criterion that would exclude of benchmark monitoring by low-risk facilities would
event mean concentration data for extreme events that provide a nonmonitoring option for oversight of these
are expected to exceed SCM design criteria. Under facilities and eliminate some of the most suspect,
extreme conditions, SCM performance will be compro- unreliable monitoring data. This approach also ensures
mised and stormwater bypass will occur. It is reasonable that high-risk industries that are more likely to be
to expect that the discharge of stormwater pollutants significant sources of stormwater pollution invest in
associated with industrial activity and the effectiveness the necessary monitoring to confirm that SCMs are
of stormwater control measures implemented are most effective in reducing pollutants and risks to receiving
representative for water quality purposes when the waters. In total, this proposed framework is expected
sampling is conducted on discharges resulting from to reduce the monitoring burden on the lowest-risk
frequent storm events and not large extreme events. facilities while increasing the quality of the data avail-
This event size may be based on a statistical review able on the overall population of industrial facilities
of long-term rainfall records to establish wet weather including the l­ argest, highest-risk facilities. Combined
precipitation conditions when they become less rel- with suggested improvements to monitoring protocols,
evant for water quality. This criterion may be a storm training, and data management discussed in this chap-
of a certain return frequency such as a 10-year storm, ter, the tiered approach is also expected to increase the
or a multiple of the 90th percentile rainfall depth, or a usefulness of the data collected toward improving
multiple of the long-term average rainfall depth for the the management of industrial stormwater.
area. Using nonrepresentative storm criteria, a permit-
tee would either not submit EMC data from storms Exemptions, Additions, and Other Permitting
that exceed the criterion or these data would not be Alternatives
evaluated against the benchmarks.
Enhanced stormwater monitoring is considered to Within the tiered framework envisioned by the
be within the financial resources and/or expertise of a committee, there are exceptions, additional monitoring,
major industrial facility and may prove beneficial to the and other permitting options as are currently applicable
industry by more accurately characterizing the storm- to the current MSGP.
water discharge than by using grab-sample first-flush
benchmark monitoring. Full-storm data can provide a No Exposure
much more complete picture of the industrial storm-
water discharge from a site. Additionally, when faced No-exposure certification is allowed under the cur-
with designing treatment SCMs for a high-risk and/ rent MSGP for sites, regardless of size or complexity, at
or complex site, the flow and water quality data col- which “all industrial materials and operations are pro-
lected by composite sampling are critical to ensuring tected by a storm resistant shelter to prevent exposure
the sizing and design are appropriate. The MS4 entity to rain, snow, snowmelt, and/or runoff ” (EPA, 2015d).
could be an active participant with Category 4 facilities, With no-exposure certification, required once every
potentially reimbursed to conduct the enhanced moni- 5 years, facilities are exempt from the requirements
toring on behalf of the larger facilities in the watershed, of the MSGP, including monitoring. Certification
so that the data are consistent and useful both at a site requires facility owners to confirm no-exposure condi-
level and on a watershed basis. tions by answering specific questions about industrial
materials or activities exposed to precipitation and to
Benefits of Tiered Monitoring Requirements allow the permitting authority to inspect the property,
although such inspections are rarely conducted.
The current MSGP includes several levels of The committee agrees that monitoring is not
monitoring based on expected sector-specific storm- needed at facilities with no exposure but recommends

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

60 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

verification of no exposure by a certified inspector or Advanced Analyses Possible Under Enhanced

the permitting authority. Maryland is an example of a Monitoring
jurisdiction that currently requires third-party verifica-
tion of no exposure. Under the AIM process (still to be developed) and
the enhanced monitoring category envisioned within
the tiered framework for large, complex sites with
Effluent Limitation Guidelines
repeated benchmark exceedances, there are opportuni-
As discussed in Chapter 1, EPA has established ties to use advanced tools and analyses to better under-
effluent limitation guidelines (ELGs) for 10 subsectors stand water quality impacts from individual facilities.
of industrial facilities (see Appendix B), with required These tools, such as wet weather dilution or the biotic
monitoring at least once per year at each outfall. This ligand model, may require monitoring of receiving
ELG monitoring, required by law, would supplement water flows or quality, more complex sampling tech-
the MSGP monitoring envisioned in Table 3-2. niques, and modeling, so they are not viewed as tools
that should be required of all permittees. Nevertheless,
for facilities struggling with repeated exceedances, these
Individual Stormwater Permit Monitoring
advanced tools and analyses can clarify where further
In its original regulatory strategy for industrial SCMs are necessary to protect receiving water quality.
stormwater (EPA, 1990, p. 48002), EPA identified
an individual permit category for situations where Wet Weather Dilution and Mixing Zones
the MSGP benchmark monitoring requirements,
SWPPPs, and SCMs may be inadequate to address Many MSGP benchmarks are based on water qual-
pollution from stormwater discharges associated ity criteria (see Table 1-3) and all MSGP benchmarks
with industrial activity. Federal regulations empower are applied at the point of discharge without dilution.
the permitting authority to exclude facilities from By its very nature, industrial stormwater discharges
the MSGP and require individual NPDES permits occur during wet weather conditions when the receiv-
when special considerations such as a large quan- ing stream is expected to be flowing at some reasonable
tity of pollutant discharge, proximity to receiving capacity above base flow, which could provide dilution
waters, and the characteristics of pollutants are at of stormwater discharges. NPDES regulations allow for
issue (40 CFR § 122.28(b)(3)). Extensive stormwater municipal and industrial process wastewater discharges
discharge characterization for conventional and non­ to incorporate dilution and an impacted mixing zone
conventional pollutants, toxic pollutants, hazardous when evaluating instream toxicity. According to EPA
substances, and treatment units must be submitted (2014), a mixing zone is “a limited area of volume of
with the permit application. Based on this informa- water where initial dilution of a discharge takes place
tion, an individual stormwater permit can require and where certain numeric water quality criteria may be
more extensive monitoring and/or a greater number of exceeded.” State regulations generally limit these areas
pollutants compared to the MSGP, where benchmark based on widths or cross-sectional areas and lengths on
monitoring is determined by standard industrial clas- a case-by-case basis, and the use of mixing zones is at
sification code. Individual permits can also be struc- the discretion of the permitting authority.
tured with enforceable discharge criteria expressed as Explicit inclusion of a dilution allowance in deriv-
numerical effluent limits, which trigger a permit viola- ing benchmark thresholds for the MSGP has not been
tion if exceeded. This stricter enforcement of pollutant done by EPA and would be challenging, given the state-
exceedances can be helpful for sites that represent a to-state variability in how mixing-zone allowances are
high public concern or that raise environmental justice included as part of state water quality standards and the
issues. Federal law authorizes any “interested person” site-specific analysis normally conducted to implement
to petition the permitting authority to require an the allowance for a discharge. However, facilities that
individual permit. repeatedly fail to reach benchmarks and are elevated to
the upper tiers of the AIM process should be permitted

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 61

a mixing-zone allowance, as is allowed with municipal Dissolved metals are used to determine acute
and industrial process wastewater dischargers, after the and chronic aquatic life criteria. According to EPA
facility has applied all reasonable SCMs. A mixing- (1996b), dissolved metals are operationally defined as
zone allowance would allow facility operators to set “that which passes through a 0.45 µm or a 0.40 µm
site-specific criteria that are protective of ambient water filter.” With this operational definition, a fraction of
quality in the receiving waters. the m ­ etals measured as dissolved consists of small
Calculating a stormwater mixing zone based on particulate or colloidal metals that are able to pass
best available science may require the use of data sets through the filter or metals that are complexed with
characterizing upstream flow and water quality condi- organic ligands, which may not be biologically avail-
tions and dynamic water quality models to understand able. Dissolved metals require field or laboratory filtra-
the impact of stormwater runoff on receiving waters. tion within 15 minutes of sample collection (40 CFR
These water quality models are typically calibrated § 136.3) because metal species continue to change
with site-specific water quality and hydrology data. between dissolved, precipitated, and sediment-sorbed
Applicable water quality model types may be “far field,” forms after the sample is collected.
where water quality is influenced by the hydrodynamics Several studies have been conducted to character-
of the receiving water, or “near field,” where pollutant ize metal concentrations in urban stormwater based
concentrations at the discharge location are determined on total metals (Pitt et al., 2004a,b; Shaver et al.,
from plumes at the facility outfalls (Gawad et al., 1996; 2007). In a number of stormwater studies, a significant
Jirka et al., 1996; Davis, 2018). EPA should develop fraction (approximately 30 to 70 percent) of copper,
guidance for using water quality models for calculating ­cadmium, and zinc was found in the dissolved form
stormwater mixing zones. (Pitt et al., 1995; Crunkilton et al., 1996; Sansalone and
Buchberger, 1997; Pitt and Clark, 2010). Differences
Alternative Metals Benchmarks in stormwater chemistry, receiving water chemistry,
temperature, and sediment composition will affect the
The 2015 MSGP requires total metals analyses fraction of metals that are bound or dissolved (Weiner,
(rather than dissolved), but questions have emerged 2008). Runoff that is collected from receiving waters
from industry about whether dissolved metal a­ nalyses will often have higher amounts of metals in particulate
or the biotic ligand model would provide a more form while stormwater collected from pipes will have a
accurate assessment of stormwater pollution. Both higher dissolved fraction (Clary et al., 2011).
approaches require more rigorous monitoring that Because dissolved metal concentrations provide a
may be a burden if applied uniformly to all permittees. more accurate measure of potential toxicity, it would
However, if permittees have repeated exceedances of be reasonable for the MSGP to allow industries that
metals benchmarks (see Appendix D), they may benefit have had repeated exceedances of benchmark levels for
from enhanced monitoring of dissolved metals or in total metals to sample for dissolved metals and compare
support of the biotic ligand model. this quantity against the existing benchmark. However,
sampling for dissolved metals requires more complex
Dissolved Metals. Dissolved metals are more biologi- sampling methodology, including filtering within
cally available than particulate-bound metals and are 15 minutes of sampling. Because rapid filtering for dis-
more important in assessing pollutant risk. According solved metals puts an additional burden on industry, the
to EPA (1996b), committee does not recommend that dissolved metals
The primary mechanism for toxicity to organisms analyses be required for all permittees covered by the
that live in the water column is by adsorption to or MSGP, but should be an option if all proper sampling
uptake across the gills; this physiological process procedures are followed.
requires metal to be in a dissolved form. This is not
to say that particulate metal is nontoxic, only that Biotic Ligand Model. As discussed in Chapter 2, the
particulate metal appears to exhibit substantially
Biotic Ligand Model (BLM) is an aquatic toxicol-
less toxicity than does dissolved metal.
ogy tool that is used to determine the bioavailability

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

62 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

of metals in aquatic ecosystems. Lethal accumulation stormwater permits in the system, and the eventuality
values of metals on the gill surface, when fish toxicity of self-reporting monitoring data into the system has
is being considered, are used to predict lethal met- taken years.
als concentration values with the BLM (Niogi and With the 2015 MSGP, EPA required as of Decem-
Wood, 2004). EPA already uses the BLM as a tool in ber 2016 that permittees submit their discharge moni-
the Ambient Water Criteria in surface waters ( Jarvis toring reports (DMRs) electronically, including those
and Wisniewksi, 2006), but to develop a BLM, site- operating under EPA or a state MSGP, into the national
specific water quality parameters, including hardness, eDMR data system, unless a waiver is obtained (EPA,
pH, and dissolved organic carbon, need to be measured. 2015a, p. 64066). Prior to 2015, monitoring was often
As with dissolved metals discussed in the previous submitted in paper format, making review of these data
section, the MSGP should allow those who exceed and permit compliance cumbersome and staff intensive.
total metals benchmarks to analyze receiving waters to When EPA reviewed benchmark monitoring data
calculate pollutant toxicity associated with a facility’s for development of the 2015 MSGP, only 485 of the
stormwater discharge. However, the facility would need 1,200 covered facilities required to perform benchmark
to do additional sampling beyond the current MSGP monitoring submitted their results electronically and
requirements to acquire the data needed by the BLM. many of the records were unusable (EPA, 2012). Data
Watershed-based collaborative relationships among collected outside of the MSGP have no single or linked
industries, municipalities, and other dischargers could repository for storage and public access.
help facilitate the characterization of receiving water As part of the information gathering conducted
chemistry, as required for use of the BLM, at reduced for this study, states acknowledged that they have
cost. Multiple dischargers could combine resources to been limited in their ability to receive, review, and
appropriately characterize the necessary water quality respond to MSGP monitoring due to staffing short-
parameters over a range of flows, seasonal variations, falls. However, many states reported that they do have
and other important conditions. With these data, BLM the capacity to review data electronically and that
modeling could be completed to establish watershed- digital reporting improves the effectiveness of staff
specific benchmark concentrations for copper for all oversight, particularly in states with limited staffing.
dischargers to the receiving water. This characterization Automated searchable data systems streamline envi-
procedure for copper and the BLM has been done in ronmental compliance. States using these systems can
Oregon (OR DEQ, 2018). Collaborative monitoring autogenerate reminders and compliance advisories. The
could be expanded to other pollutants that need receiv- level of electronic reporting is increasing as permittees
ing water quality information to determine discharge become aware of and adept at electronic reporting, state
concentrations. data systems capabilities grow, and compliance rates
increase. States are required to share MSGP monitor-
UPDATING AND UPGRADING CURRENT ing information with EPA via the national data system,
METHODS OF DATA MANAGEMENT and data sharing is increasing. Two particular advan-
tages arising from improving data management tools
Submitting, managing, and reviewing data col- are an improved capacity to screen data automatically
lected under the MSGP has been challenging. In 1995 for outliers and errors and to analyze large data sets
when the first MSGP was issued, EPA’s national data using data visualization software.
system for the NDPES program did not accommodate
stormwater permits. It could not address the need to Screening for Errors and Omissions
enter the type and numbers of sources and reporting
against benchmarks instead of enforceable numeric The new era of automated data systems and elec-
limits. The data system had been in place since 1982, tronic self-reporting offers many opportunities for
5 years prior to Congress’s action to expand stormwa- improving data quality. Illegible discharge reports are
ter permitting (EPA, 2013, p. 46011). The transition eliminated. Permittees enter results into screens pre-
to an updated national data system, the inclusion of populated with information on outfalls, sampling fre-

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 63

quencies, parameters, and units. If consistent units for quick illustration of query results. These visualizations
benchmarks are used based on the value from which the can then develop “data stories” that help to understand
benchmark was derived, as recommended in Chapter 2, industrial stormwater effluent quality and progress
unit errors could be substantially reduced. The systems made under the MSGP.
have the capability of providing permittees immediate A simple visualization example using California
electronic feedback, such as by alerting or requiring Water Boards’ tool compiles the most recent 5 years
facilities to check and correct decimal point placement of data by facility and compares the median value
and verify results that exceed the benchmark threshold, (using a minimum of three samples) to the benchmark.
helping to reduce transcription errors. Several entries Facilities where the median is below the benchmark are
among the 2015 MSGP appeared erroneously high shown on the left side of Figures 3-2 and 3-3 for lead
or low, suggesting the data management system could and TSS, respectively, and facilities where the median
improve its alerts to permittees of outlier data or “less is above the benchmark are shown on the right side
than” values that exceed the benchmark, thereby further of each figure (note that the scales are different). A
reducing errors (see Appendix D). comparative analysis by pollutant indicates that TSS
Electronic data can also improve agency oversight. is a greater water quality challenge compared to lead
EPA and states can generate automated reports, which for facilities covered by the California equivalent of
streamlines the identification of omissions and exceed- the MSGP.
ances. More complete information regarding whether a A quick temporal analysis can be made using this
facility did in fact report a discharge during the moni- visualization for lead. Given that EPA only included
toring period is becoming available. benchmarks in the 1995 MSGP in cases where the
median of all samples for a given sector exceeded the
Data Analysis and Visualization potential benchmark value, and that the benchmark
threshold for lead has not changed since the 1995
With improved data quality and more data becom- MSGP, the fact that relatively few facilities currently
ing available through application programming inter- exceed the benchmark indicates that lead pollutant
faces and web services, the ability to evaluate the data ­levels have significantly improved during the time
for patterns, trends, and correlations is expected to period the MSGP has been in place. This may be
increase. For example, California Water Boards’ data explained by improvements in pollution prevention
center contains industrial stormwater effluent water measures that have been implemented in this time
quality data and an assessment tool that can be used for period, including the removal of lead from gasoline and

FIGURE 3-2 Median lead concentrations at sites in California Water Board, Los Angeles Region, from monitoring from the past 5 years,
with results less than (left) and greater than (right) the benchmark of 82 µg/L.
SOURCE: D. Altare, California Water Boards, personal communication, 2018.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

64 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

FIGURE 3-3 Median TSS concentrations at sites in California Water Board, Los Angeles Region, from monitoring from the past 5 years,
with results less than (left) and greater than (right) the benchmark of 100 mg/L.
SOURCE: D. Altare, California Water Boards, personal communication, 2018.

paints and improved housekeeping at sites. For TSS, ­ rotocols and training to improve the quality of
p
the data visualization could be used to target compli- monitoring data. Specifically, EPA should
ance assistance efforts; for example, it could be used
to find opportunities where treatment SCMs could • Consider a training or certificate program in
supplement existing site measures to reduce pollution stormwater collection and monitoring to ensure
discharge levels. that required sampling and data collection are
The committee recommends that EPA continue representative of stormwater leaving the site to the
to compile data for facilities operating under the EPA greatest extent possible.
MSGP and state MSGPs nationally and make these • Stay abreast of advancements in monitoring, sam-
data publicly available in a timely manner. The commit- pling, and analysis technology that can provide
tee also recommends that EPA develop visualization more or better-quality information for similar or
tools that can be used by others to easily examine data reduced costs and consider these in future revisions
for patterns, trends, and correlations. of the MSGP.

CONCLUSIONS AND EPA should allow and promote the use of com-
RECOMMENDATIONS posite sampling for benchmark monitoring for all
pollutants except those affected by storage time.
The current MSGP benchmark monitoring EPA’s disallowance of composite sampling and reli-
requirement focuses on low-cost, coarse indicators ance on grab sampling in the interest of discrete char-
of site problems, and the usefulness of the data can acterization of the highest pollutant concentration is
frequently be hampered by its variability. Stormwater not warranted based on the methods used to derive
monitoring data display variability that originates from benchmark thresholds. Multiple composite sampling
many different sources, including the variability of techniques are available that provide more consistent
precipitation within and among storms and changes and reliable quantification of stormwater pollutant dis-
in operations over the course of time. In this chapter, charges compared to a single grab sample. Composite
the committee recommends improvements in sampling samplers have become common in stormwater moni-
design and procedures, laboratory analysis protocols, toring as experience with this approach has increased
and data management to reduce error and improve the and costs have declined, and the EMCs that result
reliability of monitoring results to support improved from composite sampling may reduce the likelihood of
stormwater management. exceeding the benchmark compared to first-flush grab
EPA should update and strengthen industrial sampling. Composite sampling is not appropriate for
stormwater monitoring, sampling, and analysis pollutants for which the results may vary over time with

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

STORMWATER SAMPLING AND DATA COLLECTION 65

storage, such as those that transform or degrade rapidly based on facility risk, complexity, and past perfor-
or interact with the atmosphere (e.g., pH). mance. The committee proposes four categories:
Quarterly stormwater event samples collected
over 1 year are inadequate to characterize industrial 1. Inspection only. Low-risk facilities could opt for
stormwater discharge or describe industrial SCM permit-term inspection by a certified inspector or
performance over the permit term. Under the MSGP, the permitting authority in lieu of monitoring.
if a permittee’s average of four consecutive quarterly Facilities could be classified as low risk based on
samples meets the benchmark, a waiver is granted for facility size (e.g., less than 0.5 or 1 acre of indus-
the remainder of the permit term. For permittees with trial activity), recognizing that size may not fully
average results that meet the benchmark, the MSGP represent the risk profile, or more accurately based
should require a minimum of continued annual sam- on a detailed assessment of the type and intensity
pling, to ensure appropriate stormwater management of industrial activities conducted on site, or a hybrid
throughout the remainder of the permit term. Extended approach.
sampling over the course of the permit would provide 2. Industry-wide monitoring only. All facilities in
greater assurance of continued effective stormwater sectors that do not merit additional pollutant moni-
management and help identify adverse effects from toring would conduct industry-wide monitor­ing
modifications in facility operation and personnel over for pH, TSS, and COD. These data would provide
time. Given the natural variability and the limitations broad, low-cost indicators of the effective­ness of
of grab samples, substantial uncertainty is associated stormwater control measures on site.
with using the average of only four stormwater samples. 3. Benchmark monitoring. Sectors that merit addi-
EPA should analyze industrial stormwater data and tional pollutant monitoring, based on the most
sector-specific coefficients of variation to recommend recent data and industry literature review, would
additional increases in sampling frequency, consistent conduct sector-specific benchmark monitoring in
with EPA’s determination of an acceptable level of error addition to pH, TSS, and COD, which would be
for this indicator of SCM performance. Additional collected by all facilities with chemical monitoring.
continued monitoring at a lower intensity throughout 4. Enhanced monitoring. Facilities with repeated
the permit would also increase the overall sample size benchmark exceedances or those characterized by
and thereby reduce the uncertainty in the monitoring the permitting authority as large complex sites with
results. high pollutant discharge potential would conduct
State adoption of national laboratory accredita- more rigorous monitoring, in consultation with the
tion programs for the Clean Water Act with a focus permitting authority. These facilities could collect
on the stormwater matrix and interlaboratory cali- volume-weighted composite samples at multiple
bration efforts would improve data quality and reduce outfalls if appropriate. Additional tools and moni-
error. NPDES laboratory accreditation programs and toring strategies could be used to assess the water
stormwater interlaboratory calibration efforts would quality impact to receiving waters from stormwater
improve the comparability and reliability of monitor- discharge, including wet weather mixing zones,
ing data. To support these efforts, EPA should publish dissolved metal sampling, and site-specific inter-
guidance and case studies on interlaboratory calibration pretation of water quality criteria, with additional
specifically focused on the stormwater matrix, includ- guidance from EPA. EPA should develop “non-
ing the establishment of performance quantification representative storm” criteria to exclude monitor-
levels for stormwater samples. These efforts would ing for events that would not be representative of
promote similar procedures at a national level to ensure facility stormwater discharge.
the comparability and reliability of test results reported
to permitting authorities. This tiered system would improve the overall qual-
To improve stormwater data quality while bal- ity of monitoring data to inform future iterations of the
ancing the burden of monitoring, EPA should expand MSGP while balancing the overall burden to industry
its tiered approach to monitoring within the MSGP, and permitting agencies.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

66 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

To improve the ability to analyze data nation- levels. Automated compliance reminders, improved
ally and the efficiency and capability of oversight by checks on missing or unusual data, and data analysis
permitting agencies, EPA should enhance electronic and visualization capabilities would improve the effec-
data reporting and develop data management and tiveness of staff oversight and provide new opportuni-
visualization tools. Electronic reporting has only been ties to analyze trends. EPA should develop national
required of permittees since 2016, and the data man- visualization tools that can be used to easily examine
agement capabilities are still developing to make the data for patterns, trends, and correlations.
most use of this information at the national and state

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Consideration of Retention Standards in

the Multi-Sector General Permit

T
he majority of this report has focused on improv- of the committee’s analysis, differs from other forms
ing the monitoring process as a way of confirming of stormwater retention commonly used in stormwater
appropriate stormwater management and ensur- management that aim to hold water on site for later,
ing compliance with the objectives of the Multi-Sector more gradual release, possibly after treatment.
General Permit (MSGP). This chapter focuses on a dif- Storage for stormwater retention can be provided
ferent approach to ensuring that industrial stormwater with a pond or engineered facility such as underground
is appropriately managed—retention standards. On- tankage or an underground infiltration facility. The
site stormwater retention and infiltration are already latter example is important when land is either expen-
included within the MSGP as possible stormwater sive or unavailable. Stormwater retention systems are
control measures (SCMs). Nevertheless, this commit- generally designed based on the volumetric capture
tee was asked to evaluate the feasibility of retention of a storm event of a specific size. The targeted event
standards as both technology-based and water quality- would be large enough so that exceedance of this event
based numeric effluent limitations to establish objective size would be relatively rare, because exceedance will
and transparent effluent limitations (see Statement of result in the discharge of untreated or minimally treated
Task in Chapter 1). The committee was also tasked stormwater. Retention designs must also consider the
to discuss whether the appropriate data and statistical time over which the captured stormwater would be
methods are available for establishing such standards removed, typically through infiltration or beneficial
and the merits and faults of retention versus discharge use, so that storage is again available to handle the
standards. Retention standards are not assumed to next storm.
replace monitoring, but would provide another struc- Infiltration is an attractive management option
tural approach to control pollutant discharges. for stored stormwater; it has been used widely and
successfully in municipal stormwater applications to
STORMWATER RETENTION reduce stormwater impacts on local water bodies and
to recharge groundwater. In addition to simple stor-
The process of stormwater retention, as envisioned age with infiltration, novel SCMs that are infiltration
in the committee’s task, involves storing the stormwater based, known collectively as low-impact development
on site, with the goal that at least a large portion of it or stormwater green infrastructure, including bio­
will not be discharged to surface waters but will go retention basins, permeable pavements, and vegetated
elsewhere. Possible fate pathways for stormwater after filter strips and swales, may be employed for stormwater
retention include infiltration (see Figure 4-1), some retention and infiltration (Caltrans, 2010). Infiltration
type of beneficial use, and evapotranspiration. This of stormwater requires soil and geologic conditions
definition of stormwater retention, which is the focus conducive to the infiltration process, including rela-

