Considering Cumulative Effects Under the National Environmental Policy Act Page: 40
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present actions used in air quality analyses to
determine whether new emission sources will
exceed air quality standards. The NAAQS
inventory includes all existing emission sources,
sources with Prevention of Significant
Deterioration (PSD) permits that have not yet
begun to operate, and applicants for whom a
PSD permit has not yet been issued. The
NAAQS analysis requires explicitly modeling
all existing nearby sources (as far away as 50
kilometers) be for air quality effects. In the
analysis of the cause-and-effect relationships
related to the anticipated impacts, each source
represents a cause, and their combined emis-
sions create an effect on air quality, the signif-
icance of which can be determined by comparing
the concentration of pollutants emitted to thres-
hold concentrations specified in the NAAQS.
The NAAQS thresholds are concentrations
known to cause significant human health or
other environmental effects.
The historical context and full suite of on-
going actions are not only critical for evaluating
cumulative effects, but also for developing po-
tential restoration as well. The first step in
developing a river restoration plan is to under-
stand how past actions (e.g., contributions of
contaminants to the watershed) have contrib-
uted to the current condition of the water body.
The historical trends in resource condition and
its current potential for sustained structure and
function are an essential frame of reference for
developing mitigation and enhancement mea-
sures.
Determining Magnitude
Initially, the analyst will usually determine
the separate effects of past actions, present
actions, the proposed action (and reasonable
alternatives), and other future actions. Once
each group of effects is determined, cumulative
effects can be calculated. The cumulative
effects on a specific resource, however, will not
necessarily be the sum of the effects of allactions. Knowing how a particular resource
responds to environmental change (i.e., the
cause-and-effect relationship) is essential for
determining the cumulative effect of multiple
actions. Will the effects of two or more actions
be additive, i.e., if one project would result in
the death of 25% of a trout population (within a
given level of uncertainty) and another the
death of 10% of the trout, would the two projects
together result in the loss of 35% of the trout?
Although this is sometimes the case, there are
often instances where the cause-and-effect rela-
tionship is more complex, i.e., the cumulative
effect of two projects may be greater than the
sum of the effects of each (in the trout example,
more than 35% of the trout would die) or less
than their sum (less than 35% of the trout
would die). In some cases, the resource may
better withstand additional adverse effects as
stress increases, while in others, the resource
may crash once a threshold is reached.
Once effects are identified using one of the
methodologies described in Chapter 5, a table
can be used to itemize effects into categories of
past, present, proposed, and future actions.
Tables 4-1, 4-2, and 4-3 show how these tables
can be constructed using the results from differ-
ent types of analyses. Regardless of the degree
of quantification used, such tables are useful
tools for putting the effects of the proposed
action and alternatives into proper context.
Table 4-1 illustrates the net cumulative effects
of combining fish population increases from the
proposed action with population losses from
past and future actions. The table could be ex-
panded to include the countervailing effect of
sulfate aerosols on global warming (because
they compensate for greenhouse gases) at the
same time they are degrading ambient air qual-
ity. A series of such tables (one for each altern-
ative) enables the analyst to compare alterna-
tives meaningfully.42
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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/m1/49/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .