UC Agriculture & Natural Resources

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Title
Groundwater Quality and Groundwater Pollution

Permalink
https://escholarship.org/uc/item/0vw7400h

Author
Harter, Thomas

Publication Date
2003

DOI
10.3733/ucanr.8084

Peer reviewed

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University of California
 PUBLICATION 8084 FWQP REFERENCE SHEET 11.2

Reference:
Groundwater Quality and
Groundwater Pollution
THOMAS HARTER is UC Cooperative Extension Hydrogeology Specialist, University of
California, Davis, and Kearney Agricultural Center.

UNIVERSITY OF
Groundwater quality comprises the physical, chemical, and biological qualities of
ground water. Temperature, turbidity, color, taste, and odor make up the list of physi-
CALIFORNIA
cal water quality parameters. Since most ground water is colorless, odorless, and
Division of Agriculture without specific taste, we are typically most concerned with its chemical and biologi-
and Natural Resources cal qualities. Although spring water or groundwater products are often sold as “pure,”
http://anrcatalog.ucdavis.edu their water quality is different from that of pure water.

In partnership with
Naturally, ground water contains mineral ions. These ions slowly dissolve from
soil particles, sediments, and rocks as the water travels along mineral surfaces in the
pores or fractures of the unsaturated zone and the aquifer. They are referred to as dis-
solved solids. Some dissolved solids may have originated in the precipitation water or
river water that recharges the aquifer.
A list of the dissolved solids in any water is long, but it can be divided into three
groups: major constituents, minor constituents, and trace elements (Table 1). The
http://www.nrcs.usda.gov total mass of dissolved constituents is referred to as the total dissolved solids (TDS)
concentration. In water, all of the dissolved solids are either positively charged ions
Farm Water (cations) or negatively charged ions (anions). The total negative charge of the anions
always equals the total positive charge of the cations. A higher TDS means that there
Quality Planning
A Water Quality and
Technical Assistance Program Table 1. Primary (major), secondary, and trace constituents in natural ground water.
for California Agriculture
Major Secondary Trace Trace
http://waterquality.ucanr.org
constituents constituents constituents constituents
(1.0 – 1,000 mg/l) (0.01 – 10 mg/l) (0.0001 – 0.1 mg/l) (less than 0.001 mg/l)
This REFERENCE SHEET is part of
the Farm Water Quality antimony beryllium
Planning (FWQP) series, aluminum bismuth
developed for a short course cations:______________________________ arsenic cerium
that provides training for sodium potassium barium cesium
growers of irrigated crops who calcium iron bromide gallium
are interested in implementing magnesium strontium cadmium gold
water quality protection chromium indium
practices. The short course cobalt lanthanum
teaches the basic concepts of anions:______________________________ copper niobium
watersheds, nonpoint source bicarbonate carbonate germanium platinum
pollution (NPS), self-assessment sulfate nitrate iodide radium
techniques, and evaluation chloride fluoride lead ruthenium
techniques. Management goals silica boron lithium scandium
and practices are presented for manganese silver
a variety of cropping systems. molybdenum thallium
nickel thorium
phosphate tin
rubidium tungsten
selenium ytterbium
titanium yttrium
uranium zirconium
vanadium
zinc
ANR Publication 8084 2