67

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

68 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

a b

FIGURE 4-1 Aboveground (a) and belowground (b) stormwater retention facilities with infiltration.
SOURCES: (a) Montgomery County Department of Environmental Protection; (b) Philadelphia Water Department.

tively high-permeability soils. However, for industrial s­tormwater management requirements for new con-
stormwater, concerns due to the likely presence of struction or redevelopment (see Box 4-1). The reten-
toxic pollutants that could migrate through the soil tion standards listed in Box 4-1 are specifically based
to groundwater systems require very careful consid- on stormwater volume reduction in accordance with
erations, especially in terms of pretreatment require- the maximum extent practicable standard for municipal
ments, before infiltrating. These issues are discussed in separate storm sewer system (MS4) permits rather than
more detail later in this chapter. water quality-based effluent limits (WQBELs). Water
The two other storage recovery pathways are typi- quality benefits will result due to the corresponding
cally minor for industrial stormwater. Evapotranspira- reduction in pollutant mass load. This approach specifi-
tion will require vegetation and a large amount of land cally aims to reduce discharge loads, with less emphasis
area, both of which are not common on most industrial on specific pollutant concentrations. Other consider-
sites. Beneficial use of stormwater may be feasible in ations also drive these standards, such as groundwater
areas with extreme water shortages, but its applicability recharge. Possible application of retention standards in
will be highly site specific. It is not expected that on-site the regulatory context of the MSGP is discussed later
stormwater harvesting and use (e.g., firefighting, dust in the chapter.
control, washing, toilet flushing) will be practiced at Stormwater retention systems are typically sized
many industrial sites due to the water quality treatment according to the retention standard and site-specific
requirements and/or likely small or inconsistent water information such as the drainage area, the runoff coef-
demand from these applications (NASEM, 2016). To ficient (land use), and infiltration rate. The retention
be part of a reliable retention system, the demand for standard can be based on a specific design storm (see
reclaimed water would need to be sufficiently consistent Box 4-2), under which the SCM is expected to operate
to ensure that storage is made available for the next at full efficiency. Retention systems would capture all of
precipitation event in a reasonable period of time. the small and mid-sized storms up to a specific design
storm and a portion (usually the initial fraction) of the
RETENTION STANDARDS largest storms, resulting in capture of a large fraction
of the overall runoff volume and corresponding con-
Stormwater retention standards are commonly taminant load. Events larger than the targeted storm
used in municipal stormwater applications to reduce event and some smaller events that enter the storage
overall stormwater volumes and the associated pol- when it is not completely empty (such as back-to-back
lutant mass discharge. States and localities routinely rains) will result in overflow of the storage system and
select retention standards as the basis of municipal discharge of stormwater and industrial pollutants.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

CONSIDERATION OF RETENTION STANDARDS IN THE MULTI-SECTOR GENERAL PERMIT 69

BOX 4-1 Municipal Stormwater Retention Standards

Examples of volume-based stormwater retention standards that have been developed by federal, state, and l­ocal
governments to manage municipal stormwater are provided in Table 4-1. Most of these standards apply to new
construction or substantial redevelopment of a property. California’s retention standard applies to all volume-based
SCMs used by facilities covered by the Industrial Stormwater General Permit. Environmental Protection Agency (EPA)
guidance applies to development projects on federal facilities that are leased, purchased, constructed, or renovated.
These retention standards are designed to provide multiple benefits, including improved water quality, downstream
resource protection, and peak flow control.

TABLE 4-1
Examples of Retention Standards for Municipal Stormwater

Jurisdiction Retention Criteria Reference

Phoenix, Arizona 100-year, 2-hour storm City of Phoenix (2011)
California 85th percentile, 24-hour storm California Water Boards (2018)
Federal facilities 95th percentile, 24-hour storm EPA (2009b)
Washington, DC 90th percentile storm, 24-hour storm DDOE (2014)
(equal to 1.2 inches)
Washington, Western Region 6-month, 24-hour storm State of Washington Department of Ecology (2019)
Colorado, Greater Denver area 80th percentile storm UDFCD (2018)
(equal to about 0.6 inch)
Connecticut 1.0-inch storm CT DEEP (2004)
Ohio 0.9-inch storm runoff volume Ohio Environmental Protection Agency (2018)

Depending on the retention system design, pollutant frequent back-to-back storms) or concerns over vector
concentrations in the bypass flow may be less than those (primarily mosquito) control.
that occur early in the storm because the bypass will
occur only after substantial prior rainfall, when runoff MERITS OF AND CONCERNS
concentrations are typically lower. ABOUT RETENTION FOR
Retention standards may also include a requirement INDUSTRIAL STORMWATER
of how long the captured stormwater may be stored
prior to infiltration or beneficial use, which affects the Many states and local governments have developed
volume available to capture any subsequent storms. regulations requiring retention in all new develop-
The 2018 amendment to the 2014 California Indus- ment or significant redevelopment (EPA, 2016c;
trial General Permit requires that discharge reduction see Box 4-1). These states, such as Minnesota, New
SCMs be sized for an 85th percentile, 24-hour storm ­Jersey, and Washington, promote retention by includ-
as a daily volume for on-site retention and infiltration ing descriptions of proper infiltration methods in their
or beneficial use, meaning that the captured stormwater stormwater manuals. Some states, such as California
would need to infiltrate or be used on site fully within and Oregon, have developed specific requirements
24 hours. Sites with insufficient infiltration rates to for industrial stormwater retention if it is used as
meet this requirement can increase the size of their part of stormwater management (OR DEQ, 2017;
retention storage (California Water Boards, 2018). The California Water Boards, 2018). Widespread inter-
recommended maximum storage time for stormwater est in storm­water retention has mostly focused on
may be influenced by local rainfall conditions (e.g., common m ­ unicipal stormwater source areas, such as

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

70 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

BOX 4-2 Understanding the Design Storm and Cumulative Depth

Design storms for stormwater management are defined based on the probability of occurrence. For example, a
5-year storm represents a storm of a particular rainfall depth over a particular duration that on average will occur
once over a 5-year period. A 5-year storm has a 20 percent probability of occurring in any given year. Design storms
are generally useful to describe larger events, including extreme flood events, and historically have been used to
describe drainage-design events. Usually the design storm definition is based on daily rainfall, but some states and
localities require shorter intervals, such as 6-hour or 2-hour rainfall rates (see Box 4-1) or a time interval developed
from site characteristics.
To illustrate how more common events can be related to storage requirements, the cumulative probability of rainfall
below a specific depth can be used. To determine the cumulative probability curve, daily rainfall data appropriate
to a site are required. The longest possible rainfall record is desired, and, in many cases, this should be a minimum
of 25 years, and preferably 50 years or longer to capture interannual and multidecadal climate variability. Once a
sufficient period of rainfall record has been obtained, the analysis begins by sorting the 24-hour rainfall data from
smallest to largest depth in a cumulative distribution curve. For a 90th percentile 24-hour storm event, 90 percent
of all storm events would have 24-hour precipitation totals less than or equal to that amount. As an example, the
lower curve in Figure 4-2 shows the cumulative ranking of daily rainfall depth for a 73-year record at Baltimore/
Washington International Thurgood Marshall Airport. These data indicate that 59.8 percent of the total 73-year daily
rainfall depths resulted from events that were 1 inch or less; therefore, 59.8 percent of the average annual storm-
water will be completely captured in a retention facility designed for a 1-inch capture depth (assuming 100 percent
rainfall-to-runoff ratio).
Capture also will occur during larger storms, because the retention facility will fill before bypassing. The upper
curve in Figure 4-2 includes this effect. Capturing the first 1 inch of larger storms increases the overall capture to
84.6 percent of the total rainfall. The pollutant mass fraction capture will usually be greater than the volumetric
fraction since the retention facility captures the first flush of large storms, which is usually the most contaminated.
Design storm analysis is based on historical data and assumes climate stationarity—the use of previous events
to predict those of the future. However, with climate change, historic precipitation records may not capture the full
variability or likelihood of future conditions. Forward-looking predictive models may be necessary to properly design
future SCMs, considering nonstationarity scenarios.

FIGURE 4-2 Cumulative 24-hour rainfall distribution curve (black line) at the Baltimore/Washington International Thurgood
­Marshall Airport and cumulative percent retention with a given design storm (red line).
SOURCE: Data from NOAA National Centers for Environmental Information, https://www.ncdc.noaa.gov/cdo-web.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

CONSIDERATION OF RETENTION STANDARDS IN THE MULTI-SECTOR GENERAL PERMIT 71

roofs, parking lots, and roads. Stormwater runoff from

Summary of Benefits and
industrial facilities, in contrast, differs in its greater BOX 4-3 Concerns with Industrial
potential number of contaminants that pose a risk to Stormwater Retention
groundwater and their higher concentrations. This
Benefits:
section examines the merits and concerns when using
retention standards for industrial stormwater. • If contaminants are removed through treatment
or infiltration processes, retention r­ educes con-
taminant loads to receiving ­waters.
Merits of Retention • Capture and use or infiltration reduces runoff
peak flows.
• Stormwater infiltration recharges groundwater
Retention of stormwater is a proven way of reduc- supplies.
ing the impacts of urbanization on the natural hydro- • Capture and use reduces water demand from
conventional sources.
logical cycle of a watershed, with benefits for surface
water quality and flows. If contaminants in stormwater Concerns:
are treated prior to infiltration or adsorbed or filtered
• Infiltration may cause groundwater contami-
out by the soil matrix upon infiltration, stormwater nation or mobilize existing soil or groundwater
retention reduces the mass of contaminants discharged contamination.
• Not all states have regulatory authority to
to surface water (see Box 4-3). address groundwater contamination issues, if
Some bypass will occur in the late stages of reten- they occur.
tion of extreme storms, and the amount of bypass would • Lack of maintenance or system failure can lead
to surface water contaminant discharges.
be largely determined by the design storm used to size • Infiltration requires a large amount of land,
the retention system (see Box 4-2). The amount of while capture and use requires consistent and
sufficient water demand.
bypass is also affected by the infiltration rate, which can • Under current regulations, permittees are still
change over time. The retention system can be designed responsible to meet water quality require­
to retain the first flush and, if its capacity is exceeded, ments in bypass flows that occur in large
storm events.
bypass runoff will occur later in the storm. If a first flush
of pollutants typically occurs at the site, capturing the
first part of the runoff with an empty retention system
provides a higher proportional mass removal compared
to an equivalent volume captured later in the storm
event (Han et al., 2006b). restrictions, retention could reduce the runoff from a
Stormwater retention, by preventing discharge, also substantial number of acres in a watershed.
reduces the peak rates of runoff. High flow rates and
magnitudes of stormwater are common in developed Concerns Associated with Retention
areas that have a significant percentage of impervi-
ous surfaces. These high stormwater flows can modify When evaluating the potential for stormwater
or destroy natural habitats due to erosion, known as retention at an industrial facility, extreme caution
hydromodification, and cause flood damage. It is gen- should be used to ensure that infiltration does not
erally desirable to reduce the maximum rate of runoff, result in groundwater contamination or mobiliza-
and, in some cases, regulatory agencies have required tion of existing soil or groundwater contamination.
controls that reduce peak flow rates. Stormwater Many common pollutants found in stormwater,
retention with infiltration also increases groundwater such as heavy metals and toxic organics, have some
recharge, which increases base flows in streams, provid- mobility in the soil column (Armstrong and Llena,
ing ecological benefits, reduces saltwater intrusion in 1992; Clark et al., 2010; Treese et al., 2012). Without
coastal areas, and potentially benefits water availability appropriate treatment, as well as spill prevention and
for water supply. Given that many industrial sites are containment, industrial stormwater retention can lead
highly impervious and may be quite large, if reten- to ground­water contamination well beyond the site
tion is feasible after consideration of the appropriate boundary that is difficult and costly to remediate.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

72 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

A large percentage of the U.S. population depends of the potential risks to groundwater, industrial storm-
on ground­water for water s­upply, and groundwater water infiltration is not recommended in these states.
contamination of aquifers used as water supplies can Another disadvantage of retention and infiltration
cause major health risks. Groundwater contamination basins is the large amount of land required. Existing
from stormwater infiltration has been documented in industrial facilities may not have available and suitable
various locations around the country. For example, land in which to construct an infiltration basin, and
groundwater was contaminated with organic chemicals major retrofits are costly. Retention and infiltration
from stormwater from two industrial sites in Florida are more likely to be useful for new facilities, where
(Pitt, 1996) and from drywell infiltration of storm­ construction would be less expensive.
water (Edwards et al., 2016). Infiltrated stormwater also has the potential to
Even when retention systems are designed to mobilize existing contaminants in the subsurface.
protect groundwater quality, systems can fail if not Extensive infiltration can cause existing groundwater
designed or maintained appropriately. Failure can occur contamination plumes to migrate, thereby shifting or
because of inadequate information on soil infiltration spreading their adverse impacts. This is a particular
rates, improper retention basin sizing for the design concern in highly industrial areas, which are likely to
storm, or insufficient treatment and/or pretreatment. have existing contaminant plumes in the subsurface.
Neglecting the appropriate maintenance protocols that Infiltration can also cause local or regional groundwater
enable the infiltration system to function as designed mounding that could saturate contaminated soils cur-
can also lead to failure. rently above the saturated zone or under a protective
Concerns over potential groundwater contami- cap, resulting in a release of stored pollution.
nation have led some states, such as Minnesota and A final challenge is that the regulatory framework
­W isconsin, and some authors (e.g., Pitt, 2011) to under the MSGP requires that discharge from these
suggest limiting the use of retention for industrial retention facilities, which is expected to occur only
stormwater or simply prohibiting the infiltration of under the heaviest storms, comply with the bench-
industrial stormwater in most cases. Wisconsin pro- marks. This is a deterrent to use of retention systems
hibits the infiltration of industrial stormwater, with for industrial stormwater, because bypass that exceeds
the exception of rooftops, no-exposure facilities, and benchmark thresholds under high flow conditions may
parking areas of Tier 2 (light) industries (Wis. Admin. result even after substantial investments to construct
Code NR [Natural Resources] § 212.21). Minnesota’s such systems that reduce overall pollutant loads. This
stormwater manual prohibits stormwater infiltration issue is discussed in more depth at the end of the
at “potential stormwater hotspots” that might have the chapter.
potential to produce relatively high levels of pollutants
in the case of spills, leaks, or illicit discharges, includ- CONSIDERATIONS FOR RETENTION
ing storage areas, ­refueling areas, vehicle storage, and AT INDUSTRIAL SITES
material transfer areas.1 ­California allows infiltration of
industrial stormwater in its general permit and includes Successful use of retention/infiltration at an indus-
state groundwater protection requirements for on-site trial facility for treatment of industrial stormwater
compliance (California Water Boards, 2018). depends on a full understanding of the characteristics
To protect groundwater and surface water, states of the potential stormwater pollutants, selection and
will need the regulatory authority to address failures in thorough evaluation of the infiltration site, and appro-
maintenance or performance of industrial stormwater priate use of treatment technologies as needed. Certain
retention facilities. However, not all states have the pollutant or site characteristics will make retention and
authority to manage groundwater quality and may lack infiltration inappropriate or cost prohibitive.
enforcement capacity if contamination occurs. Because

1 See https://stormwater.pca.state.mn.us/index.php?title=

Potential_stormwater_hotspots.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

CONSIDERATION OF RETENTION STANDARDS IN THE MULTI-SECTOR GENERAL PERMIT 73

Pollutant Characteristics stormwater infiltration include nutrients, metals,

organics, total dissolved solids/salts, and bacteria (Pitt,
A key factor that distinguishes industrial storm- 1996; Datry et al., 2004; Boving et al., 2008; Weiss et
water management from typical urban stormwater al., 2008).
management is the range of potential pollutants and the Persistence is an additional consideration. Chemi-
likelihood of elevated concentrations. The occurrence cals likely to biodegrade in the subsurface to harmless
and concentrations of stormwater pollutants can vary byproducts would pose a much lower risk than chemi-
widely by industrial sector or even individual facilities, cals that do not biodegrade readily and are likely to
based on the materials and chemicals used on site. persist in groundwater for years. For example, simple
Therefore, before stormwater retention and infiltration hydrocarbons can readily biodegrade in aerobic envi-
are considered, expected stormwater pollutants at a site ronments, although maintaining aerobic environments
should be carefully assessed. in infiltration zones with extended times of inundation
Several pollutant characteristics affect the contami- can be problematic. Highly chlorinated organic com-
nation risks associated with infiltration: pounds, however, are resistant to aerobic degradations
or may degrade to a product that is highly persistent
• Abundance (high concentrations and high detec- (e.g., trichloroethylene to vinyl chloride). The bio­
tion frequencies) in stormwater, degradation of some stormwater pollutants has been
• Toxicity, documented in SCMs, such as hydrocarbons in bio-
• Mobility in subsurface soils where infiltration will retention media (LeFevre et al., 2012a,b). However,
occur, and reliance on biodegradation would need to be clearly
• Persistence. demonstrated and routinely monitored if part of an
industrial stormwater management strategy.
Contaminant abundance in site runoff and the toxicity Pitt et al. (1994) identified municipal storm-
of those contaminants are initial considerations in an water pollutants with the greatest potential adverse
evaluation of the suitability of retention and infiltration. impacts on groundwater assuming sandy soils with
If the aquifer is (or could be) used as a water supply high infiltration rates, low soil organic content, and
or is hydrologically connected with such an aquifer, low ­adsorption potential (considered a worst-case
groundwater pollution is a particular concern because ­scenario for contaminant mobility). The listing is based
of the human health risks. Contaminated groundwater on the pollutant information in municipal rather than
is notoriously difficult to treat due to the inaccessibility industrial stormwater and, therefore, does not include
and corresponding lack of knowledge about the pol- all contaminants likely to be found in industrial storm-
lutant sources, aquifer travel pathways, and possible water. However, it serves as a general guideline for the
pollutant treatment mechanisms. types of contaminants that pose concerns for retention
Special care must be taken when considering and infiltration. The contaminants of moderate risk
retention and infiltration of highly soluble pollutants to groundwater from surface infiltration of municipal
with low adsorption to geomedia or vadose zone soils stormwater included
because these pollutants can be highly mobile. Low-
molecular-weight polar compounds tend to be highly • Organic compounds, such as low-molecular-
soluble and move rapidly in the soil column. Soluble weight polycyclic aromatic hydrocarbons (e.g.,
pollutants will not be strongly affiliated with particu- pyrene and fluoranthene);
late matter and will not be significantly removed via • Nutrients, such as nitrate; and
sedimentation, filtration, or other particulate matter • Chloride from road salt.
removal processes. Instead, chemically reactive filter
media may be required to adsorb the pollutant. Water These chemicals all have moderate to high mobility in
chemistry parameters such as pH, salinity, and hard- soils. In municipal environments, heavy metals, such
ness may also impact pollutant mobility (FAO, 2000). as lead, zinc, and copper, tend to occur in low concen-
Examples of known groundwater contaminants from trations and sorb to surface soils and sediments and

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

74 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

therefore would be less likely to reach groundwater Site Suitability

through infiltration (Dechesne et al., 2004), unless the
pH is very low. There are concerns, however, when The suitability of a site for detention/infiltration
the groundwater table or a perched lens is near the sur- will depend on the stormwater management and treat-
face (Squillace et al., 1996; Datry et al., 2004) or when ment processes envisioned on the site. A site evaluation
the background soil has a measurable metals content. would include determination of the suitability of the
For example, Ku and Simmons (1986) noted measur- site for infiltration and an assessment of the risk of
able concentrations of chromium in groundwater below groundwater contamination.
stormwater infiltration facilities. Also, deicing salts have Much work has gone into defining the appropriate
the potential to enhance the transport of metals in the physical characteristics of municipal stormwater and
subsurface (Kakuturu and Clark, 2015). E ­ xtrapolating highway runoff infiltration systems (NASEM, 2015;
risk from data from municipal stormwater to industrial WDNR, 2017).2 Determining the suitability of a site
settings must be done carefully, considering the many will include conducting infiltration rate measurements
differences in abundance, contaminant occurrence, and of the native soils. The average infiltration rate will
site operation. determine the size of the device relative to the size of
Pitt et al. (1994) offered general guidelines for the drainage area. Many existing industrial sites may
infiltrating industrial stormwater to minimize risk to lack the land to site an appropriately sized infiltration
groundwater, assuming that specific evaluation of con- basin; some sort of subsurface infiltration gallery with
taminant mobility and/or treatment is not provided to underground storage may instead be employed (see
remove the pollutants: Figure 4-1b).
Information about depth to groundwater or
• Runoff from industrial areas with substantial out- perched lenses, depth to bedrock, soil properties, and
door storage or with substantial uncovered outdoor existing subsurface infrastructure is also necessary.
operations with heavy machinery use should not be Many industrial areas are located near waterfronts
treated by infiltration. Such sites may be expected to or in low-lying areas, where infiltration would be
­inappropriate if the depth to groundwater is shallow.
have high concentrations of soluble pollutants and
potentially wide varieties of contaminants (especially Some states specifically prohibit municipal stormwater
organic compounds). Although much is now known infiltration systems where depth from the bottom of
about organic chemical fate and transport, many the infiltration system to groundwater (i.e., seasonal
emerging compounds still have poorly understood high water table) or bedrock is low (e.g., 3 feet in
treatment, mobility, and toxicity characteristics. Minnesota [MPCA, 2015]; 2 feet in Pennsylvania
• Runoff from critical source areas, such as vehicle [PA DEP, 2006], 10 feet in Orange County, California
service facilities and large parking areas, require [County of Orange, 2013]). Additionally, if the soil
adequate (pre)treatment to reduce groundwater texture will not support sufficient infiltration rates
contamination potential before infiltration. or if the rate is too high, leaving inadequate contact
• Snowmelt should be diverted from infiltration time for treatment, the site is not suitable without soil
devices because of its potential for having high amendments. Some states also prohibit stormwater
concentrations of soluble salts that are effectively infiltration if the groundwater is protected as a drink-
transported through soils to the groundwater. In ing water supply.
soils containing clay or high organic matter con- Soil chemical properties, such as soil organic matter
tent, salt can also reduce the soil permeability and content and soil cation exchange capacity (CEC), will
render infiltration devices inoperable. control the attenuation characteristics of stormwater
pollutants. Adsorption of pollutants, under equilibrium
Industrial stormwater containing pollutants with low conditions, can be described by a partitioning coefficient,
toxicity, low concentrations, and limited mobility in the Kd, which describes the ratio of the concentration of
subsurface environment will pose the lowest infiltration
2 See also https://www.pca.state.mn.us/water/minnesotas-
contamination risks.
stormwater-manual.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

