Assessing Environmental Impact 210 Economics-Jobs created or lost
-Property values
-JobsEach title might have several specific subtopics to be studied; for example, under
Aesthetics, “Air” may include odor, sound, and visual impacts as items in the
checklist.
Numerical ratings may be assigned to these items. One procedure is to fist estimate
the ideal or natural levels of environmental quality (without anthropogenic pollution)
and take a ratio of the expected condition to the ideal. For example, if the ideal
dissolved oxygen in the stream is 9 ma, and the effect of the proposed action is to
lower the dissolved oxygen to 3 ma, the ratio would be 0.33. This is sometimes
called the environmental quality index (EQI). Another option to this would be to make
the relationship nonlinear, as shown in Fig. 2-1. Lowering the dissolved oxygen by
a few milligrams per liter will not affect the EQI nearly as much as lowering it, for
example, below 4 ma, since a dissolved oxygen below 4 mg/L definitely has a severe
adverse effect on the fish population.
EQIs may be calculated for all checklist items that have a natural quantitative scale.
In order to assess those items that do not have a quantitative scale, like aesthetics or
historical objects, a scale based on qualitative considerations may be generated by
an expert in the particular area. For example, impact on a historic building might be
measured by the cost of recovering from certain amounts of damage. Something like
visual aesthetics can simply be assigned a scale.
The EQI values are then tabulated for each parameter. Next, weights may be
attached to the items, usually by distributing 1000 parameter importance units (PIU)
among the items. Assigning weights is a subjective exercise and is usually done by the
decision makers: those individuals who are going to make decisions about the project.
The product of EQI and PIU, called the environmental impact unit (EIU), is thus the
0
Environmental quality index
Figure 2-1. Projected environmental quality index me for dissolved oxygen.