are more cations and anions in the water. With more ions in the
CONVERSIONS water, the water’s electrical conductivity (EC) increases. By measur-
All concentrations are measured in: ing the water’s electrical conductivity, we can indirectly determine its
TDS concentration. At a high TDS concentration, water becomes
milligrams per liter (mg/l) saline. Water with a TDS above 500 mg/l is not recommended for
micrograms per liter (µg/l) use as drinking water (EPA secondary drinking water guidelines).
Water with a TDS above 1,500 to 2,600 mg/l (EC greater than 2.25
To convert between the two: to 4 mmho/cm) is generally considered problematic for irrigation
1,000 µg/l = 1 mg/l use on crops with low or medium salt tolerance.
Also used to measure concentration Except for natural organic matter originating from topsoils,
are the units parts per billion (ppb) all of these naturally occurring dissolved solids are inorganic con-
and parts per million (ppm). For stituents: minerals, nutrients, and trace elements, including trace
general purposes, one can use the metals. In most cases, trace elements occur in such low concentra-
following very simple conversion: tions that they are not a threat to human health. In fact many of
the trace elements are considered essential for the human metabo-
1 ppb ≈ 1 µg/l lism. In Europe, water from springs and wells with certain levels of
1 ppm ≈ 1 mg/l trace elements has long been considered a remedy for ailments.
Popular health spas usually are located near such areas. High con-
To convert TDS to electrical conduc- centrations of trace metals can also be found in ground water near
tivity (a measure of salinity): contaminated sources, however, posing serious health threats.
The TDS concentration in mg/l is Some trace constituents that are associated with industrial pollu-
approximately 65 percent of the tion, such as arsenic and chromium, may also occur in completely
electrical conductivity value in µS/cm pristine ground water at concentrations that are high enough to
or in µmho/cm. For example: make that water unsuitable as drinking water.
65 mg/l ≈ 100 µmho/cm Microbial matter is also a natural constituent of ground water.
130 mg/l ≈ 200 µmho/cm Just as microbes are ubiquitous in the environment around us, they
are very common in the subsurface, including ground water.
260 mg/l ≈ 400 µmho/cm
Hydrogeologists increasingly rely on these, for instance, for subsur-
520 mg/l ≈ 800 µmho/cm face bioremediation of contaminated ground water.
650 mg/l ≈ 1 mmho/cm Human activities can alter the natural composition of ground
water through the disposal or dissemination of chemicals and
microbial matter at the land surface and into soils, or through
injection of wastes directly into ground water. Groundwater pollu-
tion (or groundwater contamination) is defined as an undesirable change in groundwa-
ter quality resulting from human activities (for more information on nonpoint source
pollution from agricultural activities, see Nonpoint Sources of Pollution in Irrigated
Agriculture [UC ANR Publication 8055]).
The EPA’s drinking water program (http://www.epa.gov/ogwdw/) defines accept-
able levels of both inorganic and organic groundwater constituents and of microbial
matter, as well as other groundwater quality factors.
Groundwater pollution works differently from surface water pollution, although
they have many sources in common, such as fertilizers, pesticides, and animal wastes.
Several important concepts should be kept in mind:
1. Unlike surface water, ground water does not typically flow toward a single
outlet at the topographic bottom of the watershed, where the cumulative effect
of watershed pollution and of improvements in watershed management can be
directly measured. Groundwater discharge depends on topography (mountain-
ous, hilly, or flat), hydrogeology (confined or unconfined aquifers, fractured
rock or sediments, aquifer geometry), the sources of groundwater recharge
ANR Publication 8084 3

(precipitation; percolation of irrigation water; seepage from streams, lakes, and

canals), and the amount of ground water pumped in wells. Groundwater dis-
charge may be exclusive to wells that are distributed throughout a given
watershed or groundwater basin, or it may discharge to down-gradient stream
segments and lakes, springs, or (via the underground) directly to neighboring
groundwater basins.
Of most interest to us is the effect of farm practices on groundwater pollution
in wells (irrigation wells, domestic wells, municipal wells) and on groundwa-
ter quality in seepage to streams. As we consider the link between groundwa-
ter sources and groundwater uses, we can look at this in two ways:
• Farmer’s view. From the point of view of a farmer whose water is percolating
from the farm into ground water, the main concern is one of stewardship,
protection of water quality in the percolating waters that recharge the
ground water. This water will travel to many destinations (wells, streams,
and lakes) nearby and, potentially, far away, and may impact many different
users (domestic water users, irrigation water users, stream water users).
• User’s view. From the perspective of water users (including farmers), the
main concerns are about assessment and protection: the quality of water in
the well, spring, or stream, and who it is whose activities have an impact on
it. Ground water that is pumped from someone’s well or ground water dis-
charging into springs and streams may have originated nearby or may have
traveled several miles, or it may be a mix of both. It generally has many
sources. A careful, professional hydrogeologic assessment is typically neces-
sary to determine the extent of the exact source area of these groundwater
discharges. Depending on the depth and pumping rate of the well and
depending on local hydrogeologic conditions, the size of the source area
may range from a few acres to many tens of square miles, and it often
includes many potential nonpoint and point sources of groundwater pollu-
tion (see the California Department of Health Services Drinking Water Source
Assessment and Protection Program
(http://www.dhs.ca.gov/ps/ddwem/dwsap/DWSAPindex.htm).
2. The most prevalent forms of groundwater pollution from nonpoint sources are
salt and nitrate contamination, which adversely affect approximately 10 to 15
percent of California’s water wells, followed by pesticide and industrial contam-
ination. Pathogens are also frequently detected in ground water. These contami-
nants, often associated with septic systems and animal wastes, are transported by
water percolating from the soil to the water table, where they enter the ground
water. The degree of groundwater pollution depends on a number of factors:
• NPS sources. The number and intensity or strength of NPS pollution activi-
ties within the source area of a well or a spring. A large number of low-
grade NPS pollution sources may have a cumulative effect similar to that of
a few more-intense NPS pollution sources.
• Percolation rate. The rate of percolation from the land surface to ground
water. A significant amount of chemicals or pathogens may reach ground
water when the water percolation rate is high.
• Natural attenuation. The ability of the soil or aquifer to retain or degrade the
chemical before it reaches a well, spring, stream, or lake. The more a chemi-
cal is degraded or retained in the subsurface, the less likely it will be to
reach a nearby well or stream.
ANR Publication 8084 4