CONSIDERATION OF RETENTION STANDARDS IN THE MULTI-SECTOR GENERAL PERMIT 75

pollutant adsorbed to the concentration in solution. The Unless pretreatment is provided to reduce all pol-
higher the Kd, the greater extent the pollutant will be lutants below levels of concern, dry wells or subsurface
adsorbed to the soil and the less will remain in solution injection are not appropriate for industrial stormwater
where it could be transported to ground­water. ­Values of infiltration because these systems provide little to
Kd depend on pollutant characteristics but also on the no removal of contaminants. Pitt and Talebi (2012)
water chemistry and characteristics of the a­dsorbent. found no statistically significant concentration reduc-
The sorption of hydrophobic organic compounds will tions in stormwater contaminants (nutrients, heavy
primarily depend on the organic ­matter content of the ­metals, p­ esticides, herbicides, bacteria) after infiltrat-
soils, with Kd linearly related to the soil fraction organic ing through at least 4 feet of underlying rock and soil
matter (Schwarzenbach et al., 1993). Specific chemical beneath dry wells. Dry wells are only appropriate for
characteristics of the natural organic matter will play disposal of high quantities of water that are of good
a minor role in the adsorption of pollutants because quality and, as such, are unlikely to be appropriate for
most natural organic matter has a variety of sorption industrial runoff.
and ion-exchange sites. Soils with low organic matter In addition to site-level analyses, regional analyses
content would not be expected to provide significant of potential effects on stormwater infiltration on exist-
attenuation and removal of organic pollutants. Values ing soil or groundwater contamination may be needed.
of Kd for various pollutants have been tabulated based To reduce the likelihood of mobilizing existing con-
on soil properties (e.g., Sauvé et al., 2000). Contact time taminants, known soil contamination sites and ground-
with the soil is another important parameter that influ- water contamination plumes in the region should be
ences removal of pollutants. Although Kd can provide inventoried, and the potential impacts of increased
a gross estimate of potential removal given sufficient groundwater levels should be carefully examined.
contact time, the time-based interaction of pollutants
with soil will determine the actual fraction of pollutant On-Site Treatment Options
removed by the soil. Thus, slower infiltration rates will
usually result in higher fractions of pollutant removal Removal of particulate matter from runoff is
than faster infiltration rates. necessary for any infiltration system at an industrial
Pollutants, such as heavy metals, metalloids, and facility. Particulate matter removal protects the system
phosphorus, will adsorb onto soils via specific bonding by reducing the risk of the infiltration system media
mechanisms with chemical sites on the soil matrix. clogging, and it also removes the fraction of influent
Important factors controlling Kd include the CEC, pollutants that are associated with those particles.
hydrous oxide content, clay content, and organic If the infiltrating soil characteristics are insufficient
­matter content. The stormwater pH and other chemi- to remove the anticipated stormwater pollutants before
cal parameters can be controlling factors for attenua- they reach groundwater, a wide range of additional
tion of ionic pollutants (Stumm and Morgan, 1995). treatment options can be employed (see Box 1-1).
Soils with low CEC, hydrous oxide content, clay con- The treatment performance of conventional treat-
tent, and organic matter content would not be expected ment SCMs for industrial stormwater is summarized
to provide significant removal of ionic pollutants. by Clark and Pitt (2012) and discussed in Chapter 2
Phosphorus removal will only occur if the background and Appendix D. Soluble pollutants can be difficult
phosphorus level in the soil is low. Many common to remove, unless an absorbent highly specific to that
organic soil amendments have high phosphorus con- chemical is used. Extrapolating performance of SCMs
tents, resulting in phosphorus leaching rather than from municipal stormwater to industrial settings where
removal. Metals may also sorb to colloidal material pollutants and concentrations are not comparable will
or form complexes with organic or inorganic ligands, require careful analysis of the unit processes themselves
which can enhance their transport in the subsurface and their treatment efficiencies across a wide range of
(Fein, 1996; Nowack et al., 1997). These processes concentrations and water chemistries.
and their impact on removal are poorly understood in Any treatment of industrial stormwater will result
stormwater. in accumulation of the removed industrial pollutants in

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

76 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

the SCMs. Less-mobile pollutants, such as lead, cop- also be considered when assessing risks of infiltration
per, zinc, and hydrophobic organic contaminants, will to groundwater.5 If pollutants on this list (but not regu-
generally accumulate in sediments at the point of reten- lated under the Safe Drinking Water Act) or emerging
tion and infiltration or sorb onto geomedia (DiBlasi chemicals of concern to human health are present in
et al., 2009; Jones and Davis, 2013). Persistence of stormwater, careful consideration of pollutant removal
pollutants in the shallow soil varies depending on the or treatment options is needed.
contaminant and the local conditions. Depending on In lieu of other information on contaminant
pollutant toxicity and mobility, these soils/sediments attenuation in the groundwater of an industrial site,
may need to be managed to control risks to human the committee recommends that industrial stormwater
health and the environment. infiltrated to groundwater be treated to meet primary
Models such as the Seasonal Soil (SESOIL) drinking water standards for inorganic chemicals and
compartment model can be used to simulate the water organic chemicals, and secondary standards for chloride
transport, sediment transport, and the fate of the pol- and total dissolved solids. If the aquifer is not suit-
lutants in the subsurface beneath infiltration facili- able for use as a public water supply, this requirement
ties. SESOIL has been used to support performance could be relaxed with concurrence of state and local
results from dry pond industrial stormwater infiltration public health agencies. Additionally, other pollutants
(Eppakayala, 2015) and as a screening tool to evaluate of concern that may not currently be regulated by the
groundwater contamination potential of infiltrating Safe Drinking Water Act should be treated to drinking
MS4 stormwater (Clark and Pitt, 2007). water risk levels. The industrial facility would need to
Infiltrating industrial stormwater can carry high ensure that this level of quality is met through monitor-
risks. Risks to groundwater from infiltration of indus- ing, either before the stormwater is applied to the infil-
trial stormwater can be greatly reduced by requiring tration area or after passing through the infiltration/
that infiltrated water meet stringent water quality treatment media at the base of the unsaturated zone.
requirements, such as those for drinking water, as Some degree of stormwater treatment, possibly
defined by the Safe Drinking Water Act. This recom- advanced treatment, would be required at most indus-
mendation would put numeric limits on many pol- trial sites to meet drinking water quality standards. This
lutants of concern, including many heavy metals, a may include adsorption of toxic organic compounds
number of synthetic organic compounds, and nitrate. via activated carbon or another specialty adsorbent. If
The use of drinking water standards as cleanup goals the stormwater exceeds drinking water limits for total
for contaminated groundwater is well established. The dissolved solids, chloride, specific conductance, and/or
2018 amendments to the California equivalent of the sulfate, costly technologies, such as reverse osmosis or
MSGP allows infiltration of industrial stormwater if other desalination processes, would be required, likely
the water meets drinking water quality standards by making infiltration economically unfeasible.
the time it reaches the base of the unsaturated zone Requiring that infiltrating stormwater meet drink-
(California Water Boards, 2018). California’s amended ing water standards holds industries to a higher infil-
permit includes all primary maximum contaminant tration standard than MS4s. However, such require-
levels (MCLs)3 as well as secondary standards for ments acknowledge the wide range of pollutant types,
total dissolved solids, chloride, specific conductance, concentrations, toxicities, and properties expected in
and sulfate.4 However, drinking water standards may industrial stormwater. Stormwater from areas that are
not provide a sufficient screening tool because many not part of the industrial activity would not have to
industrial chemicals that may be highly toxic are not meet the drinking water requirement to be infiltrated;
regulated under the Safe Drinking Water Act. EPA’s segregation of such stormwater is highly encouraged.
drinking water Contaminant Candidate List 4 should EPA guidelines for infiltrating industrial storm­
water would help ensure that industries implement this
3 See https://www.epa.gov/ground-water-and-drinking-water/ stormwater management option in a way that is effec-
national-primary-drinking-water-regulations.
4 See https://www.epa.gov/dwstandardsregulations/secondary-

drinking-water-standards-guidance-nuisance-chemicals. 5 See https://www.epa.gov/ccl/chemical-contaminants-ccl-4.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

CONSIDERATION OF RETENTION STANDARDS IN THE MULTI-SECTOR GENERAL PERMIT 77

tive in reducing surface water pollution while being specific information. Given the site-specific nature of
protective of groundwater. Such guidance would ideally local rainfall patterns and stormwater production and
include the necessary treatment options and costs for quality, however, it is not possible to recommend a
different pollutant source areas, considering concentra- nationwide standard for retention.
tions, toxicity, persistence, and potential for adsorption As described in Chapter 1, the MSGP must be
onto or ion exchange with the geomedia. The potential written to include technology-based effluent limits
for dilution or attenuation in the subsurface could also (TBELs) and WQBELs. Therefore, if numeric reten-
be addressed. tion standards were to be included in the MGSP, it
would be within the context of functioning as a TBEL
Design Considerations or WQBEL, which is notably distinct from how reten-
tion standards have been applied in MS4 permits,
Readily available stormwater manuals (e.g., which has been in accordance with the maximum
CASQA, 2010) provide details on the proper instal- extent practicable standard. Given the site-specific
lation of infiltration systems. Additionally, a growing nature of the suitability of retention with infiltration at
body of practitioners has experience and knowledge industrial sites, numeric retention standards as a TBEL
to evaluate where industrial stormwater retention and could not be established in EPA’s MSGP or as best-
infiltration retention systems are appropriate. To evalu- available technology in an effluent limitation guide-
ate the feasibility of retention, a number of data sets line. However, retention with infiltration is already an
are required. As discussed previously, knowledge of the appropriate allowance within the 2015 MSGP require-
stormwater contaminants (types and concentrations) is ment to “select, design, install, and implement control
necessary on both a chronic and an episodic basis. Site measures (including best management practices) to
conditions, including soil properties, depth to ground- minimize pollutant discharges” (EPA, 2015d). Because
water table, rainfall information, and land availability, retention with infiltration reduces the overall volume of
are also required. If these properties are favorable, then a discharge, it is an effective means to minimize pol-
a preliminary design of a retention system can made. lutant discharges through reduction in pollutant mass.
Details must include a design storm and consideration Nonetheless, because of the variable nature of
on how that design affects compliance with bench- rainfall and stormwater, no retention system can be
marks for any bypass. constructed to contain all stormwater from all events.
Many design models, such as WinSLAMM and In some cases bypass discharges that occur in storms
P8, are available to add confidence to the sizing of an beyond the design storm size may be below benchmark
infiltration system. These models can describe both the thresholds, and in those cases there is a high level of
retention and water quality benefits of an infiltration assurance that the discharge that relies on infiltration
system. The models can maximize the benefits of a as a treatment SCM also complies with WQBELs. In
system by accounting for important variables, such as other cases, the bypass concentration may exceed the
soil type, drainage area size, and rainfall patterns. respective benchmark, which will be problematic to
industrial facilities desiring to implement r­etention/
REGULATORY CONTEXT FOR infiltration, triggering corrective actions. Some degree
RETENTION STANDARDS of regulatory relief during large-event bypass would
need to be implemented to encourage industrial
Retention with infiltration is an attractive method stormwater retention where it is safe and appropriate.
for stormwater control from industrial facilities when The most significant incentive would be assurance
the contaminants do not pose a risk to groundwater that installation of a well-designed retention system
and where land is available to install infiltration SCMs. provides relief from the corrective action process associ-
In general, hydrological and statistical methods and ated with episodic results above benchmark thresholds
data are available (or could be readily obtained) for associated with bypass.
determining retention requirements to achieve specific At least one state has recognized the benefits
objectives for pollutant mass reduction, given site- of industrial stormwater retention in reducing the

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

78 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

­ ollutant load on water bodies and has adapted its

p toxic amounts, can pose serious risks to groundwater;
permit to facilitate the practice. In Oregon, permittees these risks must be managed to prevent groundwater
can request a mass reduction waiver if they have imple- contamination. Based on the potential environmental
mented stormwater retention with infiltration or ben- benefits, particularly in areas of water scarcity, the
eficial use, if these practices can be shown to reduce the committee encourages the use of industrial stormwater
mass discharge of pollutants below the equivalent mass retention with infiltration or beneficial use under con-
discharge of the benchmarks. Permittees are required to ditions where groundwater is protected.
provide data and analysis of this mass discharge reduc- Rigorous permitting, (pre)treatment, and moni-
tion and to take corrective actions by reviewing their toring requirements are needed along with careful
SCMs and whether additional pollution controls are site characterization and designs to ensure ground-
needed (OR DEQ, 2017). water protection in industrial stormwater infiltration
EPA could encourage infiltration by specifically systems. In lieu of other information on the attenu-
addressing the uncertainty associated with bypass dur- ation of contaminants in groundwater before they
ing events that exceed design conditions. If retention are transported to the site boundary, infiltrated water
systems are relied on to meet a WQBEL, a wet weather should be required to meet primary drinking water
accommodation could be included that considers dilu- standards for inorganic chemicals and organic chemi-
tion or assimilative capacity during extreme storms (see cals, and secondary standards for chloride and total
Chapter 3). Allowable frequencies of stormwater dis- dissolved solids. Water quality should be monitored
charge at levels above benchmark thresholds could be and evaluated in the infiltration device or at the base of
derived from allowances of frequencies of exceedance the vadose zone. Many water quality treatment options
of water quality criteria and the duration of exposure are available ranging from natural removal employing
upon which the criteria are based. EPA could also in situ soils to standard SCMs to advanced treatment.
develop a water quality standard exceedance allowance Industries considering infiltration should evaluate
for extreme weather events or, as was contemplated in whether potential stormwater contaminants from rou-
the past (EPA, 1995), establish separate water quality tinely occurring pollutants as well as accidents and spills
criteria for wet weather events. are compatible with infiltration and what technologies
If EPA wants to encourage the use of retention are required to remove these contaminants prior to
with infiltration as a means to reduce pollutant loads infiltration. Chemicals covered by the Safe Drinking
and peak flows, assuming infiltration is suitable based Water Act and unregulated chemicals with known
on groundwater considerations, it should develop addi- human health risks at concentrations of concern should
tional guidance on appropriate design storm standards, be evaluated. Meeting stringent water quality require-
perhaps in consideration of regional precipitation pat- ments may make infiltration cost prohibitive at sites
terns. Additionally, EPA should develop guidance and with contaminants that pose a high risk of polluting
cases studies for demonstrating through the Additional groundwater. Other factors influencing the feasibility
Implementation Measure process that discharges above of a retention and infiltration system include the land
a benchmark threshold that occur only in storms larger available, soil infiltration rate, soil chemistry, and depth
than the design storm do not result in an exceedance of to groundwater.
water quality standards. Site-specific factors and water quality-based
effluent limits render national retention standards for
CONCLUSIONS AND industrial stormwater infeasible within the existing
RECOMMENDATIONS regulatory framework of the MSGP. Retention and
infiltration or beneficial use is already allowed within
Stormwater retention for infiltration or beneficial the MSGP as one of many possible SCMs. However,
use minimizes pollutant loads to receiving waters and the suitability of retention with infiltration or beneficial
reduces damaging peak flows while potentially increas- use is based on site-specific factors that cannot be gen-
ing water availability. Yet, infiltration of industrial eralized nationally into retention standards. Issues such
stormwater, which can contain hazardous pollutants in as the design storm size, stormwater quality, receiv-

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

CONSIDERATION OF RETENTION STANDARDS IN THE MULTI-SECTOR GENERAL PERMIT 79

ing water quality goals, and site conditions must be frequencies of stormwater discharge at levels above
known to ensure performance reliability. Additionally, benchmark thresholds, development of water quality
although retention could be designed using site-specific standard exceedance allowances for extreme weather
factors as a TBEL, industrial stormwater must also events, or establishment of separate water quality cri-
comply with WQBELs, which are typically concen- teria for major wet weather events. Finally, EPA could
tration based. It is impractical to design stormwater develop guidance and case studies for demonstrating
retention to capture all potential rainfall events, and for that exceeding the benchmark during storms with
storm events that exceed the design standard, discharge precipitation amounts greater than the design storm do
or bypass will occur that may exceed the benchmarks. not result in an exceedance of water quality standards.
EPA should consider incentives to encourage EPA should develop guidance for retention and
industrial stormwater infiltration or capture and use infiltration of industrial stormwater for protection of
where appropriate. The most significant incentive groundwater. The guidance should include informa-
would be assurance that installation of infiltration in tion on applied water quality, treatment offered within
accordance with EPA guidance for determining the the infiltration zone, monitoring requirements, natural
appropriate design storm provides relief from the cor- attenuation of pollutants, groundwater use designa-
rective action process associated with episodic bypass tions, and possible impacts of pollutant dilution or
that exceeds benchmark thresholds. This could be done mobilization in the subsurface. Because of the potential
through a number of regulatory measures, including risks to groundwater, industrial stormwater infiltra-
a mixing zone allowance, establishment of allowable tion is not recommended in states that lack the legal
­authority to manage and enforce groundwater quality.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

References

Abdel-Shafy, H. I., and M. S. M. Mansour. 2016. A review on California Water Boards. 2018. General Permit for Storm Water
polycyclic aromatic hydrocarbons: Source, environmental im- Discharges Associated with Industrial Activity, NPDES No.
pact, effect on human health and remediation. Egyptian Journal CAS000001, Order No. 2014-0057-DWQ, amended by Order
of Petroleum 25(1):107–123. No. 2015-0122-DWQ, amended by Order No. 2018-XXXX-
Armstrong, D. E., and R. Llena. 1992. Stormwater infiltration: DWQ. Available at https://www.waterboards.ca.gov/water_­
Potential for pollutant removal. Chicago, IL: Prepared for the issues/programs/stormwater/docs/industrial/unoff_igp_amend.
Wisconsin Department of Natural Resources (Madison) and pdf (accessed January 30, 2019).
the U.S. Environmental Protection Agency. Caltrans. 2010. Treatment BMP technology report. CSTW-
Avila, H., and R. E. Pitt. 2009. Scour in stormwater catchbasin RT-09-239-06. Sacramento, CA: California Department of
devices—experimental results from a full-scale physical model. Transportation. Available at http://www.dot.ca.gov/hq/env/
Journal of Water Management Modeling R235-19. stormwater/pdf/CTSW-RT-09-239-06.pdf (accessed Novem-
Avila, H., R. Pitt, and S. R. Durrans. 2008. Factors affecting scour ber 8, 2018).
of previously captured sediment from stormwater catchbasin Canadian CME (Canadian Council of Ministers of the Environ-
sumps. Journal of Water Management Modeling R228-13. ment). 1999. Canadian environmental quality guidelines for the
Bent, G. C., J. R. Gray, K. P. Smith, and G. D. Glysson. 2001. protection of aquatic life. Canadian Council of Ministers of the
A synopsis of technical issues for monitoring sediment in highway Environment, Winnipeg, Manitoba. Available at http://st-ts.
and urban runoff. Northborough, MA: U.S. Geological Survey ccme.ca/en/index.html (accessed November 19, 2018).
Open-File Report 00-497. CASQA (California Stormwater Quality Association). 2010.
Boving, T. B., M. H. Stolt, J. Augenstern, and B. Brosnan. 2008. Low impact development manual for Southern California: Tech-
Environmental Geology 55(3):571–582. nical guidance and site planning strategies. Prepared for the
Breault, R. F., and G. E. Granato. 2000. A synopsis of technical issues Southern California Stormwater Monitoring Coalition in
for monitoring trace elements in highway runoff and urban storm- cooperation with the State Water Resources Control Board
water. Northborough, MA: U.S. Geological Survey Open-File by the Low I­ mpact Development Center. Available at https://
Report 00-422. www.casqa.org/sites/default/files/downloads/socallid-manual-
Bulkley, J., D. LeFevre, H. Clark, A. Samples, and R. Berns. 2009. final-040910.pdf (accessed November 8, 2018).
Wet weather benchmarking report. University of Michigan, Ann City of Phoenix. 2011. Storm water policies and standards: Revisions.
Arbor. Pp. 1–383. Available at http://css.umich.edu/sites/­ Available at https://www.phoenix.gov/streetssite/Documents/
default/files/css_doc/WWfinal.pdf (accessed December 18, swpolicy.pdf (accessed November 8, 2018).
2018). Clark, S. E. 2000. Urban stormwater filtration: Optimization of
Burton, G. A., and R. E. Pitt. 2002. Chapter 5: Sampling effort and ­design parameters and a pilot-scale evaluation. Ph.D. Dissertation,
collection methods. Pp. 224–338 in Stormwater effects handbook: University of Alabama at Birmingham.
A toolbox for watershed managers, scientists, and engineers, G. A. Clark, S. E., and R. Pitt. 2007. Influencing factors and a proposed
Burton and R. E. Pitt, eds. Boca Raton, FL: Lewis Publishers. evaluation methodology for predicting groundwater contami-
Cadmus, P., S. F. Brinkman, and M. K. May. 2018. Chronic toxicity nation potential from stormwater infiltration activities. Water
of ferric iron for North American aquatic organisms: Deriva- Environment Research 79(1):29–36.
tion of a chronic water quality criterion using single species and Clark, S. E., and R. Pitt. 2008. Comparison of stormwater solids
mesocosm data. Archives of Environmental Contamination and analytical methods for performance evaluation of manufac-
Toxicology 74(4):605–615. tured treatment devices. Journal of Environmental Engineering
134(4):259–264.

81

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

82 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Clark, S. E., and R. Pitt. 2012. Targeting treatment technologies DDOE (District Department of the Environment). 2014. Over-
to address specific stormwater pollutants and numeric discharge view of 2013 stormwater rule and stormwater management
limits. Water Research 46(20):6715–6730. guidebook. Available at https://ddot.dc.gov/sites/default/files/
Clark, S. E., and R. Pitt. In press. Industrial stormwater: Sedi- dc/sites/ddot/publication/attachments/2014-0418-DDOT-
mentation and filtration effectiveness to meet benchmark GI-SWM%20Regs%20and%20GI%20Training.pdf (accessed
concentrations. Submitted to the Journal of Sustainable Water November 8, 2018).
in the Built Environment. Dechesne, M., S. Barraud, and J.-P. Bardin. 2004. Spatial distri-
Clark, S. E., and C. Y. S. Siu. 2008. Measuring solids concentra- bution of pollution in an urban stormwater infiltration basin.
tion in stormwater runoff: Comparison of analytical methods. Journal of Contaminant Hydrology 72(1):189–205.
Environmental Science & Technology 42(2):511–516. DeForest, D. K., K. V. Brix, L. M. Tear, and W. J. Adams. 2018.
Clark, S. E., C. Y. S. Siu, R. Pitt, C. D. Roenning, and D. P. Treese. Multiple linear regression models for predicting chronic alumi-
2009. Peristaltic pump autosamplers for solids measurement in num toxicity to freshwater aquatic organisms and developing
stormwater runoff. Water Environment Research 81(2):192–200. water quality guidelines. Environmental Toxicology and Chemistry
Clark, S. E., K. H. Baker, D. P. Treese, J. B. Mikula, C. Y. S. Siu, 37(1):80–90.
C. S. Burkhardt, and M. M. Lalor. 2010. Sustainable stormwater DiBlasi, C. J., H. Li, A. P. Davis, and U. Ghosh. 2009. Removal
management: Infiltration vs. surface treatment strategies. WERF and fate of polycyclic aromatic hydrocarbon pollutants in an
Report 04-SW-3. London, UK: IWA Publishing. urban stormwater bioretention facility. Environmental Science
Clary, J., M. Leisenring, and P. Hobson. 2011. International & Technology 43(2):494–502.
stormwater Best Management Practices (BMP) database pollutant Edwards, E. C., T. Harter, G. E. Fogg, B. Washburn, and H.
category summary: Metals. International Stormwater BMP Data- Hamad. 2016. Assessing the effectiveness of drywells as tools
base. Prepared by Wright Water Engineers, Inc., and Geosyntec for stormwater management and aquifer recharge and their
Consultants, Inc. Available at http://www.bmpdatabase.org/ groundwater contamination potential. Journal of Hydrology
Docs/BMP%20Database%20Metals%20Final%20August%20 539:539–553.
2011.pdf (accessed November 7, 2018). EPA (Environmental Protection Agency). 1976. Quality criteria
County of Orange, CA, Department of Public Works. 2013. for water. EPA 440-9-76-023. Available at https://www.epa.
Technical Guidance Document for the Preparation of Concep- gov/sites/production/files/2018-10/documents/quality-criteria-
tual/Preliminary/ or Project Water Quality Management Plans, water-1976.pdf (accessed November 12, 2018).
Appendix VIII, Groundwater-Related Infiltration Feasibility EPA. 1980a. Ambient water quality criteria for copper. EPA 440/4-
Criteria (see Appendix VIII.2). 80-036. Washington, DC: EPA.
Cristina, C., J. Tramonte, and J. J. Sansalone. 2002. A granulometry- EPA. 1980b. Ambient water quality criteria for polynuclear aromatic
based selection methodology for separation of traffic-generated hydrocarbons. EPA 440/5-80-069.
particles in urban highway snowmelt runoff. Water, Air, and Soil EPA. 1980c. National recommended water quality criteria for
Pollution 136(1–4):33–53. ­b eryllium. LOEL Acute Freshwater. EPA 440/5-80-024
Cross, L. M., and L. D. Duke. 2008. Stormwater regulations (October).
for industry: Linking regulatory priorities with water quality EPA. 1983. Final Report of the Nationwide Urban Runoff Program.
protection. Journal of the American Water Resources Association National Technical Information Service (NTIS) Accession
44(1):86–106. Number: PB84-185552. Washington, DC: Water Planning
Crunkilton, R., J. Kleist, J. Ramcheck, W. DeVita, and D. Division. Available at https://www3.epa.gov/npdes/pubs/
­V illeneuve. 1996. Assessment of the response of aquatic organ- sw_nurp_vol_1_finalreport.pdf (accessed November 8, 2018).
isms to long-term in situ exposures of urban runoff. Pp. 95–111 EPA. 1984. 40 CFR Part 133. Secondary treatment regulation; final
in Effects of watershed development and management of aquatic rule, 49 Federal Register No. 184, 36986–37007.
ecosystems, L. A. Roesner, ed. New York: American Society of EPA. 1985. Guidelines for deriving numerical national water ­quality
Civil Engineers. criteria for the protection of aquatic organisms and their uses.
CT DEEP (Connecticut Department of Energy and Environmental C. E. Stephen, D. I. Mount, D. J. Hansen, J. R. Gentile, G. A.
Protection). 2004. Hydrologic sizing criteria for treatment prac- Chapman, and W. A. Brungs, eds. EPA PB85-227049. Avail-
tices. Chapter 7 in Stormwater quality control manual. Available able at https://www.epa.gov/sites/production/files/2016-02/
at https://www.ct.gov/deep/cwp/view.asp?a=2721&q=325704 documents/guidelines-water-quality-criteria.pdf (accessed
(accessed September 18, 2018). November 1, 2018).
CT DEEP. 2018. General permit for the discharge of stormwater associ- EPA. 1986. Method 8100: Polycyclic aromatic hydrocarbons. Avail-
ated with industrial activity. DEEP-WPED-GP-014. able at https://www.epa.gov/sites/production/files/2015-12/
Datry, T., F. Malard, and J. Gibert. 2004. Dynamics of solutes documents/8100.pdf (accessed November 8, 2018).
and dissolved oxygen in shallow urban groundwater below a EPA. 1987. Ambient water quality criteria for selenium—1987. EPA
stormwater infiltration basin. Science of the Total Environment 440/5-87-006. Washington, DC: EPA.
329(1–3):215–229. EPA. 1988. Ambient water quality criteria for aluminum—1988.
Davis, L. R. 2018. Fundamentals of environmental discharge modeling EPA 440/5-86-008. Duluth, MN: Office of Research and
(Vol. 10). Boca Raton, FL: CRC Press. Development. Available at https://nepis.epa.gov/Exe/ZyPDF.
cgi/2000M5FC.PDF?Dockey=2000M5FC.PDF (accessed
February 14, 2019).

