Conducting the Environmental Impact Analysis and Assessment - Environmental Impact Analysis: Process and Methods - page 214

r Were indicative of potential broad-scale and significant ecosystem-
wide impacts
r Were suitable for quantitative and comparative impact prediction,
analysis, and comparison among alternatives
r Could be compared with accepted impact significance criteria
These two environmental resources were the enrichment or eutrophica-
tion of the Boston Harbor water column and the enrichment and degrada-
tion of the benthic environment. Not coincidentally, these were also two of
the critical impact areas in the entire nationwide effort to address gross pol-
lution of the waters of the United States by municipal wastewater treatment
and disposal through the Clean Water Act 201 funding to municipalities. As
a result they were also the topics of significant research to develop impact
significance criteria (see Section 5.3.3.3). So the approach to impact predic-
tion for these two critical environmental resources for the Boston Harbor
Cleanup was to determine the extent of nutrient and sediment enrichment
for each alternative and compare it with the significance criteria.
There were standard methods of hydraulic modeling available for pre-
dicting conditions in both the water column and the sediments result-
ing from wastewater discharges. Extending these methods to a system
as large and complex as Boston Harbor (e.g., numerous rivers entering
the system, extreme vertical stratification, and a tidal range up to almost
4 meters) was a challenge, but with the advice and expertise available
on the environmental analysis team and the TAC with stellar academic
credentials (see Section 4.3) the challenge was successfully met. After an
extensive affected environment data collection program to determine cur-
rents, tidal exchange, boundary conditions, and thermal stratification as
input parameters, a mathematical hydraulic model was developed, cali-
brated, and verified. The model was then used to predict the nutrient con-
centrations, sediment deposition rates, and deposition area from each of
the alternative wastewater effluent discharge locations and wastewater
treatment processes.
The results of the predictions were then compared with the impact sig-
nificance criteria developed by Clean Water Act initiated research to eval-
uate municipal wastewater manage issues (see Section 5.3.3.3). From this
comparison, the area of Boston Harbor exceeding the criteria for various
levels of nutrient and sediment enrichment impact could be determined
for a wide variety of alternative discharge locations and levels of wastewa-
ter treatment (Figures 5.17 and 5.18 are example combinations of discharge
location and level of treatment). This information then provided objective
and quantitative expression of impact in these two critical areas so that
the decision makers could make informed decisions. The decision mak-
ers and the other stakeholders were able to quantitatively consider the
tradeoffs of costs and environmental benefits of higher levels of treatment