Conducting the Environmental Impact Analysis and Assessment - Environmental Impact Analysis: Process and Methods - page 183

described general conditions in various Great Lakes areas and also provided
information for some specific locations associated with DCR discharge.
The second source of existing information was the logs of Great Lakes car-
riers where the latitude and longitude of the starting and ending points of
DCR discharge were recorded. Review and plotting of the information from
the ship's logs revealed that generally the same locations were used year
after year by the vast majority of Great Lakes carriers for deck washing and
discharge of DCR. Thus there were concentrated areas where the discharges
had occurred.
The ship's logs identified these concentrated areas but the vastness of the
Great Lakes made pinpointing the exact location of a DCR discharge difficult
at best and a further refinement was necessary to determine where, within
the concentrated areas, DCR and associated impacts could be expected. This
refinement was accomplished by the next phase of the affected environment
investigation: a towed side-scan sonar system was used (Figure 5.8) to iden-
tify discharged DCR within the concentrated areas of deck sweeping. This
device used sound waves to distinguish the hard residue of coal, iron ore,
and limestone from the relatively uniform and soft lake bottom.
Once these likely “targets” of discharged DCR were identified with the
towed sonar array, actual sediment sampling similar to the Scituate investi-
gation was conducted to both confirm the presence of DCR and describe the
environmental conditions in the vicinity of the past and potentially future
DCR discharges. These samples confirmed that the locations identified by
FIGURE 5.8
Sonar L-3Klein System 3000 towfish used to map Great Lakes affected environment (Courtesy
L-3Klein, Inc.)