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

REFERENCES 83

EPA. 1990. 40 CFR Parts 122, 123, and 124. National Pollutant EPA. 2007. Aquatic life criteria—Copper. Available at https://www.
Discharge Elimination System Permit application regulations epa.gov/wqc/aquatic-life-criteria-copper (accessed November 8,
for storm water discharges; final rule. 55 Federal Register No. 2018).
222, 47990–48091. EPA. 2008a. Multi-Sector General Permit for stormwater discharges
EPA. 1991. Technical support document for water quality-based toxics associated with industrial activity (MSGP). Available at https://
control. EPA 505/2-90-00. NTIS Accession Number: PB91- www.epa.gov/sites/production/files/2015-10/documents/
127415. Washington, DC: Office of Water (EN-336). Available msgp2008_finalpermit.pdf (accessed November 1, 2018).
at https://www3.epa.gov/npdes/pubs/owm0264.pdf (accessed EPA. 2008b. Multi-Sector General Permit for stormwater discharges
February 14, 2019). associated with industrial activity (MSGP)—Fact sheet. Avail-
EPA. 1992a. Final NPEDS general permits for storm water dis- able at https://www.epa.gov/sites/production/files/2015-10/­
charges associated with industrial activity; notice. 57 Federal documents/msgp2015_fs.pdf (accessed November 1, 2018).
Register 41236–41342. September 9, 1992. EPA. 2009a. Industrial stormwater monitoring and sampling guide.
EPA. 1992b. National Pollutant Discharge Elimination System EPA 832-B-09-003. Available at https://www3.epa.gov/­npdes/
application deadlines, general permit requirements and report- pubs/msgp_monitoring_guide.pdf (accessed November 2, 2018).
ing requirements for storm water discharges associated with EPA. 2009b. Technical guidance on implementing the storm­water
industrial activity; final rule. 57 Federal Register 11394–11413. runoff requirements for federal projects under Section 438 of
April 2, 1992. the Energy Independence and Security Act. EPA 841-B-09-
EPA. 1992c. NPDES storm water sampling guidance document. EPA 001. Available at https://www.epa.gov/greeningepa/technical-
833-8-92-001. Available at https://www3.epa.gov/npdes/pubs/ guidance-­i mplementing-stormwater-runoff-requirements-
owm0093.pdf (accessed November 8, 2018). federal-projects (accessed September 18, 2018).
EPA. 1995. Final National Pollutant Discharge Elimination System EPA. 2010. National Pollutant Discharge Elimination System
Storm Water Multi-Sector General Permit for industrial activi- (NPDES) Permit Writers’ Manual. Available at https://
ties. 60 Federal Register 50804–51319. September 1995. www.epa.gov/sites/production/files/2015-09/documents/pwm_
EPA. 1996a. Advisory Committee Charter, Urban Wet Weather chapt_05.pdf (accessed September 18, 2018).
Flows Advisory Committee, C. Browner, Administrator (signed EPA. 2012. 2008 MSGP Benchmark Discharge Monitoring
Jan. 06, 1995). 61 Federal Register 46462. Available at https:// ­Report Analysis and SWPPP Compliance Review. EPA-HQ-
www.govinfo.gov/app/details/FR-1996-09-03/96-22380 (ac- OW-2012-0803-0002.
cessed December 31, 2018). EPA. 2013. 40 CFR Parts 122,123,127, et al. NPDES electronic
EPA. 1996b. The metals translator: Guidance for calculating a total reporting rule; proposed rule. 78 Federal ­Register 46005–46116.
recoverable permit limit from a dissolved criterion. EPA 823-B- EPA. 2014. Clean Water Act National Pollutant Discharge Elimina-
96-007. Washington, DC: EPA Office of Water. Available at tion System compliance monitoring strategy 2014. Washington,
https://www3.epa.gov/npdes/pubs/metals_translator.pdf (ac- DC: Office of Enforcement and Compliance Assurance. Avail-
cessed November 7, 2018). able at https://www.epa.gov/sites/production/files/2013-09/
EPA. 1999. Report to Congress on the Phase II Storm Water Regula- documents/npdescms.pdf (accessed November 7, 2018).
tions. EPA 833-99-001. Washington, DC: EPA Office of Water. EPA. 2015a. 40 CFR Parts 9, 122, 123, et al. National Pollutant
Available at https://www3.epa.gov/npdes/pubs/­ReptoCong_ Discharge Elimination System (NPDES) electronic reporting
PhII_SWR.pdf (accessed November 7, 2018). rule; final rule. 80 Federal Register 64063–64158.
EPA. 2000. Final reissuance of National Pollutant Discharge Elimi- EPA. 2015b. Method 8310: Polynuclear aromatic hydrocarbons. Avail-
nation System (NPDES) storm water Multi-Sector ­General able at https://www.epa.gov/sites/production/files/2015-12/
Permit for industrial activities. 65 Federal Register 64746–64880. documents/8310.pdf (accessed November 1, 2018).
EPA. 2001. Update of the ambient water quality criteria for cadmium. EPA. 2015c. Multi-Sector General Permit for stormwater discharges
EPA 822/R-01-001. Washington, DC: EPA. associated with industrial activity (MSGP)—Fact sheet. Avail-
EPA. 2004. NPDES Compliance Inspection Manual. EPA 305-X-04- able at https://www.epa.gov/sites/production/files/2015-10/­
001. July 2004. Washington, DC: EPA Office of Enforcement documents/msgp2015_fs.pdf (accessed November 1, 2018).
and Compliance Assurance. Available at https://www.epa.gov/ EPA. 2015d. United States Environmental Protection Agency (EPA)
sites/production/files/2013-09/documents/npdesinspect_0.pdf National Pollutant Discharge Elimination System (NPDES)
(accessed December 31, 2018). Multi-Sector General Permit for stormwater discharges associated
EPA. 2006a. Industrial stormwater factsheet series: Sector R: Ship and with industrial activity (MSGP). Available at https://www.epa.
boat building or repair yards. EPA 833-F-06-033. Available at gov/sites/production/files/2015-10/documents/msgp2015_­
https://www.epa.gov/sites/production/files/2015-10/documents/ finalpermit.pdf (accessed November 1, 2018).
sector_r_shipbuilding.pdf (accessed November 12, 2018). EPA. 2016a. Aquatic life ambient water quality criteria cadmium—
EPA. 2006b. National ambient water quality criteria—Human health 2016. EPA 820-R-26-002. Available at https://www.epa.gov/
criteria table. EPA 822-F-01-0102006. Available at https://www. sites/production/files/2016-03/documents/cadmium-final-
epa.gov/wqc/national-recommended-water-quality-­criteria- report-2016.pdf (accessed November 12, 2018).
human-health-criteria-table (accessed November 8, 2018). EPA. 2016b. Aquatic life ambient water quality criterion for
EPA. 2006c. National recommended water quality criteria—Aquatic ­selenium—Freshwater 2016. Available at https://www.epa.gov/
life criteria table. EPA 822-F-04-010 2006-CMC. Available sites/­production/files/2016-07/documents/aquatic_life_awqc_
at https://www.epa.gov/wqc/national-recommended-water- for_selenium_-_freshwater_2016.pdf (accessed November 12,
quality-criteria-aquatic-life-criteria-table (accessed Novem- 2018).
ber 8, 2018).

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

84 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

EPA. 2016c. Summary of state post construction stormwater stan- Incardona, J. P., T. L. Linbo, and N. L. Scholz. 2011. Cardiac toxic-
dards. Available at https://www.epa.gov/sites/production/ ity of 5-ring polycyclic aromatic hydrocarbons is differentially
files/2016-08/documents/swstdsummary_7-13-16_508.pdf dependent on the aryl hydrocarbon receptor 2 isoform during
(accessed September 18, 2018). zebrafish development. Toxicology and Applied Pharmacology
EPA. 2017. 2017 Draft aquatic life criteria for aluminum in freshwater. 257(2):242–249.
Available at https://www.epa.gov/wqc/2017-draft-aquatic-life- Jarvis, C. M., and L. Wisniewksi. 2006. An introduction to the
criteria-aluminum-freshwater (accessed November 12, 2018). biotic ligand model. U.S. Environmental Protection Agency
EPA. 2018. Industrial wastewater treatment technology database presentation, May 10. EPA Office of Water, O ­ ffice of Science
(IWTT). Available at https://www.epa.gov/eg/industrial- & Technology. Available at https://acwi.gov/monitoring/­
wastewater-treatment-technology-database-iwtt (accessed conference/2006/2006_conference_mater ials_notes/­
January 24, 2019). Concurrent_­S essionI/I4Stress/I4_Wisniewski.pdf (accessed
Eppakayala, V. K. 2015. Performance evaluation of stormwater treat- November 7, 2018).
ment controls at an industrial site. Ph.D. Dissertation. Univer- Jirka, G. H., R. L. Doneker, and S. W. Hinton. 1996. User’s
sity of Alabama, Tuscaloosa. Available at https://­socwisconsin. manual for CORMIX: A hydrodynamic mixing zone model and
org/wp-content/uploads/2017/05/Eppakayala_2015_­ decision support system for pollutant discharges into surface waters.
SWTrtmtAtIndustrialSite.pdf (accessed November 1, 2018). ­Washington, DC: EPA, Office of Science and Technology.
FAO (Food and Agriculture Organization). 2000. FAO pesticide Available at https://www.epa.gov/sites/production/files/2015-
disposal series 8: Assessing soil contamination: A reference manual. 10/­documents/cormix-users_0.pdf (accessed January 24, 2019).
Rome, Italy: Food and Agriculture Organization of the United Johnson, P. D., R. Pitt, S. R. Durrans, M. Urrutia, and S. Clark.
Nations. 2003. Metals removal technologies for urban stormwater. WERF
Fein, J. B. 1996. The effect of aqueous metal-chlorophenolate 97-IRM-2. Alexandria, VA: Water Environment Research
complexation on contaminant transport in groundwater systems. Foundation.
Applied Geochemistry 11(6):735–744. Jones, P. S., and A. P. Davis. 2013. Spatial accumulation and
Fischer, H. B., E. J. List, R. C. Y. Koh, J. Imberger, and N. H. strength of affiliation of heavy metals in bioretention media.
Brooks. 1979. Mixing in inland and coastal waters. San Diego, Journal of Environmental Engineering 139(4):479–487.
CA: Academic Press. Kakuturu, S. P., and S. E. Clark. 2015. Effects of deicing salts on
Furumai, H., H. Balmer, and M. Boller. 2002. Dynamic behavior clogging of stormwater filter media and on media chemistry.
of suspended pollutants and particle size distribution in highway Journal of Environmental Engineering 141(9):04015020.
runoff. Water Science & Technology 46(11–12):413–418. Kayhanian, M., T. Young, and M. K. Stenstrom. 2005. Limitation
Gawad, S. A., J. A. McCorquodale, and H. Gerges. 1996. Near- of current solid measurements in stormwater runoff. Stormwater
field mixing at an outfall. Canadian Journal of Civil Engineering 6(7):22–30.
23(1):63–75. Kerkez, B., C. Gruden, M. Lewis, L. Montestruque, M. Quigley,
Geosyntec Consultants and Wright Water Engineers, Inc. 2009. B. Wong, A. Bedig, R. Kertesz, T. Braun, O. Cadwalader, A.
Urban Stormwater BMP Performance Monitoring. Available at Poresky, and C. Pak. 2016. Smarter stormwater systems. Envi-
www.bmpdatabase.org/monitoring-guidance.html (accessed ronmental Science & Technology 50(14):7267–7273.
February 14, 2019). Ku, H. F., and D. L. Simmons. 1986. Effect of urban stormwater
Gossett, R., and K. Schiff. 2010. Stormwater Monitoring ­Coalition runoff on ground water beneath recharge basins on Long Island, New
Laboratory Guidance Document, 3rd edition. Southern ­California York. Syosset, NY: U.S. Geological Survey Water-Resources
Coastal Water Research Project Technical Report 615, Costa Investigations Report 85-4088.
Mesa, CA. 22 pp. Available at ftp://ftp.sccwrp.org/pub/ Landsman, M. R., and A. P. Davis. 2018. Evaluation of nutrients
download/DOCUMENTS/TechnicalReports/615_SMC_­ and suspended solids removal by stormwater control measures
LabGuide3rdEdition.pdf (accessed November 19, 2018). using high flow media. Journal of Environmental Engineering
Graczyk, D. J., D. M. Robertson, W. J. Rose, and J. J. Steuer. 2000. 144(10):04018106.
Comparison of water-quality samples collected by siphon samplers LeFevre, G. H., R. M. Hozalski, and P. J. Novak. 2012a. The role
and automatic samplers in Wisconsin. Middleton, WI: U.S. Geo- of biodegradation in limiting the accumulation of petroleum
logical Survey Fact Sheet FS-067-00. hydrocarbons in raingarden soils. Water Research 46(20):6753–
Gray, J. R., G. D. Glysson, L. M. Turcios, and G. E. Schwarz. 6762.
2000. Comparability of suspended-sediment concentration and LeFevre, G. H., P. J. Novak, and R. M. Hozalski. 2012b. Fate of
­total suspended solids data. Denver, CO: U.S. Geological Survey naphthalene in laboratory-scale bioretention cells: Implications
Water-Resources Investigation Report 00-4191. for sustainable stormwater management. Environmental Science
Han, Y., S.-L. Lau, M. Kayhanian, and M. K. Stenstrom. 2006a. & Technology 46(2):995–1002.
Characteristics of highway stormwater runoff. Water Environ- LeFevre, G. H., K. H. Paus, P. Natarajan, J. S. Gulliver, P. J. Novak,
ment Research 78(12):2377–2388. and R. M. Hozalski. 2015. Review of dissolved pollutants in
Han, Y.-H., S.-L. Lau, M. Kayhanian, and M. K. Stenstrom. 2006b. urban storm water and their removal and fate in bioretention
Correlation analysis among highway stormwater pollutants and cells. Journal of Environmental Engineering 141(1):04014050.
characteristics. Water Science & Technology 53(2):235–243. Li, Y., S.-L. Lau, M. Kayhanian, and M. K. Stenstrom. 2005.
Harcum, J., S. Adair, and J. Collins. 2005. Review of discharge Particle size distribution in highway runoff. Journal of Environ-
monitoring report data from the MSGP 2000. Technical mental Engineering 131(9):1267–1276.
Memorandum to Jack Faulk, February 10.

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

REFERENCES 85

Ma, J.-S., J.-H. Kang, M. Kayhanian, and M. K. Stenstrom. Ott, R. L., and M. T. Longnecker. 2015. An introduction to ­statistical
2009. Sampling issues in urban runoff monitoring programs: methods and data analysis, 7th edition. Boston, MA: Cengage
Composite versus grab. Journal of Environmental Engineering Learning.
135(3):118–127. PA DEP (Pennsylvania Department of Environmental Protection).
McIntyre, J., R. Edmunds, B. Anulacion, J. Davis, J. Incardona, J. 2006. Pennsylvania stormwater best management practices manual.
D. Stark, and N. Scholz. 2016. Severe coal tar sealcoat runoff Bureau of Watershed Management. 363-0300-002.
toxicity to fish is prevented by bioretention filtration. Environ- Pitt, R., with contributions from S. Clark, R. Field, and K. Parmer.
mental Science & Technology 50:1570–1578. 1996. Groundwater contamination from stormwater infiltration.
MDEP (Maryland Department of the Environment). 2014. General Chelsea, MI: Ann Arbor Press.
permit for discharges from stormwater associated with industrial Pitt, R. 2011. Water quality objectives and failure modes of green infra-
activities. Discharge Permit No. 12-SW. NPDES Permit No. structure stormwater components. Presented at 84th Annual Water
MDR0000. Envi­ronment Federation Technical Exhibition and Conference
MPCA (Minnesota Pollution Control Agency). 2015. National Pol- (WEFTEC). Los Angeles, CA, October 15–19.
lutant Elimination System (NPDES)/State Disposal System (SDS) Pitt, R., and S. E. Clark. 2010. Evaluation of biofiltration
General Permit MNR050000 for Industrial Stormwater Multi- ­media for engineered natural treatment systems. Prepared
Sector (ISW). Authorization to discharge stormwater associated for G ­ eosyntec Consultants. Available at http://unix.eng.
with industrial activity under the National Pollutant Discharge ua.edu/~rpitt/­Publications/5_Stormwater_Treatment/­Media_
Elimination System (NPDES)/State Disposal System (SDS) for_­s tormwater_treatment/media%20report%20SSFL%20
permit program. Available at https://www.pca.state.mn.us/sites/ May%2010%202010.pdf (accessed November 19, 2018).
default/files/wq-strm3-67a.pdf (accessed November 14, 2018). Pitt, R., and L. Talebi. 2012. Evaluation and demonstration of storm-
NASEM (National Academies of Sciences, Engineering, and water dry wells and cisterns in Millburn Township, New Jersey.
Medicine). 2015. Volume reduction of highway runoff in urban EPA 600/R-12/600. Cincinnati, OH: EPA O ­ ffice of Research
areas: Guidance manual. Washington, DC: The National Acad- and Development.
emies Press. Pitt, R., S. Clark, and K. Parmer. 1994. Protection of groundwater
NASEM. 2016. Using graywater and stormwater to enhance l­ ocal ­water from intentional and nonintentional stormwater infiltration. EPA
supplies: An assessment of risks, costs, and benefits. ­Washington, DC: 600/SR-94/051. Cincinnati, OH: EPA O ­ ffice of Research and
The National Academies Press. Development.
Niogi, S., and C. M. Wood. 2004. Biotic ligand model, a flexible Pitt, R., R. Field, M. Lalor, and M. Brown. 1995. Urban storm­water
tool for developing site-specific water quality guidelines for toxic pollutants: Assessment, sources, and treatability. Water
metals. Environmental Science & Technology 38(23):6177–6192. Environment Research 67(3):260–275.
Nowack, B., H. Xue, and L. Sigg. 1997. Influence of natural and Pitt, R. E., R. Bannerman, S. Clark, and D. Williamson. 2004a.
anthropogenic ligands on metal transport during infiltration of Chapter 23: Source of pollutants in urban areas (Part 1)—
river water to groundwater. Environmental Science & Technology older monitoring projects. Pp. 465–484 in CHI Monograph 13,
31(3):866–872. ­Effective modeling of urban water systems, W. James, K. N. Irvine,
NRC (National Research Council). 2009. Urban stormwater man- E. A. McBean, and R. E. Pitt, eds. Guelf, ON: Computational
agement in the United States. Washington, DC: The National ­Hydraulics International.
Academies Press. Pitt, R., R. Bannerman, S. E. Clark, and D. Williamson. 2004b.
O’Donnell, J. O. 2005. Memorandum re: Review of 2000 MSGP Chapter 24: Source of pollutants in urban areas (Part 2)—­recent
monitoring requirements and suggested changes, March 15. sheetflow monitoring. Pp. 485–530 in CHI Monograph 13,
Fairfax, VA: Tetra Tech, Inc. ­Effective modeling of urban water systems, W. James, K. N. Irvine,
Ohio Environmental Protection Agency. 2018. General permit E. A. ­McBean, and R. E. Pitt, eds. Guelf, ON: Computational
authorization for storm water discharges associated with construc- ­Hydraulics International.
tion activity under the National Pollutant Discharge Elimination RI DEM (Rhode Island Department of Environment Manage-
System. Available at http://epa.ohio.gov/portals/35/permits/ ment). 2013. Multi-Sector General Permit Rhode Island Pollutant
OHC000005/Final_OHC000005.pdf (accessed November 8, Discharge Elimination System storm water discharge associated with
2018). industrial activity (excluding construction activity). RIR500000.
Okamoto, A., M. Yamamuro, and N. Tatarazako. 2014. Acute toxic- Available at http://www.dem.ri.gov/programs/benviron/water/
ity of 50 metals to Daphnia magna. Journal of Applied Toxicology permits/ripdes/pdfs/msgp.pdf (accessed November 14, 2018).
35(7):824–830. Sansalone, J. J., and S. G. Buchberger. 1997. Characterization of
OR DEQ (Oregon Department of Environmental Quality). 2017. solid and metal element distributions in urban highway storm-
National Pollutant Discharge Elimination System: Stormwater water. Water Science & Technology 36(8–9):155–160.
Discharge General Permit No. 1200-Z. Available at https:// Sansalone, J. J., and C. M. Cristina. 2004. First flush concepts for
www.oregon.gov/deq/FilterPermitsDocs/Final1200Zpermit. suspended and dissolved solids in small impervious watersheds.
pdf (accessed November 8, 2018). Journal of Environmental Engineering 130(11):1301–1314.
OR DEQ. 2018. National Pollutant Discharge Elimination System Saunders, T. G., R. C. Ward, J. C. Loftis, T. D. Steele, D. D. Adrian,
Industrial Stormwater Permit Evaluation Report No. 1200-Z. and V. Yevjevich. 1983. Design of networks for monitoring water
October, Final Action. Available at https://www.oregon. quality. Highlands Ranch, CO: Water Resources Publications.
gov/deq/FilterPermitsDocs/1200-Zevalreport.pdf (accessed
Decem­ber 31, 2018).

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

86 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Sauvé, S., W. Hendershot, and H. E. Allen. 2000. Solid-solution van Dam, R. A., A. C. Hogan, C. D. McCullough, M. A. Houston,
partitioning of metals in contaminated soils: Dependence on C. L. Humphrey, and A. J. Harford. 2010. Aquatic toxicity of
pH, total metal burden, and organic matter. Environmental Sci- magnesium sulfate, and the influence of calcium, in very low
ence & Technology 34(7):1125–1131. ionic concentration water. Environmental Toxicology and Chem-
Schiff, K. C., and L. L. Tiefenthaler. 2011. Seasonal flushing of istry 29(2):410–421.
pollutant concentrations and loads in urban stormwater. Journal Vuorinen, P. J., M. Keinänen, S. Peuranen, and C. Tigerstedt. 1998.
of the American Water Resources Association 47(1):136–142. Effects of iron, aluminium, dissolved humic material and acidity
Schwarzenbach, R. P., P. M. Gschwend, and D. M. Imboden. 1993. on grayling (Thymallus thymallus) in laboratory exposures, and a
Environmental organic chemistry. New York: John Wiley & Sons. comparison of sensitivity with brown trout (Salmo trutta). Boreal
Selbig, W. R., A. Cox, and R. T. Bannerman. 2012. Verification Environmental Research 3:405–419.
of a depth-integrated sample arm as a means to reduce solids Water Quality Program. 2011. Technical guidance manual for
stratification bias in urban stormwater sampling. Journal of evaluating emerging stormwater treatment technologies: Technology
Envi­ronmental Monitoring 14(4):1137–1143. assessment protocol—ecology (TAPE). No. 11-10-061. Olympia,
Shaver, E., R. Horner, J. Skupien, C. May, and G. Ridley. 2007. WA: Washington State Department of Ecology.
Fundamentals of urban runoff management: Technical and institu- Waterkeeper Alliance v. U.S. EPA. 2016. Settlement agreement. Avail-
tional issues, 2nd edition. North American Lake Management able at http://waterkeeper.org/wp-content/uploads/2016/08/
Society. Waterkeeper_Alliance_Settlement_Agreement_08162016-
Squillace, P. J., J. S. Zogorski, W. G. Wilber, and C. V. Price. 1996. EPA-MSGP.pdf (accessed November 1, 2018).
Preliminary assessment of the occurrence and possible sources WDNR (Wisconsin Department of Natural Resources). 2017.
of MTBE in groundwater in the United States, 1993−1994. Wisconsin Department of Natural Resources conservation prac-
Environmental Science & Technology 30(5):1721–1730. tice standard site evaluation for storm water infiltration 1002.
State of Vermont. 2008. General permit 3-9020 (2006) for stormwater Available at https://dnr.wi.gov/topic/stormwater/documents/­
runoff from construction sites as amended February 2008. Agency SiteEvalForInfiltr1002.pdf (accessed November 8, 2018).
of Natural Resources Department of Environmental Conserva- Weiner, E. R. 2008. Applications of environmental aquatic chemistry,
tion. Available at https://dec.vermont.gov/sites/dec/files/wsm/ 2nd edition. Boca Raton, FL: CRC Press.
stormwater/docs/StormwaterConstructionDischargePermits/ Weiss, P. T., G. LeFevre, and J. S. Gulliver. 2008. Contamination
sw_cgp_amended_final.pdf (accessed November 8, 2018). of soil and groundwater due to stormwater infiltration practices:
State of Washington Department of Ecology. 2019. Stormwater man- A literature review. University of Minnesota St. Anthony Falls
agement manual for Western Washington—Draft. https://fortress. ­Laboratory: Engineering, Environmental and G ­ eophysical F
­ luid
wa.gov/ecy/ezshare/wq/permits/Flare/­Draft2019SWMMWW. Dynamics. Project Report No. 515. Available at https://www.
htm (accessed on January 27, 2019). pca.state.mn.us/sites/default/files/stormwater-r-weiss0608.pdf
Stumm, W., and J. Morgan. 1995. Aquatic chemistry: Chemical (accessed November 19, 2018).
­equilibria and rates in natural waters, 3rd edition. New York: Winterstein, T. A., and H. G. Stefan. 1983. Suspended sediment
Wiley. sampling in flowing water: Laboratory study of the effects of
Swamikannu, X., M. Mullin, and L. D. Duke. 2000. Industrial storm nozzle orientation, withdrawal rate and particle size. External
water discharger identification and compliance evaluation in the city Memorandum No. M-168. Minneapolis, MI: University of
of Los Angeles. Sacramento, CA: California Water Resources Minnesota–St. Anthony Falls Hydraulic Laboratory.
Control Board. Zahedi, S., H. Vaezzade, M. Rafati, and M. Z. Dangesaraki. 2014.
Treese, D. P., S. E. Clark, and K. H. Baker. 2012. Nutrient release Acute toxicity and accumulation of iron, manganese and alumi-
from disturbance of infiltration system soils during construction. num in Caspian kutum fish (Rutilus kutum). Iranian Journal of
Advances in Civil Engineering 2012:393164. Toxicology 8(24):1028–1033.
UDFCD (Urban Drainage and Flood Control District). 2018.
Urban storm drainage criteria manual: Vol. 3, Stormwater quality.
Available at https://udfcd.org/volume-three (accessed Novem-
ber 8, 2018).

