Considering Cumulative Effects Under the National Environmental Policy Act Page: 41
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The separation of effects into those attribu-
table to the proposed action or a reasonable
alternative versus those attributable to past
and future actions also allows the analyst to
determine the incremental contribution of each
alternative. Situations can arise where an
incremental effect that exceeds the threshold of
concern for cumulative effects results, not from
the proposed action, but from reasonably fore-
seeable but still uncertain future actions.
Although this situation is generally unexplored,
the decisionmaker is faced with determining
whether to forgo or modify the proposed action
to permit other future actions. Identifying in-
cremental effects, therefore, is an important
part of informing the decisionmaker.
Most cumulative effects analyses will iden-
tify varying levels of beneficial and adverse
effects depending on the resource and the indi-
vidual action. Aquatic species will experience
entirely different effects from terrestrial ones.
A warm water fishery (e.g., largemouth bass)
may benefit from a change that is detrimental
to a cold water fishery (e.g., trout), and effects
that are beneficial to the well being of a human
community (e.g., provision of social services)
may be detrimental to natural systems (e.g.,
wetlands lost during construction of a hospital).Because of this mixture of beneficial and
adverse effects, the decisionmaker is often hard
pressed to determine which alternative is envir-
onmentally preferred. To overcome this prob-
lem, indices of overall cumulative effect can be
developed. Some of the matrix methods used in
cumulative effects analysis were developed
specifically to address this need. These methods
use unitless measures of effect (e.g., scales or
ranks) to get around the problem of combining
results from a variety of resources.
Presentation of overall cumulative effects
can be controversial. Intentional or uninten-
tional manipulation of assumptions can dra-
matically alter the results of aggregated indices
(Bisset 1983), and experience indicates that
complex quantitative methods for evaluating
cumulative effects make it more difficult for the
public to understand and accept the results.
Effects on resources are usually presented
separately, and professional judgment is used
in determining the reasonable alternative with
the greatest net positive cumulative effect. The
U.S. EPA has developed guidelines for address-
ing specific kinds of risks (including cancer
risks and the risks posed by chemical mixtures)
and for comparing disparate kinds of risks (U.S.
EPA 1993).43
Table 4-1. Example table using quantitative description of effects (within a given level of
uncertainty) on various resources
Cumulative
Resource Past Actions Present Actions Proposed Action Future Actions Cumulative
Effect
Air Quality No effect on SO2 20% increase in SO2 10% increase in SO, 5% increase in SO, 35% increase in
S02
Fish 50% of 1950 2% of fish 5% increase in fish 1% of fish 48% of 1950 fish
population lost population lost population population lost population lost
Wetlands 78% of presettlement 1% of existing 0.5% of existing 1.5% of existing 95% of preset-
wetlands lost wetlands lost wetlands lost wetlands lost annu- tlement wetlands
annually for 5 years ally for 10 years lost inl0 years
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Council on Environmental Quality (U.S.). Considering Cumulative Effects Under the National Environmental Policy Act, text, January 1997; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc31126/m1/50/: accessed May 5, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .