Environmental Engineering FOURTH EDITION

18 ENVIRONMENTALENGINEERING

impacts of the other variables, and is arbitrarily assigned a negative value, so that its
impact is subtracted from the other impacts.
In order to determine a FONSI in this example, a value that indicates the lower
limit of significance can be assigned to either importance or magnitude, or to their
product. Thus, if the absolute value of the product of importance and magnitude of
any variable is considered significant only if that product is 10% of the total, odor and
noise would not be deemed significant and need not be considered further in assessing
environmental impact. Alternatively, if the absolute value of the product of importance
and magnitude of any variable is considered significant only if that product is greater
than 5, the same conclusion is reached.

The interaction matrix technique is a two-dimensional listing of existing charac-

teristics and conditions of the environment and detailed proposed actions that may

affect the environment. This technique is illustrated in Example 2.2. For example, the

characteristics of water might be subdivided into:

0 surface

0 ocean

0 underground

0 temperature

0 groundwater

0 recharge

0 snow, ice, and permafrost.

0 quantity

Similar characteristics must also be defined for air, land, socioeconomic conditions, and

so on. Opposite these listings in the matrix are lists of possible actions. In our example,

one such action is labeled resource extraction, which could include the following

actions:

0 blasting and drilling

0 surface extraction

0 subsurface extraction

0 well drilling

0 dredging

0 timbering

0 commercial fishing and hunting.

The interactions, as in the checklist technique, are measured in terms of magnitude

and importance. The magnitudes represent the extent of the interaction between the

environmental characteristics and the proposed actions and typically may be measured.

The importance of the interaction, on the other hand, is often a judgment call on the

part of the engineer.

If an interaction is present, for example, between underground water and well

drilling, a diagonal line is placed in the block. Values may then be assigned to the

interaction, with 1 being a small and 5 being a large magnitude or importance, and these