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Appendixes

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Appendix A

State Industrial Stormwater Permit

Benchmark Monitoring Comparison

Tables on following pages

89

Copyright National Academy of Sciences. All rights reserved.

 90

EPA 2015 Alaska 2015 California 2014 Connecticut 2016 Maryland 2014
Benchmark (BM) monitoring for BM monitoring for some BM monitoring for some BM monitoring for some BM monitoring for some
some sectors. sectors. sectors. Many facilities identify sectors. sectors.
additional site-specific
monitoring parameters.
Frequency: Quarterly. Frequency: Quarterly. Frequency: Twice every 6 Frequency: Once every 6 Frequency: Quarterly.
months. Compliance Group months.
participants monitor once every
6 months.
BM Monitoring Waiver: BM Monitoring Waiver: BM Monitoring Reduction: BM Monitoring Waiver: BM Monitoring Waiver:
Average four consecutive Average four consecutive After four consecutive results Average four consecutive Average four consecutive
results below BM. results below BM. with no numeric action level results below BM results below BM.
Natural background. Natural background. exceedances, reduce to once Natural background. Natural background.
No further pollutant reductions No further pollutant reductions every 6 months (once per year Run-on entering from off site. No further pollutant reductions
are technologically available are technologically available for Compliance Group). No further pollutant reductions are technologically available
and economically practicable and economically practicable are technologically available and economically practicable
and achievable, reduce to once and achievable, reduce to once and economically practicable and achievable, reduce to once
per year. per year. and achievable, reduce to once per year.
per year.
Additional Sectors Covered Additional Sectors Covered Additional Sectors Covered Additional Sectors Covered Additional Sectors Covered
(not in EPA MSGP): (not in EPA MSGP): (not in EPA MSGP): (not in EPA MSGP): Small- (not in EPA MSGP): School
N/A. A coal loading facility (Sector Preproduction plastics facilities scale composting facilities; bus maintenance facilities;
AD). which manufacture, handle, public works and Department Department of Public Works
or transport plastics including of Transportation garages; salt and highway maintenance
Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

resin pellets and color powder storage facilities. facilities, hydrodemolition, and
material. salt terminals.
Mandatory Baseline Monitoring Mandatory Baseline Monitoring Mandatory Baseline Monitoring Mandatory Baseline Monitoring Mandatory Baseline Monitoring
for All Sectors: None. for All Sectors: None. for All Sectors: Total suspended for All Sectors: Chemical for All Sectors: None.
solids (TSS), oil and grease, and oxygen demand (COD), TSS,
pH. oil and grease, pH, total
phosphorus, total nitrogen,
nitrate, copper, lead, zinc.
Infiltration: Permittees may Infiltration: Permittees may Infiltration: Permittees may Infiltration: Permittees may Infiltration: Permittees may
consider infiltration to minimize consider infiltration to minimize consider infiltration to minimize consider infiltration to minimize consider infiltration to minimize
pollutants in stormwater pollutants in stormwater pollutants in stormwater pollutants in stormwater pollutants in stormwater
discharge. discharge. discharge, with local municipal discharge. discharge.
government approval.

Copyright National Academy of Sciences. All rights reserved.



EPA 2015 Minnesota 2015 Rhode Island 2013 Washington 2015 West Virginia 2014 Wisconsin 2017
BM monitoring for some BM monitoring for all BM monitoring for some BM monitoring for some BM monitoring for some Monitoring for some
sectors. sectors. sectors. sectors. sectors. sectors. No benchmarks.

Frequency: Quarterly. Frequency: Quarterly. Frequency: Once every 6 Frequency: Quarterly. Frequency: Once per Frequency: Annual.
months. 6-month period (collected
at least 3 months apart).
BM Monitoring Waiver: BM Monitoring Waiver: BM Monitoring Waiver: BM Monitoring Waiver: BM Monitoring Waiver: BM Monitoring Waiver:
Average four consecutive Average four consecutive Average four consecutive Eight consecutive results Average four consecutive Facility inactive or
results below BM. results below BM. results below BM. below BM. results below BM. remote. Contamination off
Natural background. Natural background. Natural background. site and not associated
No further pollutant Run-on entering from No further pollutant with facility.
reductions are off site. Infiltration and reductions are
technologically available ponding waiver. technologically available
and economically and economically
practicable and practicable and
achievable, reduce to achievable, reduce to
once per year. once per year.
Additional Sectors Additional Sectors Additional Sectors Additional Sectors Additional Sectors Additional Sectors
Covered (not in EPA Covered (not in EPA Covered (not in EPA Covered (not in EPA Covered (not in EPA Covered (not in EPA
MSGP): N/A. MSGP): None. MSGP): None. MSGP): MSGP): Motorsports MSGP): None.
Puget Sound sediment racing complexes; shale
cleanup sites. mining only where the
shale mined is not used
Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

in manufacturing; salt
storage—limited to under
50,000 tons; transloading
facilities.

Mandatory Baseline Mandatory Baseline Mandatory Baseline Mandatory Baseline Mandatory Baseline Mandatory Baseline
Monitoring for All Monitoring for All Monitoring for All Monitoring for All Monitoring for All Monitoring for All
Sectors: None. Sectors: TSS. Sectors: None. Sectors: Turbidity, oil Sectors: None. Sectors: None.
sheen, pH, copper, and
zinc.
Infiltration: Permittees Infiltration: Specific Infiltration: Permittees Infiltration: Does Infiltration: All facilities Infiltration: Stormwater
may consider infiltration requirements must be may consider infiltration not cover facilities must have a groundwater infiltration excluded from
to minimize pollutants in met where used for a to minimize pollutants in who infiltrate all their protection plan. permit coverage.

Copyright National Academy of Sciences. All rights reserved.

stormwater discharge. BM monitoring waiver. stormwater discharge. stormwater. Stormwater infiltration
Prohibits new/expanded authorized unless
infiltration at five considered significant
subsectors based on risk sources of pollutants.
to groundwater.
91
Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

92 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

SOURCES Minnesota Pollution Control Agency. 2015. National Pollutant

Elimination System (NPDES)/State Disposal System (SDS) gen-
Alaska Department of Environmental Conservation. 2015. Multi- eral permit MNR050000 for industrial stormwater multi-sector
sector general permit for storm water discharges associated with (ISW). Authorization to discharge stormwater associated with
indus­trial stormwater (MSGP). General Permit No. AKR060000. industrial activity under the National Pollutant Discharge
Ashton, W., Alaska DEC, personal communication, 2018. Elimination System (NPDES)/State Disposal System (SDS)
Bertolacini, J., Wisconsin Department of Natural Resources, per- permit program. Available at https://www.pca.state.mn.us/sites/
sonal communication, 2018. default/files/wq-strm3-67a.pdf (accessed November 14, 2018).
Burch, P., West Virginia Department of Environmental Protection, Porter, T., Washington Department of Ecology, personal com-
personal communication, 2018. munication, 2018.
California Water Boards. 2018. Preproduction plastics debris program. Rhode Island Department of Environmental Management. 2013.
California Water Code, Division 7, Chapter 5.2, Section 13367. Multi-sector general permit Rhode Island pollutant discharge
Chatterton, M., Rhode Island Department of Environmental elimination system storm water discharge associated with indus­trial
Management, personal communication, 2018. activity (excluding construction activity). RIR500000.
Connecticut Department of Energy and Environmental Protection. State of Washington Department of Ecology. 2014. Industrial
2016. General permit or the discharge of stormwater associated with stormwater general permit: A National Pollutant Discharge Elimi-
industrial activity. DEEP-WPED-GP-014. nation System (NPDES) and state waste discharge general permit for
EPA (Environmental Protection Agency). 2015. United States stormwater discharges associated with industrial activities.
Environmental Protection Agency (EPA) National Pollutant State of West Virginia Department of Environmental Protection.
Discharge Elimination System (NPDES) Multi-Sector General 2014. General National Pollution Discharge Elimination System
Permit for stormwater discharges associated with industrial activity water pollution control permit. Permit no. WV0116025.
(MSGP). Available at https://www.epa.gov/sites/production/ Stone, C., Connecticut Department of Energy and Environmental
files/2015-10/documents/msgp2015_finalpermit.pdf (accessed Protection, personal communication, 2018.
November 1, 2018). Walddrip, L., California State Water Resources Control Board,
Gearheart, G., California State Water Resources Control Board, personal communication, 2018.
personal communication, 2018. Wenzel, M., Minnesota Pollution Control Agency, personal com-
Hlavinka, P., Maryland Department of the Environment, personal munication, 2018.
communication, 2018. Wisconsin Administrative Code NR [Natural Resources] 2.16.002.
Maryland Department of the Environment. General permit for Stormwater discharge permits: Definitions.
discharges from stormwater associated with industrial activities.
Discharge permit no. 12-SW, NPDES permit no. MDR0000.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Appendix B

Lists of Pollutants from Which Industries Self-Identified

the Need for Monitoring in the 1992 Group Applications,
Adapted from EPA Form 2F, 1992

TABLE 2F-2 
Conventional and Nonconventional Pollutants

Bromide
Chlorine
Total residual color
Fecal coliform fluoride nitrate/nitrite
Nitrogen
Total organic oil and grease
Phosphorus, total radioactivity
Sulfate/sulfite surfactants
Aluminum, total
Barium, total
Boron, total
Cobalt, total
Iron, total
Magnesium, total
Molybdenum, total
Manganese, total
Tin, total
Titanium, total

93

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

94 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE 2F-3
Toxic Pollutants

Toxic Pollutants and Total Phenol

Antimony, Total Copper, Total Silver, Total
Arsenic, Total Lead, Total Thallium, Total
Beryllium, Total Mercury, Total Zinc, Total
Cadmium, Total Nickel, Total Cyanide, Total
Chromium, Total Selenium, Total Phenols, Total
GC/MS Fraction Volatiles Compounds
Acrolein Dichlorobromomethane 1,1,2,2-Tetrachloroethane
Acrylonitrile 1,1-Dichloroethane Tetrachloroethylene
Benzene 1,2-Dichloroethane Toluene
Bromoform 1,1-Dichloroethylene 1,2-trans-Dichloroethylene
Carbon Tetrachloride 1,2-Dichloropropane 1,1,1-Trichloroethane
Chlorobenzene 1,3-Dichloropropylene 1,1,2-Trichloroethane
Chlorodibromomethane Ethylbenzene Trichloroethylene
Chloroethane Methyl bromide Vinyl chloride
2-Chloroethylvinyl ether Methyl chloride
Chloroform Methylene chloride
Acid Compounds
2-Chlorophenol 2,4-Dinitrophenol Pentachlorophenol
2,4-Dichlorophenol 2-Nitrophenol Phenol
2,4-Dimethylphenol 4-Nitrophenol 2,4,6-Trichlorophenol
4,6-Dinitro-O-Cresol p-Chloro-m-Cresol 2-Methyl-4,6-Dinitrophenol
Base/Neutral
Acenaphthene 2-Chloronaphthalene Fluorene
Acenaphthylene 4-Chlorophenyl phenyl ether Fluroranthene
Anthracene Chrysene Hexachlorobenzene
Benzidine Dibenzo[a,h]anthracene Hexachlorobutadiene
Benzo[a]anthracene 1,2-Dichlorobenzene Hexachloroethane
Benzo[a]pyrene 1,3-Dichlorobenzene lndeno[1,2,3-cd]pyrene
3,4-Benzofluoranthene 1,4-Dichlorobenzene Isophorone
Benzo[ghi]perylene 3,3-Dichlorobenzidine Napthalene
Benzo[k]fluoranthene Diethyl phthalate Nitrobenzene
Bis(2-chloroethoxy)methane Dimethyl phthalate N-Nitrosodimethylamine
Bis(2-chloroethyl)ether Di-N-butyl phthalate N-Nitrosodi-N-Propylamine
Bis(2-chloroisopropyl)ether 2,4-Dinitrotoluene N-Nitrosodiphenylamine
Bis(2-ethylyhexyl)phthalate 2,6-Dinitrotoluene Phenanthrene
4-Bromophenyl phenyl ether Di-N-octyphthalate Pyrene
Butylbenzyl phthalate 1,2-Diphenylhydrazine (as Azobenzene) 1,2,4-Trichlorobenzene
Pesticides
Aldrin Dieldrin PCB-1254
Alpha-BHC Alpha-Endosulfan PCB-1221
Beta-BHC Beta-Endosulfan PCB-1232
Gamma-BHC Endosulfan sulfate PCB-1248
Delta-BHC Endrin PCB-1260
Chlordane Endrin aldehyde PCB-1016
4,4’-DDT Heptachlor Toxaphene
4,4’-DDE Heptachlor epoxide
4,4’-DDD PCB-1242

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

APPENDIX B 95

TABLE 2F-4
Hazardous Substances

Toxic Pollutant
Asbestos
Hazardous Substances
Acetaldehyde Dinitrobenzene Napthenic acid
Allyl alcohol Diquat Nitrotoluene
Allyl chloride Disulfoton Parathion
Amyl acetate Diuron Phenolsulfonate
Aniline Epichlorohydrin Phosgene
Benzonitrile Ethion Propargite
Benzyl chloride Ethylene diamine Propylene oxide
Butyl acetate Ethylene dibromide Pyrethrins
Butylamine Formaldehyde Quinoline
Carbaryl Furfural Resorcinol
Carbofuran Guthion Strontium
Carbon disulfide Isoprene Strychnine
Chlorpyrifos Isopropanolamine Styrene
Coumaphos Kelthane Tetrachlorodiphenyl ethane
Cresol Kepone 2,4,5-TP [2-(2,4,5-
Crotonaldehyde Malathion  Trichlorophenoxy) propanoic acid]
Cyclohexane Mercaptodimethur Trichlorofan
2,4-D (2,4-Dichlorophenoxyacetic acid) Methoxychlor 2,4,5-Trichlorophenoxyacetic acid
Diazinon Methyl mercaptan Triethylamine
Dicamba Methyl methacrylate Trimethylamine
Dichlobenil Methyl parathion Uranium
Dichlone Mevinphos Vanadium
2,2-Dichloropropionic acid Mexacarbate Vinyl acetate
Dichlorvos Monoethyl amine Xylene
Diethyl amine Monomethyl amine Xylenol
Dimethyl amine Naled Zirconium

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Appendix C

Monitoring Parameters Required in Environmental

Protection Agency 2015 Multi-Sector General Permit

Parameter Sectors Benchmark ELG

Alpha-terpineol Hazardous waste (K), Landfills (L) 0.033–0.042 mg/L, daily
maximum; 0.016–0.019 mg/L,
monthly avg. maximum
Ammonia Hazardous waste (K, K1), Vehicle maintenance areas 2.14 mg/L (S) 10 mg/L, daily maximum; 4.9
and airports (S), Landfills (L) mg/L, monthly avg. maximum
(K, L); 14.7 mg/L as N, daily
maximum (S)
Aniline Hazardous waste (K) 0.024 mg/L, daily maximum;
0.015 mg/L, monthly avg.
maximum
Benzoic acid Hazardous waste (K), Landfills (L) 0.119–0.12 mg/L, daily
maximum; 0.071–0.073 mg/L,
monthly avg. maximum
Biochemical Hazardous waste (K), Landfills (L), Food (U2), Vehicle 30 mg/L (S, U2) 140–220 mg/L, daily
oxygen demand maintenance areas, Air transportation facilities (S) maximum; 37–56 mg/L,
(5 day) (BOD5) monthly avg. maximum (K, L)
Chemical oxygen Paper (B1), Timber (A1), Food (U2), Hazardous waste 120 mg/L
demand (COD) (K1), Metal mining (G1), Scrap and waste recycling
(N1), Timber (A4), Vehicle maintenance areas, Air
transportation facilities (S)
Fluoride Chemical and allied products (C) 75.0 mg/L, daily maximum;
25.0 mg/L, 30-day avg.
Napthalene Hazardous waste treatment (K) 0.059 mg/L, daily maximum;
0.022 mg/L, monthly avg.
maximum
Nitrate plus Chemical and allied products (C1, C2, C3), Fabricated 0.68 mg/L
nitrite nitrogen metals (AA1), Food (U2), Metal mining (G1), Mineral
mining (J1)
Oil and grease Asphalt paving and roofing (D) 15.0 mg/L, daily maximum; 10
mg/L, 30-day average
p-Cresol Hazardous waste treatment (K), Landfills (L) 0.024–0.025 mg/L, daily
maximum; 0.014–0.015 mg/L,
monthly avg. maximum
pH Metal mining (G2), Vehicle maintenance or deicing 6.0–9.0 s.u. 6.0–9.0 s.u.
at air transportation facilities (S), Asphalt paving (G2, S) (D, E, K, L, J, O, A)
(D), Grass, clay, cement, concrete, and gypsum (E),
Hazardous waste (K), Landfills (L), Mineral mining (J),
Electric power (O), Timber (A)
continued

97

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98 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Parameter Sectors Benchmark ELG

Phenol Hazardous waste treatment (K), Landfills and land 0.026–0.048 mg/L, daily
application sites (L) maximum; 0.015–0.029 mg/L,
monthly avg. maximum
Phosphorus Chemical and allied products (C1) 2.0 mg/L
Pyridine Hazardous waste treatment (K) 0.072 mg/L, daily maximum;
0.025 mg/L, monthly avg.
maximum
Total aluminum Automobile salvage yards (M1), Chemical and allied 0.75 mg/L
products (C2), Coal mines (H1), Fabricated metals
(AA1), Glass, clay, cement, concrete, and gypsum (E1),
Primary metals (F1, F2), Water transportation facilities
(Q1), Scrap recycling (N1)
Total antimony Metal mining (G2) 0.64 mg/L
Total arsenic Hazardous waste treatment (K, K1), Metal mining (G2), FW: 0.15 mg/L (K1, 1.1 mg/L, daily maximum;
Timber (A2) G2, A2) 0.54 mg/L, monthly average
SW: 0.069 mg/L maximum (K)
(K1, G2, A2)
Total beryllium Metal mining (G2) 0.13 mg/L
Total cadmium Hazardous waste treatment (K1), Metal mining (G2) SW: 0.04 mg/L
FW: hardness
dependent (dep.)
(0.0005 to 0.0053
mg/L)
Total chromium Hazardous waste treatment (K) 1.1 mg/L, daily maximum; 0.46
mg/L, monthly avg. maximum
Total copper Metal mining (G2), Primary metals (F2, F3, F4), Scrap SW: 0.0048 mg/L
recycling and waste recycling (N1), Timber (A2) FW: hardness dep.
(0.0038 to 0.0332
mg/L)
Total cyanide Hazardous waste treatment (K1) FW: 0.022 mg/L
SW: 0.001 mg/L
Total iron Automobile salvage yards (M1), Chemical and allied 1.0 mg/L
products (C1, C2), Coal mines (H1), Fabricated metals
(AA1), Glass, clay, cement, concrete, and gypsum (E2),
Landfills (L2), Metal mining (G2), Scrap recycling and
waste recycling (N1), Primary metals (F2), Electric
power (O1), Water transport facilities (Q1)
Total lead Automobile salvage yards (M1), Chemical and allied FW: hardness dep.
products (C1), Hazardous waste (K1), Metal mining (0.014 to 0.262)
(G2), Scrap recycling (N1), Water transportation SW: 0.21 mg/L
facilities (Q1)
Total magnesium Hazardous waste treatment (K1) 0.064 mg/L
Total mercury Hazardous waste treatment (K1), Metal mining (G2) FW: 0.0014 mg/L
SW: 0.0018 mg/L
Total nickel Metal mining (G2) FW: hardness dep.
(0.15 to 1.02 mg/L)
SW: 0.074 mg/L
Total phosphorus Chemical and allied products (C) 105.0 mg/L, daily maximum;
35 mg/L, 30-day avg.
Total selenium Hazardous waste treatment (K1), Metal mining (G2) FW: 0.005 mg/L
SW: 0.29 mg/L
Total silver Hazardous waste treatment (K1), Metal mining (G2) FW: hardness dep.
(0.0007 to 0.0183
mg/L)
SW: 0.0019 mg/L
continued

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

APPENDIX C 99

Parameter Sectors Benchmark ELG

Total suspended Timber (A), Asphalt paving and roofing (D), Glass, 100 mg/L 23.0–88 mg/L, daily maximum;
solids (TSS) clay, cement, concrete, and gypsum (E), Primary (A, D1, E2, F2, G, 15.0–50 mg/L, 30-day avg.
metals (F2), Metal mining (G1, G2), Coal mines H1, J, M1, N1, U) (D, E, J, K, L, O)
(H1), Mineral mining (J1, J2), Hazardous waste
treatment (K), Landfills and land application sites (L),
Automobile salvage yards (M1), Scrap recycling and
waste recycling (N1), Steam electric power (O), Food
(U1, U2)
Total zinc Hazardous waste treatment (K), Landfills and land FW: hardness 0.20–0.535 mg/L, daily
application sites (L), Timber (A1), Chemical and dep. (0.04 to 0.26 maximum; 0.11–0.296 mg/L,
allied products (C1, C3, C4), Fabricated metals mg/L) monthly avg. maximum (K, L)
(AA1), Metal mining (G2), Primary metals (F1, F2, SW: 0.09 mg/L
F3, F4), Scrap recycling and waste recycling (N1), (A1, C, AA1, G2, F,
Water transportation facilities (Q1), Rubber products N1, Q1, Y1)
manufacturing (Y1)
Turbidity Metal mining (G2) 50 NTU
Woody debris Timber (A) None >1 in.

