ASSESSMENT OF CUMULATIVE AND
SYNERGISTIC IMPACTS
Types of Cumulative Impacts and Their Concepts
Presented by: Minhaj Aijaz Memon
TABLE OF CONTENTS
1. Introduction:
1.1. Cumulative Impact
1.2. Direct and Indirect Impacts
2. Types of Cumulative Impacts:
2.1. Concept of Additive Impacts
2.2. Concept of Synergistic Impacts
3. Conclusion
4. References
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1. INTRODUCTION
The US Council on Environmental Quality (CEQ) has indicated that there is increasing
evidence that the most destructive environmental effects may actually result not from the
direct and indirect effects of a given action, but instead from the combination of individual
minor effects of numerous actions over time.
Image Source: https://www.cleanwateraction.org/features/cumulative-impacts-pollution 3
Source: CEQ NEPA
1.1. CUMULATIVE IMPACT
Definition of Cumulative Impact According to the U.S Council on Environmental
Quality:
“…the impact on the environment that
results from the incremental impact of the
action when added to other past, present,
and reasonably foreseeable future actions,
regardless of which agency (federal or non-
federal) or person undertakes such other
actions. Cumulative impacts can result
from individually minor but collectively
significant actions taking place over a
period of time.”
Source: CEQ NEPA (40 CFR § 1508.7)
1.1. CUMULATIVE IMPACT
Proposed
Project
Direct &
Indirect Impact
Direct & Direct &
Past Actions Indirect
Cumulati Future
Indirect
Impact ve Impact Impact Actions
Direct &
Indirect Impact
Other Present 5
Actions
Source: CEQ NEPA (40 CFR § 1508.7)
1.2. DIRECT AND INDIRECT IMPACTS
Definition of Direct and Indirect Impacts According to the U.S Council on
Environmental Quality:
Direct Impacts Indirect
“…are caused Impacts
by the action and are
later in time or farther removed in
distance, but are still reasonably
foreseeable. Indirect effects may
include growth inducing effects and
“…are caused by the action and
other effects related to induced
occur at the same time and place.”
changes in the pattern of land use,
population density or growth rate,
and related effects on air and water
and other natural systems, including
ecosystems.”
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Source: CEQ NEPA (40 CFR § 1508.8)
1.2. DIRECT AND INDIRECT IMPACTS
Direct Passage of Indirect
Project Time/Remot
Environment Environment
Action e in
al Impacts al Impacts
Distance
Source: CEQ NEPA (40 CFR § 1508.8)
Direct Impact: Habitat Loss Due to Roadbed Indirect Impact: Habitat Losses Due to Construction of
Grading Interchange
Cumulative Impact: Habitat Losses Due to Various
Images Source: Google Images Construction Activities
2. TYPES OF CUMULATIVE IMPACTS
Cumulative
Impacts
Synergisti
Additive c
Impacts (Interactiv
e)
Source: Eccleston, 2011
2.1. CONCEPT OF ADDITIVE IMPACTS
Additive impacts occur when the magnitude of combined effects is equal to the sum of
individual effects. Common examples encountered in CIA for NEPA EIAs & EISs include:
Image Source: https://www.nps.gov/subjects/air/sources.htm Image Source:
http://www.uky.edu/WaterResources/public/watersheds. 10
php
Source: Eccleston, 2011
2.2. CONCEPT OF SYNERGISTIC IMPACTS
When effects are combined, the result
may be substantially greater or less than
that expected based on additivity.
A greater than expected result can be
described as synergistic.
Less than expected result can be
described as antagonistic.
Synergism is only considered in CIA.
Often cannot be expressed quantitatively
and are much more complex and difficult
to assess than additive effects.
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Source: Eccleston, 2011
2.2. CONCEPT OF SYNERGISTIC IMPACTS
Example of Synergistic Impact in the
Scope of EIA:
Consider that the combined effects of
terrestrial habitat losses can be expected
to be additive when the habitat type is
abundant in the region, but can be
synergistic when the habitat type is
regionally scarce.
For example, bird species favoring large
expanses of forest cover become
increasingly scarce once landscapes lack
forest tracts below area thresholds that
differ by species.
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Source: Eccleston, 2011 Image Source: Cornell University
3. CONCLUSION
Cumulative impacts can alter environmental systems through multiple pathways of effect:
Growth Induced Pathway:
Each new project or action can induce further projects or actions to occur, sometimes
referred to as “spin-off” effects (e.g., an off-trail road resulting in increased hunting and
fishing)
Physical or Chemical Transport Pathway:
A physical or chemical constituent is transported away from the activity under review,
where it then interacts with another activity (e.g., air emissions, sedimentation,
wastewater effluent). Multiple point and non-point discharges of water pollutants are
typical examples.
Nibbling Loss Pathway:
This is the gradual disturbance and loss of land or habitat (e.g., clearing of land for a new
subdivision and new roads into a forested area).
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Source: Cumulative Effects Working Group et al., 1999
4. REFERENCES
CEQ (Council for Environmental Quality), Considering Cumulative Effects under the
National Environmental Policy Act, January 1997
Eccleston, C. H., Environmental Impact Assessment: A Guide to Best Professional Practices,
Taylor & Francis Group, 2011.
Cumulative Effects Working Group et al., 1999.
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