Environmental Engineering FOURTH EDITION

22 ENVIRONMENTAL ENGINEERING

magnitude of the impact multiplied by the importance:

EIU = PIU x EQI.

This method has several advantages. We may calculate the sum of EIUs and evaluate
both the cumulative impact of the proposed project and the “worth” of many alterna-
tives, including the do-nothing alternative. We may also detect points of severe impact,
for which the EIU after the project may be much lower than before, indicating severe
degradation in environmental quality. Its major advantage, however, is that it makes it
possible to input data and evaluate the impact on a much less qualitative and a much
more objective basis.

EXAMPLE 2.3. Evaluate the effect of a proposed lignite strip mine on a local stream.

Use^10 PIU and linear functions for EQI.

The first step is to list the areas of potential environmental impact. These may be:

0 appearance of water

0 suspended solids

0 odor and floating materials

0 aquatic life

0 dissolved oxygen.

Other factors could be listed, but these will suffice for this example. Next, we need to

assign EQIs to the factors. Assuming a linear relationship, we can calculate them as in

Table 2-3.

Table 2-3.

Item Condition before project Condition after project EQI

Appearance of water 10 3 0.33

Odor 10 5 0.50

Suspended solids 10 mgL 1000 mg/L 0.01

Aquatic life 10 2 0.20

Dissolved oxygen 9 mgn^8 mgL 0.88

Note that we had to put in subjective quantities for three of the items - “Appearance of

water,” “Odor,” and “Aquatic life” - based on an arbitrary scale of decreasing quality

from^10 to 1. The actual magnitude is not important since a ratio is calculated. Also

note that the sediment ratio had to be inverted to make its EQI indicate environmental

degradation, i.e., EQI < 1. The EQI indices are weighted by the 10 available PIU, and

the EIU are calculated.

In Table 2-4, The EIU total of 2.72 for this alternative is then compared with the

total EIU for other alternatives.