NOTE: FW = freshwater; SW = saltwater.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Appendix D

2015 Multi-Sector General Permit (MSGP) Data Analysis

T
he committee obtained MSGP monitoring data compliance information to the NetDMR database. The
that have been reported in the Environmental period of record for reported results was from mid-2015
Protection Agency (EPA) Network Discharge through February 13, 2018. The data include more
Monitoring Report (NetDMR) database in response than 17,000 reported results from MSGP sites in the
to the monitoring and reporting requirements of the four states where EPA has primacy for the regulations
2015 MSGP (see Table 1-1). Stormwater samples are (Idaho, Massachusetts, New Hampshire, and New
collected by the permittees at stormwater outfalls. An Mexico), the District of Columbia, all U.S. territories,
individual facility may have multiple outfalls at a site at Indian country, and some federal facilities throughout
which samples are collected. The samples are analyzed the United States. The data analyzed by the committee
for sector-specific pollutants and any additional local are available upon request to the National Academies
requirements, typically by contract analytical laborato- Public Access Records Office.
ries, and the results are reported by the permittee using
NetDMR. Permittees certify the data to be accurate ANALYSIS METHODOLOGY
and maintain laboratory reports on file, which are avail-
able for review upon request and during site inspec- The outfall monitoring data were analyzed by pol-
tions. In addition to MSGP benchmark monitoring, lutant and sector or subsector (see Table D-1 for sector
local monitoring requirements are often prescribed to classifications). Standard industrial classification (SIC)
inform compliance with effluent limitation guidelines, codes were used to identify the appropriate sector or
local or state regulations, or development or implemen- subsector for each data point. Where SIC codes were
tation of total maximum daily loads (TMDLs; labeled lacking, other identifying information (e.g., “primary
“required monitoring” in the database). The committee permit SIC description”) was used to identify the
analyzed the data to assess the general extent to which appropriate sector or subsector. One SIC code (1021,
individual reported results were above the benchmarks copper ores) fell under both G1 and G2 and in these
and whether there are sectors or subsectors that have circumstances, the code was assigned G1.
a large percentage of facilities for which individual All results reported in the NetDMR database and
reported results exceed benchmark threshold values. delivered to the committee were used in the analysis,
The results of this data analysis are presented in this unless key data or identifying information was lacking.
appendix through a series of graphs and tables of Where reported results lacked SIC codes and the sec-
descriptive statistics, organized by pollutant. Summary tor could not be determined through other identifying
tables are provided in Chapter 2. information, those results were excluded from the
The data obtained from EPA represented sites that analysis. Results were also excluded in cases where no
were required under the 2015 MSGP to report their units were provided and where the units associated

101

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102 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE D-1
Industrial Sectors and Subsectors

Subsector Subsector Detail

A1 General sawmills and planing mills
A2 Wood preserving
A3 Log storage and handling
A4 Hardwood and wood product facilities; sawmills
B1 Paperboard mills
B2 Pulp and paper mills
C1 Agricultural chemicals
C2 Industrial inorganic chemicals
C3 Soaps, detergents, cosmetics, and perfumes

C4 Plastics, synthetics, and resins

C5 Industrial organic chemicals, paints, lacquers, pharmaceuticals
D1 Asphalt paving and roofing materials
D2 Miscellaneous products of petroleum and coal
E1 Clay product manufacturers
E2 Concrete and gypsum product manufacturers
E3 Glass and stone products
F1 Steel works, blast furnaces, and rolling and finishing mills
F2 Iron and steel foundries
F3 Rolling, drawing, and extruding of nonferrous metals
F4 Nonferrous foundries
F5 Smelting and refining of nonferrous metals, miscellaneous primary metal products
G1 Active copper ore mining and dressing facilities
G2 Active metal mining facilities
H Coal mines and related areas
I Oil and gas extraction facilities
J1 Sand and gravel mining
J2 Mining of dimension and crushed stone and nonmetallic minerals
J3 Clay, chemical, and fertilizer mineral mining
K1 Hazardous waste treatment storage, or disposal facilities
L1 Landfills, land application sites, and open dumps
L2 L1 except municipal solid waste landfill areas closed
M Automobile salvage yards
N1 Scrap recycling and waste recycling facilities
N2 Source separated recycling facilities
O Steam electric generating facilities
P Motor freight transportation facilities
Q Water transportation facilities
R Ship and boat building or repair yards
S Airports
T Treatment works
U1 Grain mill products
U2 Fats and oils products
continued

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

APPENDIX D 103

TABLE D-1
Continued

Subsector Subsector Detail

U3 Meat, dairy, and other food products and beverages
V Textile mills, apparel, and other fabric products
W Furniture and fixture manufacturing facilities
X Printing and publishing facilities
Y1 Rubber products manufacturing
Y2 Miscellaneous plastic products and manufacturing industries
Z Leather tanning and finishing facilities
AA1 Fabricated metal products, except coating
AA2 Fabricated metal coating and engraving
AB Transportation equipment, industrial, or commercial machinery manufacturing facilities
AC Electronic and electrical equipment and components, photographic, and optical goods manufacturing facilities

with the result could not be reasonably determined. data were labeled as “greater than,” the value used in the
The number of these excluded reported results are analysis was the value reported, which may represent
noted as footnotes to the tables that follow. The com- the upper limit of detection or a reporting error.
mittee excluded a few reported results that were several The committee performed several levels of veri-
orders of magnitude below known detection limits fication on this analysis. Three committee members
based on the current capabilities of chemical analysis. helped review the methodology, and this appendix
These exclusions are described in the pollutant-specific was reviewed by staff from the National Academies of
descriptions that follow. No high reported results were Sciences, Engineering, and Medicine’s Committee on
excluded because although some of the results appear National Statistics and one independent reviewer. The
suspect, it was not possible to associate the result with spreadsheets containing the calculations were reviewed
a reporting error with a high level of confidence. There in detail by National Academies’ staff to check for
could be additional reporting errors that are masked by errors. A few minor errors were detected that were dis-
the wide range of reported results. cussed with the committee and subsequently corrected.
The reported results for each pollutant were con- In the tables that follow, descriptive statistics are
verted to consistent units (e.g., mg/L, µg/L), based on presented for all sectors and subsectors with at least
Table 1-3. In the analysis, results that were labeled as one reported value (statistics generated from Excel),
“less than” a specific value (e.g., some form of analyti- including
cal detection limit) are analyzed as the value reported.
Therefore, “less than 0.01 µg/L” becomes 0.01 µg/L • The number of reported values,
for this analysis. In some cases the “less than” values • The minimum and maximum (e.g., the range of
reported were higher than the benchmark. For example, concentrations observed),
among the silver data reported, four of the reported • The median (to highlight the center value of the
results were higher than the hardness-specific bench- data), and
mark (including <20 µg/L and <25 µg/L). For the • The 75th percentile to show a relatively common
purposes of this analysis, those results were analyzed upper concentration.
and graphed as the value reported. Occurrences of
“less than” values exceeding benchmarks are noted in The data were also analyzed by subsector to calculate
footnotes in the tables where they occur. Similar to the the percentage of individual reported results that were
“less than” values, for the few reported values where the below the benchmark limit (or four or eight times

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104 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

the benchmark, consistent with suggested Addi- ANALYSIS LIMITATIONS

tional Implementation Measure [AIM] thresholds; see
Box 1-3). The data were not processed to determine The data set has the following limitations. No
whether data exceeding benchmarks warranted cor- information is known about the elements of the storm-
rective action in accordance with the permit, because water pollution prevention plan, including whether
that determination is based on the average of four structural stormwater control measures were operat-
quarterly monitoring values. For pollutants with a ing on site (and if so, whether those were designed or
benchmark threshold not dependent on hardness, all maintained appropriately) or whether the stormwater
reported results were analyzed. This includes data quality was managed using nonstructural activities,
(flagged “required monitoring” in the database) that such as good housekeeping and site sweeping. In addi-
may have been reported for other reasons, such as tion, no information is provided in the database on the
TMDLs. For the six metals where the benchmark hydrological characteristics of the site or the storm
threshold is dependent on receiving water hardness event, such as rainfall intensity or drainage area.
(cadmium, ­copper, lead, nickel, silver, and zinc), the In addition, it was apparent that the data set con-
reported results were compared to the facility-specific tained some errors related to reporting. For example,
hardness-based benchmark entered into the database. one reported concentration for copper was in the range
Reported results flagged as “required monitoring” that of 10−7 μg/L, which is not achievable given current
lacked information on a hardness-based benchmark or detection limits for instrumentation. Some very low or
permit limit or sufficient receiving water quality infor- high concentrations may have been the result of unit
mation to determine the appropriate benchmark were conversion errors.
excluded from the analysis. The number of excluded The data represent more than 2 years of data col-
reported results is noted in a footnote to each pollutant lection, including the early part of the MSGP permit
with hardness-dependent benchmarks. when all permittees are required to monitor and sample
The box-plot figures that follow illustrate the 25th, outfalls quarterly, at a minimum. If the average of four
50th, and 75th percentiles of the data, with the whiskers quarterly monitoring results meets the benchmark,
identifying the 10th and 90th percentiles. Data points those permittees are allowed to discontinue monitor-
outside of the 10th and 90th percentiles are shown as ing for the remainder of the permit cycle. Those that
individual data points. Graphs for each pollutant only did not meet the benchmark continued to sample for
include sectors with at least eight reported results. Fewer an additional year. Therefore, the results are likely to
than eight reported results were considered to be too few be biased toward the higher concentrations, based on
to provide a reasonable graphical representation of the those permittees that had to collect additional samples.
data range and where the data were primarily centered. Therefore, the data should not be used to determine
For pollutants with a single benchmark, the benchmark the percentage of permittees that had data exceeding
value as well as four and eight times the benchmark are benchmarks. The committee chose to analyze this
plotted for ease of comparison. For the six metals ­longer data set rather than only the first year of reported
where each sample benchmark is based on the receiv- results to capture more storms, given inherent variabil-
ing water hardness (cadmium, copper, lead, nickel, ity in stormwater quality (see Chapter 3). A primary
silver, and zinc), two benchmarks (plus eight times the objective of the analysis was to identify sectors and
benchmark) are presented for comparison based on pollutants with recurrent benchmark exceedances, and
generic hardness values—one representing soft water longer periods of reported results were helpful in this
(60 mg/L as CaCO3) and the other representing hard regard. For this 2-plus-year period, the effect would
water (200 mg/L CaCO3). The graphs include “required be less than it would be in past MSGP data analyses,
monitoring” reported results that were not included in which included data over either only year 2 or 4 of the
the comparison against hardness-related benchmarks. permit (Harcum et al., 2005) or up to 4 years of the per-
mit (EPA, 2012).

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

APPENDIX D 105

ANALYSIS RESULTS measured, the benchmark (750 μg/L) was achieved by

at least 50 percent of the reported results. The excep-
The following sections summarize the 2015 MSGP tion was Sector H (coal mines and coal-mine-related
reported results for individual pollutants, sorted by sec- facilities), which had no reported results that met
tor or subsector. Some of the pollutants (e.g., antimony, the benchmark and for which most reported results
cadmium, cyanide, mercury, nickel, selenium) had a exceeded a value eight times the benchmark. Among
relatively small data set, and some of the subsectors the sectors with at least eight reported results, Sectors
only represent a single facility. The largest data sets N (scrap recycling), P (motor freight transportation),
include aluminum, copper, iron, lead, total suspended Q (water transportation facilities), and R (ship build-
solids (TSS), and zinc. The results are summarized in ing) all had frequent individual results that exceeded
Chapter 2 and in Tables 2-3 and 2-4. the benchmark (>35 percent) and many (7–13 percent)
had very high values reported. The complete data set is
Aluminum summarized in Table D-2.

Figure D-1 shows the NetDMR 2015 MSGP data

for aluminum. For most sectors where aluminum was

FIGURE D-1
Aluminum results from NetDMR 2015 MSGP reported results through February 2018.
NOTES: Orange line denotes benchmark of 750 μg/L. Dashed purple lines represent four and eight times the benchmark.

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106 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE D-2
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Aluminum

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM
C2 89 5 54 274,000 270 540 19 7 3

C5 10 2 <100 1,150 178 310 10 0 0

D1 2 1 70 120 95 108 0 0 0

E2 2 1 1,900 2,100 2,000 2,050 100 0 0

F1 31 2 38 3,000 255 571 23 0 0

F2 52 4 <50 9,470 412 1,080 27 12 8

F3 6 2 37 270 50 73 0 0 0

H 22 3 3,410 315,000 26,700 99,975 100 100 95

J2 1 1 80 80 80 80 0 0 0

M 190 39 3 41,000 255 588 18 5 2

N1 318 53 13 130,000 649 2,290 46 21 13

O 2 1 <100 100 100 100 0 0 0

P 91 10 64 800,000 460 1,450 38 15 9

Q 577 66 <6 144,000 470 1,600 38 17 12

R 335 37 10 628,000 500 1,670 38 16 7

S 10 2 7.8 509 144 261 0 0 0

T 3 1 130 810 310 560 33 0 0

U3 2 1 690 9,670 5,180 7,425 50 50 50

V 4 2 <50 2,320 1,397 1,646 75 0 0

Y2 36 3 <50 920 50 153 3 0 0

AA1 387 35 26 46,100 292 1,100 30 8 4

AB 8 1 70 370 195 263 0 0 0

NOTE: Twenty-seven reported results were not included because they did not have units or the sector/subsector could not be identi-
fied (9 without units, 18 without sector/subsector information).

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APPENDIX D 107

Ammonia

Figure D-2 highlights the NetDMR 2015 MSGP

data for ammonia. In general, most reported results met
the benchmark (2.14 mg N/L). The complete data set
is summarized in Table D-3.

FIGURE D-2
Ammonia results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 2.14 mg N/L. Dashed purple lines represent four and eight times the benchmark.

TABLE D-3
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Ammonia

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (mg/L) (mg/L) (mg/L) (mg/L) >BM >4× BM >8× BM

C5 8 2 <0.05 1.3 0.16 0.28 0 0 0

K 78 7 0.06 <1,000 0.42 0.61 3a 1a 1a

L1 81 14 0.02 8.2 0.16 0.34 4 0 0

L2 1 1 0.04 0.04 0.04 0.04 0 0 0

N1 2 2 0.10 0.75 0.42 0.59 0 0 0

P 38 8 0.02 1.9 0.16 0.29 0 0 0

Q 4 1 0.10 0.45 0.24 0.39 0 0 0

S 11 2 0.09 1.6 0.42 0.66 0 0 0

AA1 2 1 0.05 3.5 1.8 2.6 50 0 0

AC 4 1 0.11 1.1 0.17 0.43 0 0 0

NOTE: Four reported results were not included because the sector/subsector could not be identified.
 aIncludes one reported result with reported detection limit exceeding the benchmark (2.14 mg N/L).

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108 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Antimony shows the results for the limited results reported for
Sectors G1 and G2, metal mining (ore mining and
Data were reported for antimony for only two sec- dressing), both of which were able to meet the bench-
tors and the data were not graphed because there were mark (640 µg/L).
fewer than eight reported results for each. Table D-4

TABLE D-4
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Antimony

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

G1 2 1 <7.5 <20 <14 <17 0 0 0

G2 4 2 <20 <500 <260 <500 0 0 0

Arsenic (150 µg/L) was achieved by greater than 75 percent of

the reported data. Sectors K (hazardous waste facilities)
Figure D-3 shows the NetDMR 2015 MSGP data and P (motor freight transportation) had one result
for arsenic. For all sectors where arsenic was measured and two results, respectively, that did not meet the
and a sufficient number of reported results were in the benchmark. The complete data set is summarized in
database to allow graphing, the freshwater benchmark Table D-5.

FIGURE D-3
Arsenic results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 150 μg/L. Dashed purple lines represent four and eight times the benchmark.

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APPENDIX D 109

TABLE D-5
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Arsenic

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

A2 34 3 <1 81.5 8.5 13 0 0 0

C3 4 1 <10 <10 <10 <10 0 0 0

C5 8 2 <2 <11 <6.5 <11 0 0 0

G1 4 1 <25 <25 <25 <25 0 0 0

G2 4 2 <25 <40 <32.5 <40 0 0 0

K1 88 7 <1 1,800 3.9 5.5 1 1 1

N1 1 1 <5 <5 <5 <5 0 0 0

O 3 1 <5 18 <5 12 0 0 0

P 21 8 0.5 <500 6.1 20 10a 0 0

R 5 1 <1 14 1.4 1.9 0 0 0

AA1 3 1 0.22 2.5 0.69 1.6 0 0 0

AC 2 1 <5 6 5.5 5.8 0 0 0

 a All exceedances were for reported results that had stated detection limits above the benchmark (150 μg/L).

Beryllium eight reported results for each. Table D-6 shows the
limited results for the data reported, which were able
Data were reported for beryllium for only Sectors
to meet the benchmark of 130 μg/L.
G1 and G2, metal mining (ore mining and dressing),
and were not graphed because there were fewer than

TABLE D-6
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Beryllium

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

G1 1 1 <2 <2 <2 <2 0 0 0

G2 4 2 <2 <100 <51 <100 0 0 0

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110 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Biochemical Oxygen Demand (5 day) (BOD5) transportation facilities), but 44 percent of reported
results in Sector S (airports) were not able to meet the
Figure D-4 highlights the NetDMR 2015 MSGP benchmark. The complete data set is summarized in
data for BOD5. Of the three sectors with at least eight Table D-7.
reported results, most of the data met the benchmark of
30 mg/L for Sectors L1 (landfills) and P (motor freight

FIGURE D-4
Five-day biochemical oxygen demand (BOD5) results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 30 mg/L. Dashed purple lines represent four and eight times the benchmark.

TABLE D-7
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for BOD5

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (mg/L) (mg/L) (mg/L) (mg/L) >BM >4× BM >8× BM

L1 68 12 <2 163 7.4 15 13 1 0

L2 1 1 78 78 78 78 100 0 0

N1 1 1 9 9 9 9 0 0 0

P 36 4 <0.0015 318 4 13 8 3 3

Q 1 1 13 13 13 13 0 0 0

S 18 2 2.9 322 20 61 44 11 6

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APPENDIX D 111

Cadmium specific hardness values. Of the sectors with at least

eight reported results, only Sector K (hazardous waste
Figure D-5 highlights the NetDMR 2015 MSGP facilities) exhibited benchmark exceedances (14 percent
data for cadmium. The cadmium benchmark value is of reported results).
determined by the hardness in the receiving water (see The figure and the descriptive statistics in Table D-8
Analysis Methodology). For the graphs, two cadmium include data collected for required monitoring that
benchmark values are presented—one representing did not include hardness-specific benchmarks. Data
soft water (60 mg/L as CaCO3; 1.3 μg/L Cd) and entries without a hardness-specific benchmark were not
one representing hard water (200 mg/L as CaCO3; included for the evaluation of the percentage of reported
4.5 μg/L Cd), although these are presented for visual- results that met benchmark thresholds.
ization purposes only. Benchmarks are based on site-

FIGURE D-5
Cadmium results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange lines denote the soft-water benchmark of 1.3 μg/L or eight times the benchmark and the purple lines denote the hard-
water benchmark of 4.5 μg/L or eight times the benchmark. Benchmark compliance is assessed based on site-specific water quality data.

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112 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

TABLE D-8
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Cadmium

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

C5 8 2 0.001 1 0.50 1 0 0 0

G1 2 1 2 2 2 2 0 0 0

G2 5 2 0.1 0.44 0.44 0.44 0 0 0

K 26 5 0.2 12 1 2 14a 7 0

N1 7 2 0.5 41 1 4.4 NA NA NA

O 2 1 5 5 5 5.0 NA NA NA

P 10 4 0.056 20 2 3.4 0 0 0

T 4 1 2 10 3.5 6.3 25a 0 0

AA1 3 1 0.00047 2.9 0.23 1.6 0 0 0

NOTES: NA, required monitoring for purpose other than MSGP benchmark compliance; no regulatory limit established for those sites.
Fifteen reported results were excluded from the hardness analysis because no regulatory limit was established for these sites. In ad-
dition to the sectors noted with NA above, the following were excluded from the analysis (the number of data points in parentheses):
G2(1), P(5).
 a Includes one reported result with stated detection limit exceeding the benchmark.

Chemical Oxygen Demand (COD) at least 50 percent of the reported results. The com-
plete data set is summarized in Table D-9. Sectors A2,
Figure D-6 highlights the NetDMR 2015 MSGP A4 (hardwood, sawmills), N1 (scrap recycling), and S
data for COD. Only Sector A2 (wood preserving) was (airports) reported 5 percent or more of data points in
unable to meet the COD benchmark of 120 mg/L for excess of four times the benchmark.

FIGURE D-6
Chemical oxygen demand (COD) results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 120 mg/L. Dashed purple lines represent four and eight times the benchmark.

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APPENDIX D 113

TABLE D-9
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for COD

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (mg/L) (mg/L) (mg/L) (mg/L) >BM >4× BM >8× BM

A1 220 28 3.6 740 37 109 21 2 0

A2 19 1 30 640 135 337 58 5 0

A4 66 9 9 711 79 176 36 5 0

B1 22 1 <5 80 22 29 0 0 0

C5 8 2 <5 103 17 66 0 0 0

G1 8 1 5.8 21 14 16 0 0 0

K 88 8 2.2 3,000 69 99 19 3 1

M 6 2 22 150 78 122 33 0 0

N1 303 52 <5 3,700 75 198 36 7 2

P 66 6 0.2 440 46 85 15 0 0

Q 4 2 14 147 23 56 25 0 0

S 18 3 20 625 57 240 33 11 0

AA1 36 2 22 164 47 59 8 0 0

NOTE: Eighteen reported results were not included because they did not have units.

Copper determined by the hardness in the receiving water.

For the graphs, two copper benchmark values are
Figure D-7 highlights the NetDMR 2015 MSGP presented—one representing soft water (60 mg/L
data for copper. The copper benchmark value is as CaCO3; 9 μg/L) and one representing hard water

FIGURE D-7
Copper results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange lines denote the soft-water benchmark of 1.3 μg/L or eight times the benchmark and the purple lines denote the hard-
water benchmark of 4.5 μg/L or eight times the benchmark. Benchmark exceedance is assessed based on site-specific water quality data.

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114 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

(200 mg/L as CaCO3; 28.5 μg/L). Two reported results the benchmark for at least 50 percent of the reported
were deleted from the analysis because their reported results. Most of these sectors (A2, F4, N1, Q, R, and
values were several orders of magnitude below the AA1) also had a large percentage of reported results (at
expected detection limit for copper. Of the sectors that least 25 percent) exceeding eight times the benchmark,
had at least eight reported results, many sectors (A2 and many results were reported that were orders of
[wood preserving], F2 [iron and steel foundries], F4 magnitude higher than this level.
[nonferrous foundries], M [auto salvage], N1 [scrap The figure and the descriptive statistics in Table D-10
recycling], Q [water transportation], R [ship building], include data collected for required monitoring that
AA1 [fabricated metal products]) were unable to meet did not include hardness-specific benchmarks. Data

TABLE D-10
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Copper

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

A2 32 3 15 888 129 213 97 84 81

A4 2 1 7 21 14 18 NA NA NA

B2 2 1 <2 8 5 6.5 NA NA NA

C4 1 1 22 22 22 22 NA NA NA

C5 6 3 6 57 18 22 NA NA NA

E2 2 2 20 42 31 37 NA NA NA

F2 35 4 <1 83 13 22 63a 9 0

F3 100 9 0.003 195 7.8 17 40b 22 14

F4 10 2 5 279 31 169 70 60 50

G1 2 1 24 67 45 56 50 0 0

G2 4 2 <3 9.5 6.2 9.5 25 0 0

M 11 2 0.016 64 24 36 82 9 0

N1 330 49 <0.001 4,380 19 62 62b 37a 26

O 4 1 18 86 30 52 NA NA NA

P 97 14 0.007 250 13 22 32c 1 1

Q 415 60 1 12,000 100 360 86d 73c 61b

R 335 38 0.01 796,000 219 615 96d 90c 81

S 22 1 0.0020 37 5.3 8.3 18 0 0

U3 27 6 2.8 357 11 31 NA NA NA

V 2 1 18 28 23 26 NA NA NA

Y2 1 1 17 17 17 17 NA NA NA

AA1 93 5 0.033 74,000 76 370 82 60 46

AC1 7 1 4 30 6 21.5 NA NA NA

NOTES: NA, required monitoring for purpose other than MSGP benchmark compliance; no regulatory limit established for those sites.
Nine reported results were not included because they did not have units; 82 reported results were excluded from the hardness-based
benchmark analysis because no regulatory limit was established for these sites. In addition to the sectors noted with NA above, the
following were excluded from the benchmark analysis: A4(2), B2(2), C4(1), C5(6), E2(2), N1(1), O(4), P(24), Q(1), R(1), U3(27), V(2),
Y2(1), AA1(1), and AC1(7).
 a Includes one result with reported detection limit exceeding the benchmark.
 b Includes two to four results with reported detection limit exceeding the benchmark.
 c Includes nine results with reported detection limit exceeding the benchmark.
 d Includes five to seven results with reported detection limit exceeding the benchmark.

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APPENDIX D 115

entries without a hardness-specific benchmark were not Cyanide

included for the evaluation of the percentage of reported
results that met benchmark thresholds. Figure D-8 highlights the NetDMR 2015 MSGP
data for cyanide. The sectors with sufficient reported
results to graph were able to meet the cyanide bench-
mark of 22 μg/L for all but one of the reported results in
the database (from Sector K, hazardous waste facility).
The complete data set is summarized in Table D-11.

FIGURE D-8
Cyanide results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 22 μg/L. Dashed purple lines represent four and eight times the benchmark.

