Conducting the Environmental Impact Analysis and Assessment - Environmental Impact Analysis: Process and Methods - page 203

5.3.4.1 Impact Prediction Approach

Impact prediction for the DCR EIS had an advantage that most EIS scientific
investigations do not share. The practice of dry cargo sweeping has occurred in
the Great Lakes for over 100 years and any impact on ecological receptors from
DCR discharge has occurred and continues to occur with the continuing DCR
sweeping practice. Thus in this case measurement of existing conditions is an
excellent predictor of future impacts of various DCR management alternatives.
An impact hypothesis was proposed: the degree of impact was generally
proportional to the mass of DCR discharged. This hypothesis was tested
through literature review and discussions within the project team and other
experts. It was also tested through the design and implementation of numer-
ous scientific investigations and found to be supportable. Thus if an EIS
alternative for DCR management reduced discharge by 25% compared with
existing practices, any ongoing or historic impact measured would similarly
be reduced by approximately 25%.
Even with this impact prediction advantage, not shared by most scientific
studies for EISs, there was uncertainty in the impact prediction. In order to
address this uncertainty, a series of scientific investigations was conducted
(Table 5.4), which identified at least two types of investigations to address
potential impacts on each ecological receptor (as identified in the impact pre-
diction conceptual model, Figure 5.10) and more in areas where the impact
was considered to be more likely or more severe.
For example, because the discharged DCR was much heavier than water,
the iron, coal, and limestone sweepings would rapidly descend to the lake
floor and accumulate over time. Thus the organisms that live on the bot-
tom (the benthos) were at the greatest risk of impact, and multiple studies
were designed to evaluate impacts on this segment of the aquatic ecosystem.
These investigations and the predicted mechanism of impact on the benthos
tested were as follows (Table 5.4):

r Sweepings Characterization : The physical and chemical attributes of
iron ore (as partially processed taconite), coal, and limestone (the
three types combined represent over 95% of the dry cargo shipped
on the Great Lakes and also the dry cargo with the greatest impact
potential) transported and potentially discharged by Great Lakes
carriers were determined. These characteristics were then compared
with the requirements and tolerances of the critical benthic species
inhabiting the floor of the Great Lakes. The degree to which DCR
attributes were incompatible with the species' chemical tolerance
(i.e., sensitivity to toxicity) or habitat requirements (physical attri-
butes) represented the degree of impact.
r Physical characterization of deposition area : As discussed above, the
physical characterization of the actual DCR is important to predict
impacts to benthic organisms, but the characteristics of the in situ