3. Groundwater pollution occurs on a different time scale than surface water pol-
lution. Ground water naturally flows at a speed that may range from a few
tens of feet per year in poorly producing aquifers to a few thousand feet per
year in very productive aquifers. In very sandy or gravelly aquifers and in
some highly porous or cavernous volcanic and karstic aquifers, groundwater
speed may be 10,000 feet (roughly 2 miles) per year or more. Nonpoint source
pollution therefore does not typically appear in domestic or irrigation wells
until years or even decades after it was released from its source. By the same
token, groundwater cleanup takes a matter of years or decades, during which
expensive treatment methods must be applied to render the ground water use-
able, especially if it is used as drinking water. The large time spans involved in
groundwater pollutant transport must be kept in mind when assessing ground-
water pollution sources and defining monitoring programs.
4. As with surface water pollution, there is a large natural variability in the gen-
eration and fate of groundwater NPS pollution. As a result, concentrations of
NPS pollutants typically vary significantly from well to well, making it difficult
to delineate the contamination plumes.
Whether contaminant concentrations change quickly over time is a function of
the well type. Large production wells (municipal wells or irrigation wells) mix water
from a large cross-section of the aquifer and concentrations stay relatively stable over
periods of months to years. Small domestic wells and monitoring wells that intersect
only short thicknesses of an aquifer may vary significantly in pollutant concentrations
over a period of less than 6 months, particularly if a strong but variable source is
nearby.
ANR Publication 8084 5

F O R M O R E I N F O R M AT I O N
You’ll find detailed information on many aspects of field crop production and
resource conservation in these titles and in other publications, slide sets, CD-
ROMs, and videos from UC ANR:
Nutrients and Water Quality, slide set 90/104
Protecting Groundwater Quality in Citrus Production, publication 21521
Sediments and Water Quality, slide set 91/102
To order these products, visit our online catalog at http://anrcatalog.ucdavis.edu.
You can also place orders by mail, phone, or FAX, or request a printed catalog of
publications, slide sets, CD-ROMs, and videos from
University of California
Agriculture and Natural Resources
Communication Services
6701 San Pablo Avenue, 2nd Floor
Oakland, California 94608-1239
Telephone: (800) 994-8849 or (510) 642-2431
FAX: (510) 643-5470
E-mail inquiries: danrcs@ucdavis.edu
An electronic version of this publication is available on the DANR Communication Services
Web site at http://anrcatalog.ucdavis.edu.
Publication 8084
© 2003 by the Regents of the University of California, Division of Agriculture and Natural
Resources. All rights reserved.
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pr-01/03-WJC/VFG ISBN 978-1-60107-259-7

This publication has been anonymously peer reviewed for technical accuracy by University of
California scientists and other qualified professionals. This review process was managed by the
ANR Associate Editor for Natural Resources.