TABLE D-11
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Cyanide

No. 75th
Reported No. Max. Median Percentile Percent Percent Percent
Results Facilities Min. (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

A4 1 1 1.9 1.9 1.9 1.9 0 0 0

B2 2 1 <1 <1 <1 <1 0 0 0

C3 4 1 <0.8 2.1 1.4 2.0 0 0 0

C5 11 3 <0.001 3.2 <1 1 0 0 0

E2 2 2 <1 2.4 1.7 2.1 0 0 0

J1 2 2 <2 <2 <2 <2 0 0 0

K 61 4 <0.00001 25 1.7 1.7 2 0 0

P 7 3 1 17 2.8 5.1 0 0 0

Q 1 1 1 1 1 1 0 0 0

U3 14 6 0.9 2.3 1 1.3 0 0 0

Y2 1 1 2.2 2.2 2.2 2.2 0 0 0

AC 4 2 <1 1.6 1 1.2 0 0 0

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116 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Iron P (motor freight transportation) exceeded the bench-

mark for ≥50 percent of the reported results. At least
Figure D-9 shows the NetDMR 2015 MSGP 10 percent of the reported results in Sectors C1 (agro-
data for iron. For sectors with at least eight reported chemicals), E2, H, L2, N1, Q (water transportation),
results, Sectors A1 (general sawmills), E2 (concrete and and S (airports) exceeded eight times the benchmark.
gypsum), E3 (glass and stone products), F2 (iron and About 95 percent of the reported results from Sector H
steel foundries), H (coal mines), L2 (landfills, exclud- exceeded eight times the benchmark. The complete
ing municipal solid waste), N1 (scrap recycling), and data set is summarized in Table D-12.

FIGURE D-9
Iron results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 1,000 μg/L. Dashed purple lines represent four and eight times the benchmark.

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APPENDIX D 117

TABLE D-12
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Iron

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM
A1 12 1 100 4,880 1,320 2,398 58 17 0
B2 1 1 <50 <50 <50 <50 0 0 0
C1 23 2 19 24,000 181 752 17 17 13
C2 84 5 25 39,100 280 776 21 5 1
C5 14 2 <50 2,940 240 750 21 0 0
D1 9 3 <50 7,290 380 1,220 33 11 0
E2 228 46 24 510,000 1,500 4,613 59 28 17
E3 8 2 27 3,600 994.5 1,700 50 0 0
F2 34 4 <50 15,400 1,165 2,950 53 24 9
F3 4 1 370 1,010 695 1,003 25 0 0
G1 1 1 <100 100 100 100 0 0 0
G2 4 2 <1,000 1,270 1,135 1,270 50 0 0
H 22 3 5,980 604,000 34,650 113,650 100 100 95
J1 9 2 190 540 280 340 0 0 0
J2 35 7 <50 9,100 250 905 20 11 3
L2 120 14 <50 92,000 1,300 3,493 59 22 17
M 196 39 <13 34,000 400 1,000 24 4 2
N1 329 53 13 164,000 2,360 5,800 69 31 18
O1 420 34 4 203,000 564 1,700 33 15 9
P 89 9 30 54,100 1,010 2,110 52 11 9
Q 626 70 5 126,000 800 2,983 45 20 12
R 345 37 13 881,000 656 2,420 41 17 9
S 19 3 14 45,000 842 2,575 42 16 16
U3 2 1 5,600 13,000 9,300 11,150 100 100 50
Y1 4 1 620 1,800 880 1,200 25 0 0
Y2 30 3 <50 1,600 110 289 7 0 0
AA1 430 35 <5 44,100 566 1,795 37 14 6

NOTE: Twenty-four reported results were not included because they did not have units.

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118 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Lead cent), with 7 percent exceeding eight times the bench-

mark, while Sector R (ship and boat building or repair
Figure D-10 highlights the NetDMR 2015 MSGP years) had about 25 percent exceedances compared to
data for lead. The lead benchmark value is determined the soft-water benchmark.
by the hardness in the receiving water. For the graphs, The figure and the descriptive statistics in
two lead benchmark values are presented—one repre- Table D-13 include data collected for required monitor-
senting soft water (60 mg/L as CaCO3; 45 μg/L) and ing that did not include hardness-specific benchmarks.
one representing hard water (200 mg/L as CaCO3; Data entries without a hardness-specific benchmark
213 μg/L). Of the sectors that had at least eight were not included for the evaluation of the percentage
reported results, Sector N1 (scrap recycling) had the of reported results that met benchmark thresholds.
largest percentage of benchmark exceedances (41 per-

FIGURE D-10
Lead results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange lines denote the soft-water benchmark of 45 μg/L or eight times the benchmark and the purple lines denote the hard-
water benchmark of 213 μg/L or eight times the benchmark. Benchmark exceedance is assessed based on site-specific water quality data.

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APPENDIX D 119

TABLE D-13
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Lead

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

C1 7 2 <3 10 <3 <10 0 0 0

C5 11 3 <2 57 <3 9 10 0 0

D1 2 1 <5 <8 <6.5 <7.2 NA NA NA

G1 2 1 <7.5 <7.5 <7.5 <7.5 0 0 0

G2 5 2 <4 44 44 44 0 0 0

K 28 5 <0.5 140 5 13 7 0 0

M 172 39 0.75 230 7.5 16 6 1 1

N1 311 51 0.6 18,000 28 77 41 16 7

O 2 1 <10 <10 <10 <10 NA NA NA

P 81 10 0.35 742 6 20 2 2 0

Q 410 57 0.08 2,467 15 29 4a 0 0

R 276 35 0.5 1,300 25 50 8b 1 0

S 6 1 0.22 3.5 0.47 2.0 0 0 0

T 9 2 2 54 19 29 0 0 0

U3 16 4 <0.5 <10 2 4.0 NA NA NA

AA1 6 1 0.37 3.6 1.5 2.9 0 0 0

AC 1 1 4 4 4 4 NA NA NA

NOTES: NA, required monitoring for purpose other than MSGP benchmark compliance; no regulatory limit established for those sites.
Nine reported results were not included because they did not have units; 67 reported results were excluded from the hardness-based
benchmark analysis because no regulatory limit was established for these sites. In addition to the sectors noted with NA above, the
following were excluded from the benchmark analysis (the number of reported results in parentheses): C5(1), D1(2), G2(1), M(2), N1(8),
O1(2), P(28), Q(1), T(5), U3(16), and AC1(1).
 a Includes one result with reported detection limit exceeding the benchmark.
 b Includes five results with reported detection limit exceeding the benchmark.

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Magnesium (hazardous waste facilities) exceeded the benchmark of

64 μg/L. A large proportion of these reported results
Figure D-11 shows the NetDMR 2015 MSGP also exceeded eight times the benchmark. The complete
data for magnesium. Only two sectors had at least data set is summarized in Table D-14. None of the
eight reported results for graphical analysis. Half of reported results submitted in either Sectors P (motor
the reported results in Sectors C5 (industrial organic freight transportation facilities) or AA1 (fabricated
chemicals) and all the reported results in Sector K metal) were able to meet the magnesium benchmark.

FIGURE D-11
Magnesium results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 64 μg/L. Dashed purple lines represent four and eight times the benchmark.

TABLE D-14
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Magnesium

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

C5 8 2 0.99 13,900 2,031 7,168 50 50 50

K 95 8 110 70,000 2,520 7,295 100a 98a 83a

P 5 1 300 16,000 2,600 11,000 100 100 80

AA1 3 1 505 1,590 1,410 1,500 100 100 67

 a Includes two results with reported detection limit exceeding the benchmark (and eight times the benchmark).

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APPENDIX D 121

Mercury >1,000 μg/L, three orders of magnitude greater than

other results in the database. One potential explanation
Figure D-12 shows the NetDMR 2015 MSGP for this is that the result was in micrograms per liter and
data for mercury. For sectors with at least eight reported was reported in milligrams per liter and was converted
results, Sectors K (hazardous waste facilities) and U3 for these purposes to micrograms per liter by multiply-
(food and beverage production) each exceeded the ing by 1,000. Because the analytical result is possible,
benchmark of 1.4 μg/L in only one reported result. The although highly unlikely, it was retained in the analysis.
plot for Sector U3 was skewed by a single result that was The complete data set is summarized in Table D-15.

FIGURE D-12
Mercury results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 1.4 μg/L. Dashed purple lines represent four and eight times the benchmark.

TABLE D-15
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Mercury

No. 75th
Reported No. Max. Median Percentile Percent Percent Percent
Results Facilities Min. (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

C3 4 1 <0.2 <4 <0.3 <1.3 25a 0 0

C5 8 2 <0.00001 0.06 <0.026 <0.05 0 0 0

G1 4 1 <0.2 <0.2 <0.2 <0.2 0 0 0

G2 24 3 <0.2 1 <1 <1 0 0 0

K 90 8 <0.002 <2 <0.067 0.12 1a 0 0

N1 5 3 <0.1 0.2 <0.2 <0.2 0 0 0

P 4 1 0.027 0.062 0.044 0.049 0 0 0

Q 4 1 0.06 0.29 0.095 0.17 0 0 0

R 5 1 <0.1 <0.15 <0.1 <0.1 0 0 0

U3 8 3 <0.05 5000 <0.05 0.10 13 13 13

NOTE: One result was not included because the sector/subsector could not be identified.
 a Includes one result with reported detection limit exceeding the benchmark.

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Nickel with hardness-specific benchmarks identified all met

the benchmarks. Even for the data entries that lacked
The NetDMR 2015 MSGP data for nickel are hardness-specific limits, all the reported results submit-
summarized in Table D-16. No sector had at least eight ted met both the soft-water benchmark of 320 μg/L
reported results for graphical analysis. Those facilities and the hard-water benchmark of 890 μg/L.

TABLE D-16
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Nickel

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

G1 1 1 <10 <10 <10 <10 0 0 0

G2 4 2 <10 <10 <10 <10 0 0 0

N1 1 1 13 13 13 13 NA NA NA

O1 2 1 <25 <25 <25 <25 NA NA NA

P 5 3 <25 <250 <25 <250 NA NA NA

NOTES: NA, required monitoring for purpose other than MSGP benchmark compliance; no regulatory limit established for those sites.
These eight reported results were excluded from the benchmark analysis.

Nitrite Plus Nitrate more than 50 percent of the reported results, although
Sectors C1, J2 (crushed stone), and AA2 (fabricated
Figure D-13 shows the NetDMR 2015 MSGP metal coating) exceeded four times the benchmark in
data for nitrite plus nitrate. For sectors with at least at least 10 percent of the reported results. The complete
eight reported results, only Sector C1 (agricultural data set is summarized in Table D-17.
chemicals) exceeded the benchmark of 0.68 mg/L in

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APPENDIX D 123

FIGURE D-13
Nitrite plus nitrate nitrogen results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 0.68 mg/L. Dashed purple lines represent four and eight times the benchmark.

TABLE D-17
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Nitrite Plus Nitrate Nitrogen

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (mg/L) (mg/L) (mg/L) (mg/L) >BM >4× BM >8× BM

C1 8 1 0.18 3.1 1.2 2.0 63 13 0

C2 62 3 <0.02 1.3 0.35 0.60 16 0 0

C3 42 3 0.081 1.8 0.48 0.5 10 0 0

C5 15 2 0.14 1.9 0.44 0.90 33 0 0

D1 11 4 0.02 0.9 0.5 0.5 9 0 0

E2 13 4 <0.02 0.89 0.09 0.16 15 0 0

F2 4 1 0.14 0.46 0.29 0.39 0 0 0

F3 4 1 <0.2 0.97 0.47 0.64 25 0 0

G1 8 1 0.21 1.9 0.43 0.88 38 0 0

J1 99 25 <0.02 7 0.35 0.89 29 4 2

J2 38 5 <0.05 24 <0.5 <0.5 21 13 11

P 3 2 <0.1 0.48 0.17 0.33 0 0 0

Q 31 2 0.021 0.94 0.35 0.52 13 0 0

Y2 26 2 0.089 0.8 0.5 0.5 12 0 0

AA1 357 30 <0.001 <68 0.5 0.68 21a 6b 2b

AA2 20 4 0.07 22 0.32 0.49 20 10 5

 a Includes seven results with reported detection limit exceeding the benchmark.
 b Includes one result with reported detection limit exceeding four and eight times the benchmark.

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pH percentage of reported results in each sector that were

below pH of 6.0 or above pH of 9.0. Sectors with at
Figure D-14 shows the NetDMR 2015 MSGP least eight reported results where ≥10 percent of the
data for pH. The pH benchmark is set based on main- data were outside the pH benchmark range were G1
taining an optimum range for water quality between (copper mining), L1 (landfills), and AB (transportation
pH 6 and 9. The complete data set is summarized in equipment).
Table D-18, including the last column which sums the

FIGURE D-14
pH results from NetDMR 2015 MSGP reported data through February 2018.
NOTE: Orange lines denote minimum and maximum of optimal pH range of 6.0–9.0.

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APPENDIX D 125

TABLE D-18
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for pH

Percent
No. Reported No. 75th Percent Percent Outside BM
Results Facilities Min. Max. Median Percentile <6 >9 Range

A1 78 6 5.9 7.8 7.1 7.3 4 0 4

A3 2 1 6.4 6.8 6.6 6.7 0 0 0

D1 21 8 4.2 8.4 7.8 7.8 5 0 5

E2 24 6 5.6 8.4 7.2 7.6 4 0 4

E3 1 1 7.8 7.8 7.8 7.8 0 0 0

F3 2 1 7.1 7.3 7.2 7.3 0 0 0

G1 10 1 7.8 9.1 7.8 9.1 0 40 40

G2 6 2 7.1 7.4 7.2 7.4 0 0 0

J1 44 8 7.0 <9 7.8 8.2 0 0 0

J2 104 20 5.5 <9 7.4 7.8 2 0 2

L1 118 14 1.6 10.1 7.2 7.6 10 2 12

L2 1 1 7.5 7.5 7.5 7.5 0 0 0

N1 14 6 6.1 7.6 7.1 7.4 0 0 0

O 4 1 6.8 7.2 7.0 7.2 0 0 0

P 57 10 5.8 8.9 7.2 7.4 2 0 2

Q 2 2 0.9 9.2 5.1 7.1 50 50 100

S 38 4 5.5 7.5 6.9 7.1 5 0 5

U3 22 7 6.6 9.4 7.8 8.1 0 9 9

V 4 2 5.9 6.7 6.3 6.5 25 0 25

Y2 7 2 6.4 8.1 7.7 7.9 0 0 0

AA1 2 1 8.3 8.9 8.6 8.7 0 0 0

AB 12 2 5.3 7.4 6.5 6.6 17 0 17

NOTE: Twenty-four reported results were not included because they did not have units or the sector/subsector could not be identified
(6 reported results without units; 18 without sector/subsector information).

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Phosphorus to have a sample that was an anomaly at 187 mg/L. No

other reported result in that sector exceeded 10 mg/L.
Figure D-15 shows the NetDMR 2015 MSGP However, because no information was available to
data for total phosphorus. For sectors with at least indicate that this was an incorrect entry, the reported
eight reported results, only Sector C1 (agricultural result was retained in the analysis. The complete data
chemicals) exceeded the benchmark of 2 mg/L in set is summarized in Table D-19.
≥10 percent of the reported results. Sector U3 seemed

FIGURE D-15
Total phosphorus results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 2 mg/L. Dashed purple lines represent four and eight times the benchmark.

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APPENDIX D 127

TABLE D-19
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Total Phosphorus

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (mg/L) (mg/L) (mg/L) (mg/L) >BM >4× BM >8× BM
A4 7 2 0.027 0.77 0.25 0.28 0 0 0
C1 24 2 0.064 3.3 0.85 1.4 13 0 0
C5 6 2 0.037 0.69 0.13 0.17 0 0 0
D1 16 5 0.091 1.2 0.14 0.38 0 0 0
E2 2 2 <0.05 1.2 0.64 0.94 0 0 0
J1 45 9 <0.05 0.8 0.12 0.17 0 0 0
J2 2 2 <0.05 0.29 0.17 0.23 0 0 0
N1 9 3 <0.1 0.29 0.14 0.22 0 0 0
O1 12 2 <0.1 1.9 0.38 1.2 0 0 0
P 25 8 0.06 2.6 0.27 0.92 4 0 0
S 16 5 0.06 0.65 0.23 0.36 0 0 0
T 6 1 0.07 2.6 0.33 0.42 17 0 0
U1 4 1 1.1 7.9 5.9 7.9 75 0 0
U3 42 8 0.012 187 0.62 0.89 7 2 2
AA1 15 2 0.03 0.6 0.07 0.19 0 0 0
AC 11 4 <0.01 0.5 0.13 0.20 0 0 0

NOTE: Two reported results were not included because they did not have subsector/sector information.

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128 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Selenium above the benchmark (5 μg/L) that were not assigned

a “less than” descriptor. All reported results for C5
Figure D-16 shows the NetDMR 2015 MSGP were below the stated detection limit, although some
data for selenium for sectors with at least eight reported of these detection limits were above the benchmark.
results. Overall, high reported detection levels make The complete data set is summarized in Table D-20.
the data more difficult to interpret. Only a few results
(7 out of 87) across all sectors were reported that were

FIGURE D-16
Selenium results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes benchmark of 5 μg/L. Dashed purple lines represent four and eight times the benchmark.

TABLE D-20
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Selenium

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM

A2 3 1 10 10 10 10 100 0 0

C5 8 2 <1 <11 <6 <11 50a 0 0

G2 4 2 <0.003 <5 4 5 0 0 0

J3 8 2 <0.5 6 <1 <2.2 25 0 0

K 60 6 0.61 100 <2 <5 18b 3 3

Q 1 1 <3 <3 <3 <3 0 0 0

T1 1 1 <6.1 <6.1 <6.1 <6.1 100a 0 0

U3 2 2 2 <3 2.5 2.8 0 0 0

 a All exceedances were for results that had reported detection limits above the benchmark of 5 μg/L.
 b Includes nine results with reported detection limit exceeding the benchmark.

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APPENDIX D 129

Silver one representing soft water (60 mg/L as CaCO3; 1.7

μg/L) and one representing hard water (200 mg/L
Figure D-17 highlights the NetDMR 2015 MSGP as CaCO3; 13.8 μg/L). All reported results noted as
data for silver. The benchmark value for silver is deter- exceeding the benchmark also were reported as “less
mined by the hardness in the receiving water. For the than” a detection level that was above the benchmark
graphs, two silver benchmark values are presented— (see Table D-21).

FIGURE D-17
Silver results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange lines denote the soft-water benchmark of 1.7 μg/L or eight times the benchmark and the purple lines denote the hard-
water benchmark of 13.8 μg/L or eight times the benchmark. Benchmark exceedance is assessed based on site-specific water quality data.

TABLE D-21
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Silver

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM
C5 8 2 <0.001 1 0.50 1 0 0 0

G1 1 1 <0.1 <0.1 <0.1 <0.1 0 0 0

G2 4 2 0.17 <0.3 0.24 <0.3 0 0 0

K 26 5 0.1 <20 <1 2.7 12a 4b 0

P 5 1 0.02 0.52 0.15 0.32 0 0 0

T 4 1 <5 <25 <9 <16 25b 0 0

 a Includes three results with reported detection limit exceeding the benchmark.
 b Includes one result with reported detection limit exceeding the benchmark.

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130 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Total Suspended Solids (TSS) results in Sectors A2 (wood preserving), E3 (glass

and stone products), and H (coal mines) exceeded the
Figure D-18 highlights the NetDMR 2015 MSGP benchmark, and more than 10 percent of the reported
data for total suspended solids compared to the bench- results in Sectors A2 and H exceeded eight times the
mark of 100 mg/L. Considering sectors with at least benchmark. The complete data set is summarized in
eight reported results, at least 60 percent of reported Table D-22.

FIGURE D-18
Total suspended solids results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes the benchmark of 100 mg/L. Dashed purple lines represent four and eight times the benchmark.

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APPENDIX D 131

TABLE D-22
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Total Suspended Solids

No. 75th
Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (mg/L) (mg/L) (mg/L) (mg/L) >BM >4× BM >8× BM

A1 374 28 0.4 2,680 23 90 22 7 3

A2 24 2 10 3,810 172 550 63 21 13

A3 53 10 <2 2,700 13 45 11 2 2

A4 74 9 <4 1,240 58 108 27 3 3

B2 4 1 6.4 63 23 44 0 0 0

D1 150 27 0.5 988 15 36 11 1 1

E2 269 46 0.66 16,000 51 150 35 14 7

E3 11 2 37 820 300 340 82 9 0

F2 27 4 4 553 7 19 11 4 0

G1 12 1 <5 529 17 36 17 8 0

G2 8 3 <5 100 6.5 54 0 0 0

H 22 3 80 25,200 2,250 4,730 95 77 55

J1 211 28 0.11 558 7 19 4 2 0

J2 428 35 0.19 5,000 15 49 14 6 3

L1 461 31 1 20,000 32 145 30 14 8

L2 1 1 1,510 1,510 1,510 1,510 100 0 0

M 185 39 0.63 1,200 15 29 6 1 1

N1 305 52 <2 5,140 35 93 24 7 2

O 6 2 <5 30 6 11 0 0 0

P 138 12 <2 1,400 16 54 18 7 3

Q 4 2 1.5 28 5.6 13 0 0 0

R 5 1 <1 4.8 2.6 2.9 0 0 0

S 20 5 <2 122 1.5 42 5 0 0

T 3 2 20 57 55 56 0 0 0

U1 37 7 9 486 76 149 38 3 0

U3 123 7 4 415 81 150 43 1 0

Y2 1 1 11 11 11 11 0 0 0

AA1 64 1 <4 70 5 19 0 0 0

AB1 3 1 5 30 7 19 0 0 0

AC1 6 2 <5 20 5.5 15 0 0 0

NOTE: Twenty reported results were not included because they did not have units or the sector/subsector could not be identified (19
results without units; 1 result without sector/subsector information).

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132 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Turbidity with at least eight reported results, only D1 (asphalt

paving) exceeded the benchmark in more than 25 per-
Figure D-19 highlights the NetDMR 2015 MSGP cent of the reported results. The complete data set is
data for turbidity compared to the benchmark of 50 summarized in Table D-23.
nephelometric turbidity units (NTU). Of the sectors

FIGURE D-19
Turbidity results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange line denotes the benchmark of 50 NTU. Dashed purple lines represent four and eight times the benchmark.

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APPENDIX D 133

TABLE D-23
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Turbidity

No.
­Reported No. Min. Max. Median 75th Percen- Percent Percent Percent
Results Facilities (NTU) (NTU) (NTU) tile (NTU) >BM >4× BM >8× BM
A2 3 1 9.4 27 11 19 0 0 0
A3 2 1 7.1 28 18 23 0 0 0
A4 4 1 0.39 77 13 32 25 0 0
B2 11 4 0.19 27 13 17 0 0 0
C5 5 2 4.3 17 4.9 11 0 0 0
D1 10 2 1.4 94 18 70 40 0 0
E2 4 3 <4 236 27 96 25 25 0
G1 1 1 12 12 12 12 0 0 0
G2 3 2 30 50 50 50 0 0 0
J2 2 1 2.2 2.3 2.3 2.3 0 0 0
J3 1 1 11 11 11 11 0 0 0
M 1 1 0.71 0.71 0.71 0.71 0 0 0
N1 8 3 1.1 33 16 28 0 0 0
O 12 2 3.9 28 17 26 0 0 0
P 10 5 3.3 37 10 32 0 0 0
Q 6 3 0.9 36 2 3.1 0 0 0
S 6 2 2.1 110 5.0 15 17 0 0
T 10 2 1.1 970 4.4 27 20 10 10
U3 29 8 0.79 240 12 42 17 3 0
AA1 4 2 0.83 12 7.1 11 0 0 0
AC 11 3 1.5 15 10 42 0 0 0

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134 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

Zinc for other data. However, for Sector Q, the reported

result is more than 10 times greater than the other data.
Figure D-20 highlights the NetDMR 2015 MSGP Again, there was insufficient information to remove
data for zinc. Zinc’s benchmark value is determined by these from the analysis, so they were retained. Of the
the hardness in the receiving water. For the graphs, two sectors with at least eight reported results, in Sectors
zinc benchmark values are presented—one representing C1 (agrochemicals), C4 (plastics), F1 (steel works), F2
soft water (60 mg/L as CaCO3; 80 μg/L) and one rep- (iron and steel foundries), F3 (rolling, drawing, and
resenting hard water (200 mg/L as CaCO3; 230 μg/L). extruding of nonferrous metals), N1 (scrap recycling),
Reviewing the data set, several reported results may Q (water transportation), Y1 (rubber products), AA1
be outliers because their reported values are below the (fabricated metal), and AA2 (fabricated metal coat-
detection limits of most common methods of analysis. ing), the reported values exceeded the hardness-specific
This is likely a transcription error. However, because benchmark for at least 50 percent of the reported
these values were within the limit of detection of results, and at least 10 percent of the reported results in
research instruments, the reported results were retained Sectors C1, C4, F3, F4 (nonferrous foundries), N1, and
in the analysis. There also were two results that were Y1 exceeded eight times the benchmark. The complete
reported as ≥10,000 μg/L (10 mg/L). The result in Sec- data set is summarized in Table D-24.
tor F3 does not appear to be outside the range reported

FIGURE D-20
Zinc results from NetDMR 2015 MSGP reported data through February 2018.
NOTES: Orange lines denote the soft-water benchmark of 80 μg/L or eight times the benchmark and the purple lines denote the hard-
water benchmark of 230 μg/L or eight times the benchmark. Benchmark compliance is assessed based on site-specific water quality data.

