Conducting the Environmental Impact Analysis and Assessment - Environmental Impact Analysis: Process and Methods - page 193

r Impacts to vastly different environmental resources can be compared
on a common basis. In the above example, it is impossible to deter-
mine which is more detrimental, 20 trips or 0.5 mg/l reduction given
just the measurement. But if both are assigned a level of significance
on a common scale they can be directly compared.
r The process allows another avenue for stakeholder participation
by encouraging input in defining threshold levels of impact. It is
an active role for the stakeholders and by supplying meaningful
input to impact determination they receive a positive return on their
investment of time and energy into the process. It also enhances sup-
port for the final comparison and decision because of their accep-
tance and participation in defining the level of impact.
r It helps the team focus on the environmental impact analysis.
Whether conducted internally by the environmental analysis team
or with a larger set of stakeholders, trying to determine what con-
stitutes a significant impact forces a close look at the resource and
places an emphasis on quantitative analysis. Identification of thresh-
old levels of impact can also point out that a resource will not really
be impacted and can be eliminated from detailed consideration.
r Impact prediction and affected environment investigations can be
structured around the impact significance thresholds. For example,
if a significance threshold for impact on a vegetated wetland from a
water supply well is less than a 0.3 meter drawdown of the ground-
water table, field investigations and groundwater modeling can be
designed to determine the level of withdrawal that will reduce the
groundwater elevation at each alternative location.

As discussed below, there are numerous approaches to developing impact
significance criteria in order to maximize these advantages.

5.3.3.1 Environmental Standards as Significance Criteria

Standards and criteria are ready-made impact significance criteria. For exam-
ple, water quality criteria are generally available for most contaminants and
often expressed as concentrations associated with both acute and chronic
effects. A predicted concentration below both levels could be designated as
no impact, a concentration above both a significant impact, and a concentra-
tion between the two as a moderate impact. There are similar criteria for other
environmental resources including air, public health, sediment quality, noise,
risk of cancer, and radiation exposure. Thresholds established for environmen-
tal permits and regulations are another source of predetermined significance
criteria. For example, the impacts to wetland resources can be easily predicted
from a layout of the project and description of the affected environment if the