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APPENDIX D 135

TABLE D-24
Statistical Summary and Benchmark Comparison of 2015 MSGP Reported Results for Zinc

No. 75th
­Reported No. Min. Max. Median Percentile Percent Percent Percent
Results Facilities (μg/L) (μg/L) (μg/L) (μg/L) >BM >4× BM >8× BM
A1 210 28 1 1,630 36 87 35 7 1

C1 24 2 17 1,300 243 365 83 29 25

C3 60 4 15 3,210 72 132 48 17 7

C4 44 5 20 1,580 150 284 89 41 16

C5 1 1 675 675 675 675 100 100 100

F1 34 2 <20 650 159 200 91a 21 3

F2 36 4 <5 1,000 158 337 61 28 6

F3 111 9 <0.01 27,200 129 268 52 27 12

F4 10 2 <10 587 59 301 50 30 30

G1 2 1 <10 10 10 10 0 0 0

G2 5 2 <30 76 76 76 0 0 0

L1 80 14 0.00016 1,110 42 83 6 1 1

L2 1 1 312 312 312 312 100 0 0

M 6 2 24 160 63 85 0 0 0

N1 316 49 5.2 3,100 150 328 68 32 13

O 4 1 72 855 416 614 NA NA NA

P 132 11 0.14 1,600 41 107 11 3 0

Q 564 65 0.000033 102,000 89 253 48b 19 8

R 59 4 0.029 1,240 0.56 2.4 12 3 3

S 27 2 0.0096 619 51 165 33 8 0

Y1 64 5 14 2,200 110 290 91 38 23

Y2 34 3 21 992 61 110 68 15 9

AA1 374 35 0.019 3,380 97 235 57 19 7

AA2 24 4 10 5,200 85 165 29 13 8

NOTES: NA, required monitoring for purpose other than MSGP benchmark compliance; no regulatory limit established for those
sites. Fourteen reported results were not included because they did not have units. An additional four reported results from P1 were
­excluded from the hardness-based benchmark evaluation because no regulatory limit was established for those sites.
 a Includes three results with reported detection limit exceeding the benchmark.
 b Includes one result with reported detection limit exceeding the benchmark.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Appendix E

Additional Data on Technical Achievability of

Treatment Stormwater Control Measures

T
his appendix expands on the discussion of tech- for removing site-specific pollutants, especially metals,
nical achievability in Chapter 2. The results of from the stormwater runoff through physical strain-
stormwater control measure pollutant removal ing and potentially through adsorption and/or ion
performance for several additional pollutants—­ exchange. For consistency, label numbers for different
aluminum, copper, lead, zinc, and chemical oxygen pollutant data from the same site remain the same,
demand (COD)—are presented here to supplement even if not all sites monitored the same pollutants (e.g.,
the data on total suspended solids and iron presented in Site HDS 3 is HDS 3 for all pollutants, regardless of
Chapter 2. Some additional information on the Clark whether data for HDS 2 are available for all po­llutants).
and Pitt (in press) study design is also provided. All samples were flow-weighted composites and were
collected generally in accordance with the 2009 guid-
ADDITIONAL STUDY DETAILS ance for collecting data suitable for inclusion in the
International Stormwater BMP Database (Geosyntec
The Clark and Pitt (in press) studies were col- Consultants and Wright Water Engineers, 2009).
lected from various locations in the United States and The analysis of whether the treatment system was
represent Environmental Protection Agency Rainfall able to remove the pollutant of interest and whether it
Zones 1, 2, 3, 6, and 7. Seven studies were of devices was considered for inclusion in the graphical analysis
whose primary treatment mechanism was sedimenta- was tested statistically using the nonparametric analyses
tion, while six studies primarily relied on filtration for available in SigmaPlot/SigmaStat (Systat Software,
water quality treatment, and two were treatment trains Inc.). The selected test was the Wilcoxon signed-rank
that relied on sedimentation pretreatment prior to fil- test using a one-tailed analysis of whether effluent
tration. For the sedimentation systems, the devices were was less than the influent. Significance was assumed
categorized into three classifications: (1) hydrodynamic if the reported p value was ≤0.05. The signed-rank
separator (HDS) devices, (2) ponds, and (3) wetlands. test examines the pairs of data for difference. If the
The HDS devices studied were proprietary sediment Shapiro-Wilk test for normality was passed, then the
retention systems; HDS 4 is an inclined plate separator program defaulted to a paired t-test. The assumption of
and the remainder are traditional HDS swirl concen- nonparametric testing was used since stormwater data
trators. The two ponds were conventional dry ponds rarely are normally distributed (Burton and Pitt, 2002).
as described in most state stormwater manuals. The One committee member performed this analysis,
wetland also was designed similarly to the engineered which was then reviewed in detail by another commit-
wetlands described in most state manuals. For the fil- tee member to check for errors. Any errors identified
tration systems, the filtration media were proprietary were corrected.
combinations that had been optimized by the vendor

137

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138 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

ADDITIONAL POLLUTANT RESULTS sites. One benchmark (9 μg/L) is based on 60 mg/L

total hardness, shown in Figure E-3, and the other
Total Aluminum (28.5 μg/L) is based on 200 mg/L total hardness, shown
in Figure E-4. One of the three media filters and the
At the industrial study sites, only one proprietary treatment train were able to meet the lower soft-water
sedimentation system and one filtration system were benchmark for at least 50 percent of the storm events.
able to meet the benchmark concentration for more The treatment train was able to meet the benchmark
than 50 percent of the monitored storm events. For for >90 percent of the monitored events. For systems
the sedimentation system, this was also the site with with eight storm events with influent concentrations
the lowest influent concentrations, which may have above the hard-water benchmark, only the media filter
influenced the treatability (see Figure E-1). The Inter- was able to meet the hard-water benchmark for at least
national Stormwater BMP Database contained limited 50 percent of the events monitored.
data on ­aluminum and only for retention ponds, which For the International Stormwater BMP Database
were shown to meet the benchmark for between 25 (see Figures E-5 and E-6), all the treatment systems
and 50 percent of the monitored storm events (see except the dry detention pond were able to meet the
Figure E-2). soft-water benchmark of 9 μg/L for between 50 and
75 percent of the monitored storm events. For the
Total Copper hard-water benchmark, the dry detention pond and
media filter could meet the hard-water benchmark for
The copper benchmark is hardness dependent; between 50 and 75 percent of the monitored storm
therefore, two sample benchmarks are shown in Fig- events, while the wet retention pond and bioretention
ures E-3 and E-4 for the evaluation of technology system met the hard-water benchmark for between 75
performance at individual industrial storm­water study and 90 percent of the monitored storm events.

FIGURE E-1
Total aluminum influent versus effluent concentrations.
NOTES: DP = dry detention pond; HDS = hydrodynamic separator; MF = media filter; TT = treatment train. The number of storm-event
samples used in each analysis is shown on the graph below each treatment system.

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APPENDIX E 139

FIGURE E-2
International Stormwater BMP Database comparison of influent and effluent concentrations for total aluminum.
NOTE: RP = wet retention ponds.

FIGURE E-3
Total copper influent versus effluent concentrations compared to the soft-water benchmark concentration of 9 µ/L.
NOTES: DP = dry retention pond; HDS = hydrodynamic separator; MF = media filter; TT = treatment train. The number of storm-event
samples used in each analysis is shown on the graph below each treatment system.

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140 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

FIGURE E-4
Total copper influent versus effluent concentrations compared to the hard-water benchmark concentration of 28.5 µg/L.
NOTES: DP = dry retention pond; HDS = hydrodynamic separator; MF = media filter. The number of storm-event samples used in each
analysis is shown on the graph below each treatment system.

FIGURE E-5
International Stormwater BMP Database comparison of influent and effluent concentrations for total copper against the soft-water
benchmark of 9 µg/L.
NOTE: BR = bioretention; DP = dry detention ponds; MF = media filters; RP = wet retention ponds; WB = wetlands.

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APPENDIX E 141

FIGURE E-6
International Stormwater BMP Database comparison of influent and effluent concentrations for total copper against the hard-water
benchmark of 28.5 µg/L.
NOTE: BR = bioretention; DP = dry detention ponds; MF = media filters; RP = wet retention ponds; WB = wetlands.

Total Lead events. However, when comparing to the hard-water

benchmark, only the wet retention pond had sufficient
For lead, the benchmark is set based on the receiv- influent-effluent pairs where the influent concentra-
ing water hardness. None of the treatment systems was tion exceeded the hard-water benchmark of 213 μg/L
able to meet the soft-water benchmark of 45 µg/L for (see Figure E-10). The wet retention pond was able to
more than 50 percent of the monitored storm events meet the hard-water benchmark for >90 percent of the
(see Figure E-7). For the two systems with sufficient reported storm events.
data pairs where the influent concentration exceeded
the hard-water benchmark of 213 μg/L (see Fig-
Total Zinc
ure E-8), one dry detention pond and one treatment
train were able to meet the effluent hard-water bench- Of the three sedimentation devices, four filtra-
mark for between 75 and 90 percent of the monitored tion systems, and two treatment trains examined at
storm events. The improved performance when only industrial stormwater sites, only one treatment train
analyzing data where the influent exceeds the hard- had effluent concentrations that consistently met the
water benchmark likely is due to the strong association soft-water benchmark of 80 μg/L (>90 percent of the
of high concentrations of lead with particulate mat- monitored storm events; see Figure E-11). One sedi-
ter that easily settled or was filtered. However, since mentation device and three filtration devices were able
particle size analysis was not included, this cannot be to meet the soft-water benchmark for between 25 and
confirmed from the data set. 50 percent of the storm events. All other devices were
The International Stormwater BMP Database data unable to meet the benchmark for more than 10 percent
(see Figure E-9) highlight the ability of other catego- of the storm events.
ries of stormwater treatment systems to remove total Three sedimentation systems, one media filter, and
lead from the influent runoff. The dry pond was able one treatment train had influent concentrations that
to meet the soft-water benchmark for between 50 and exceeded the hard-water benchmark of 230 µg/L for
75 percent of the storm events monitored, while the eight or more storm events (see Figure E-12). For one
wet retention pond and media filter were able to meet of the three sedimentation systems and one treatment
the benchmark for >90 percent of the monitored storm

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142 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

FIGURE E-7
Total lead influent versus effluent concentrations comparison against the soft-water benchmark.
NOTES: DP = dry detention pond; TT = treatment train. The number of storm-event samples used in each analysis is shown on the graph
below each treatment system.

FIGURE E-8
Total lead influent versus effluent concentrations comparison against the hard-water benchmark.
NOTES: DP = dry detention pond; TT = treatment train. The number of storm-event samples used in each analysis is shown on the graph
below each treatment system.

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APPENDIX E 143

FIGURE E-9
International Stormwater BMP Database comparison of influent and effluent concentrations for total lead compared to the soft-water
benchmark.
NOTE: DP = dry detention ponds; MF = media filters; RP = wet retention ponds.

FIGURE E-10
International Stormwater BMP Database comparison of influent and effluent concentrations for total lead compared to the soft-water
benchmark.
NOTE: RP = wet retention ponds.

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144 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

FIGURE E-11
Total zinc influent versus effluent concentrations comparison to the soft-water benchmark.
NOTES: DP = dry detention pond; HDS = hydrodynamic separator; MF = media filter; TT = treatment train. The number of storm-event
samples used in each analysis is shown on the graph below each treatment system.

FIGURE E-12
Total zinc influent versus effluent concentrations comparison to the hard-water benchmark.
NOTES: HDS = hydrodynamic separator; MF = media filter; TT = treatment train. The number of storm-event samples used in each analysis
is shown on the graph below each treatment system.

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APPENDIX E 145

train, the systems’ performance met the hard-water Chemical Oxygen Demand (COD)
benchmark of 230 μg/L for between 50 and 75 per-
cent of the storm events. The other three systems (two Chemical oxygen demand has been used in the
sedimentation and one filtration) met the hard-water MSGP as a surrogate for other organic contaminants
benchmark for between 25 and 50 percent of the moni- such as hydraulic oils and organic chemicals. For the
tored storm events. individual site analysis, only one hydrodynamic separa-
The International Stormwater BMP Database tor and one treatment train had sufficient sample pairs
results (see Figure E-13) highlight the ability of with influent concentrations that exceeded the bench-
the five types of systems to remove total zinc from mark of 120 mg/L (see Figure E-15). The analysis
the influent runoff in comparison to the soft-water showed that the hydrodynamic separator was able to
benchmark. Four of the five systems examined (media meet the effluent benchmark concentration for less
filter, bioretention systems, wetland, and wet retention than 25 percent of the monitored events while the
ponds) were able to meet the soft-water benchmark of treatment train could meet the effluent benchmark
80 μg/L for >50 percent of the monitored storm events concentration for between 50 and 75 percent of the
with the media filter and bioretention system able to monitored storm events.
meet the benchmark for >90 percent of the monitored Data from the International Stormwater BMP
storm events. Database show that wet retention pond and bio­
Figure E-14 highlights the ability of the treatment retention systems were able to reduce COD effluent
systems to meet the hard-water benchmark of 230 µg/L concentrations to less than the benchmark for >90 per-
when their influent concentrations exceeded the hard- cent of the storm events (see Figure E-16). The dry
water benchmark. The wet retention pond, the media detention pond was able to reduce the influent COD
filters, and the bioretention system were able to meet concentrations to below the benchmark concentration
the hard-water benchmark for >90 percent of the storm for between 25 and 50 percent of the storm events
events monitored, while the detention pond and wet- monitored.
land were able to meet the hard-water benchmark for
between 50 and 75 percent of the storm events.

FIGURE E-13
International Stormwater BMP Database comparison of influent and effluent concentrations for total zinc compared to the soft-water
benchmark.
NOTE: BR = bioretention; DP = dry detention ponds; MF = media filters; RP = wet retention ponds; WB = wetlands.

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146 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

FIGURE E-14
International Stormwater BMP Database comparison of influent and effluent concentrations for total zinc compared to the hard-water
benchmark.
NOTE: BR = bioretention; DP = dry detention ponds; MF = media filters; RP = wet retention ponds; WB = wetlands.

FIGURE E-15
Chemical oxygen demand influent versus effluent concentrations.
NOTES: HDS = hydrodynamic separator; MF = media filter; TT = treatment train. The number of storm-event samples used in each analysis
is shown on the graph below each treatment system.

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APPENDIX E 147

FIGURE E-16
International Stormwater BMP Database comparison of influent and effluent concentrations for chemical oxygen demand.
NOTE: BR = bioretention; DP = dry detention ponds; MF = media filters; RP = wet retention ponds; WB = wetlands.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Appendix F

Biographical Sketches of Committee Members and Staff

Allen P. Davis, Chair, is professor of civil and envi- pollutants in stormwater, toxicity of stormwater pol-
ronmental engineering and Charles A. Irish, Sr. Chair lutants, effectiveness of different stormwater control
in Civil Engineering at the University of Maryland, practices, sources of stormwater pollutants, selection
­College Park. Dr. Davis’s interests are in aquatic and of cost-effective control practices, and benefits of low-
interfacial environmental chemistry. For two decades, he impact development. He has applied these results to
has been investigating sources and treatment of pollut- management plans developed for most urban areas in
ants in urban stormwater runoff with a focus on nature- Wisconsin. This includes the calibration of the urban
based practices, particularly bioretention. In 2010, he runoff model called the Source Loading and Manage-
was awarded the A. James Clark School of Engineer- ment Model. The results of his research projects have
ing Faculty Outstanding Research Award, recognizing been used to develop Wisconsin’s administrative rules
exceptionally influential research accomplishments that regulate stormwater management. Mr. Bannerman
related to urban stormwater quality, its management, received his B.S. degree in chemistry from Humboldt
and the concept of low-impact development. He is State College and an M.S. degree from the University
author or co-author of more than 120 peer-reviewed of Wisconsin in water chemistry.
journal articles and a text on stormwater management
for smart growth. From 2001 to 2010, he was director Shirley E. Clark is a professor of environmental engi-
of the Maryland Water Resources Research Center. neering at Penn State Harrisburg and chair of Penn
He is currently editor-in-chief of the new ASCE State Harrisburg’s graduate programs in environmental
Journal of Sustainable Water in the Built Environment. and civil engineering. Dr. Clark’s research has primarily
He is a Licensed Professional Engineer in Maryland, focused on improving the effectiveness of stormwater
Fellow of the American Society of Civil Engineers treatment systems. She has evaluated two manufac-
(ASCE), ­Fellow of the ASCE Environmental and tured treatment systems—inclined plate settlers and
Water Resources Institute, and a Diplomate, Water upflow filter systems—to document their performance
Resources Engineer. Dr. Davis holds B.S., M.C.E., and for the Environmental Protection Agency’s Environ-
Ph.D. degrees from the University of Delaware. mental Technology Verification Program. Her labora-
tory in mesocosm studies optimized bioretention media
Roger T. Bannerman worked as an environmental to treat stormwater runoff at Boeing’s Santa Susana
specialist for the Wisconsin Department of Natural facility, including determining media performance for
Resources for 41 years. For much of that time, he removing pollutants such as dioxin and radionuclides.
directed research projects investigating urban runoff. Her recent industrial stormwater research focused on
Topics addressed by his studies over the years include determining the performance of various treatment
the quality of urban streams, identification of problem systems (hydrodynamic separators, ponds, filters, and

149

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

150 IMPROVING THE EPA MULTI-SECTOR GENERAL PERMIT

chemical treatment systems) in operation at multiple stormwater permit compliance, management of con-
recycling facilities. Dr. Clark holds a B.S. degree in tracted operations to recycle captured aircraft deicing
chemical engineering from Washington University, an fluid, and planning and designing new infrastructure
M.S.C.E. degree in environmental engineering, and to support collection, storage, recycling, and disposal
a Ph.D. degree in environmental health engineering, of spent aircraft deicing fluid. Ms. Kieler also worked
both from the University of Alabama at Birmingham. for 6 years in environmental consulting. Ms. Kieler
earned her B.S. in environmental engineering from
L. Donald Duke is a professor of environmental ­studies Northwestern University.
at Florida Gulf Coast University. He has worked in
energy efficiency, water quality analyses, and storm- John D. Stark is a professor of ecotoxicology at the
water management for private consulting firms and Washington State University (WSU). Dr. Stark is also
served for 2 years in the total maximum daily load unit the director of the Washington Stormwater Center
of the California Water Board, Los ­Angeles region. and a member of the Puget Sound Partnership Sci-
Dr. Duke’s research interests are in water resources ence Panel. He also runs the WSU Salmon Toxicology
including water quality assessments of natural systems; Research Laboratory. Dr. Stark specializes in ecological
watershed-scale and regional-scale planning and man- risk assessment of threatened and endangered species
agement strategies; and federal, state, and local policies with particular emphasis on salmon and their food,
and programs for flood control. He applies quantitative and has conducted research on the effects of polluted
methods and engineering analyses to environmental stormwater runoff on salmon and aquatic invertebrate
data as a means to assess public policies with the intent health. He holds a B.S. degree in biology from Syracuse
to assess effectiveness of environmental policies and University, a B.S. degree in forest biology from SUNY
decision making. Dr. Duke has worked with various Environmental Science and Forestry School, an M.S.
federal, state, and local agencies on local and regional- degree in entomology from Louisiana State University,
scale management tools, including hazardous waste and a Ph.D. degree in entomology and pesticide toxi-
mitigation and stormwater compliance plans. Dr. Duke cology from the University of Hawaii.
earned his B.S. degree in civil engineering and B.A.
degree in English from the University of Pennsylvania, Michael K. Stenstrom is Distinguished Professor in the
and his M.S. and Ph.D. degrees from Stanford Univer- Civil and Environmental Engineering Department at
sity in civil and environmental engineering with a focus the University of California, Los Angeles. His research
on resources planning. and teaching are in the environmental engineering area
with emphasis on biological treatment methods and
Janet S. Kieler is the director of environmental pro- applications of computing technologies to environ-
grams for Denver International Airport. In this role, mental engineering research. Over the past 15 years,
Ms. Kieler is responsible for directing environmental Dr. Stenstrom has performed research to characterize
compliance and performance including environmental stormwater and minimize its impacts on the environ-
planning and analysis related to air quality, water qual- ment. Dr. Stenstrom’s expertise is in process devel­
ity, waste, wetlands, and endangered species. Previously, opment for stormwater management and wastewater
Ms. Kieler served for 11 years as the permits section treatment systems, including mathematical model­ing
manager for the Water Quality Control Division and optimization. He applies these mathematical tech-
of the Colorado Department of Public Health and niques along with statistical methods to urban runoff
Environment, where she oversaw the issuance of state and stormwater issues. Through his research, he has
and National Pollutant Discharge Elimination Sys- developed several models for estimating pollutant dis-
tem permit actions, compliance monitoring through charges in stormwater runoff. Dr. Stenstrom received
field inspection and review of self-reported data, data his B.S. in electrical and computer engineering and his
management, and business processes. Ms. Kieler also M.S. and Ph.D. in environmental systems engineering
previously worked for the Denver International Airport from Clemson University.
for 8 years, where she was responsible for industrial

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

APPENDIX F 151

Xavier Swamikannu is an assistant adjunct professor STAFF

at the Institute of Environment and Sustainability at
the University of California, Los Angeles (UCLA). Stephanie E. Johnson, study director, is a senior
He previously worked for more than 20 years at the program officer with the Water Science and Technol-
California Water Board, Los Angeles region, and ogy Board. Since joining the National Academies in
served as its chief of stormwater programs, partnering 2002, she has worked on a wide range of water-related
with UCLA faculty to fund research and bring science ­studies, on topics such as desalination, wastewater
into public decision making. His research interests reuse, contaminant source remediation, coal and
include the progress of regulatory policy for water uranium mining, coastal risk reduction, and eco­
quality protection, its implementation, and effective- system restoration. Dr. Johnson received her B.A. from
ness in the United States and California. Areas of focus Vanderbilt University in chemistry and geology, and
include the potential water quality impacts of hydraulic her M.S. and Ph.D. in environmental sciences from
fracturing, eliminating barriers to the implementa- the University of Virginia.
tion of green infrastructure, better understanding of
the effectiveness of stormwater control measures, and Carly Brody is a senior program assistant for the
standardizing water quality modeling methods for use National Academies’ Water Science and Technology
by local governments in surface water pollution con- Board and the Board on Earth Sciences and Resources.
trol planning. Dr. Swamikannu was a U.S. Fulbright Prior to joining the National Academies in 2017, she
Senior Environmental Leadership Fellow at the Gov- interned with the Center for Transboundary Water
ernment of India’s Central Pollution Control Board. Management at the Arava Institute for Environmental
He received his B.S degree in natural and chemical Studies. She received a B.A. in environmental science
sciences from St. Joseph’s College in Bangalore, India, and policy and American studies from the University
his M.S. degree in environmental sciences from Texas of Maryland, College Park.
­Christian University, and his doctorate degree (D.Env.)
in environmental science and engineering from UCLA.

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Copyright National Academy of Sciences. All rights reserved.

Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Appendix G

Disclosure of Conflict of Interest

T
he conflict of interest policy of the National was established, its membership must include at least
­Academies of Sciences, Engineering, and Medi- one person with current experience in, and knowledge
cine (http://www.nationalacademies.org/coi) of, statistical and numerical methods in the analyses
prohibits the appointment of an individual to a com- of industrial stormwater data. As described in his
mittee authoring a Consensus Study Report if the indi- biographical summary, Dr. Stenstrom has extensive
vidual has a conflict of interest that is relevant to the current experience developing models to estimate pol-
task to be performed. An exception to this prohibition lutant discharges in stormwater runoff, and in applying
is permitted if the National Academies determines that mathematical modeling and statistical methods to the
the conflict is unavoidable and the conflict is publicly analysis of urban and industrial stormwater data. 
disclosed. A determination of a conflict of interest for The National Academies determined that the
an individual is not an assessment of that individual’s experience and expertise of Dr. Stenstrom was needed
actual behavior or character or ability to act objectively for the committee to accomplish the task for which it
despite the conflicting interest.  has been established. The National Academies could
Michael Stenstrom was determined to have a not find another available individual with the equiva-
conflict of interest in relation to his service on the lent experience and expertise who does not have a
Committee on Improving the Next-Generation EPA conflict of interest. Therefore, the National Academies
Multi-Sector General Permit for Industrial Storm­ concluded that the conflict was unavoidable. 
water Discharges because he serves on the Santa Susana The National Academies believed that Dr. Stenstrom
Stormwater Expert Panel, a committee constituted would serve effectively as a member of the committee,
to provide guidance to Boeing and the Los Angeles and the committee can produce an objective report,
Regional Water Quality Control Board on stormwater taking into account the composition of the committee,
management at the Santa Susana site.  the work to be performed, and the procedures to be
The National Academies concluded that in order followed in completing the study.
for the committee to accomplish the tasks for which it

153

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Improving the EPA Multi-Sector General Permit for Industrial Stormwater Discharges

Copyright National Academy of Sciences. All rights reserved.