Environmental Analysis Tools - Environmental Impact Analysis: Process and Methods - page 289

7.2.9.2 ExposureCharacterization
The conceptual site model for the marine disposal of mine tailings identified
two types of stress: chemical and physical. The chemical exposure could be
determined from a relatively simple analysis of the potentially toxic con-
stitutes (primarily heavy metals) of the mine tailings. This was enhanced
by experiments to determine the dissolution of the contaminants into the
water column and the sediment pore waters. It was also verified by sam-
pling and analysis of sediments, pore water, and overlying water column
of nearby historic mine tailings marine disposal areas. These analyses pro-
duced consistent characterization of exposure expressed as milligrams of
contaminants per liter of water (mg/l) in the overlying water column and
pore water. Similarly, chemical exposure to the mine tailings in the sedi-
ments was expressed as milligrams of contaminants per kilogram of sedi-
ment (mg/kg). These exposure concentrations could then be compared with
the concentrations developed through the effects characterization.
Characterization of physical exposure was not as simple as the chemical char-
acterization. The degree of physical exposure, and thus stress, was dependent
on the mass (converted to accumulated depth in centimeters) of tailings depos-
ited per unit area of the seafloor. This was determined by sophisticated hydrau-
lic modeling that factored in the settling rate of the tailings particles, the density
of the water, the currents, stability of tailings piles on the sea floor, and several
other physical/geological oceanographic variables. The output of the model was
a prediction of where the discharged mine tailings would come to lie on the
seafloor at the cessation of mining activities and the depth of total tailings accu-
mulation (Figure 7.7). Similar to the chemical exposure conclusions, the degree
of physical exposure to mine tailing (both total accumulation and rates of depo-
sition) could be compared with the physical effects characterization.
7.2.9.3 EffectsCharacterization
Paralleling the exposure characterization, the degree of stress causing
adverse effects to ecological receptors had to be determined for both chemical
and physical stressors. Similar to the exposure analysis, the determination of
effects from chemical effects was relatively straightforward and utilized tried
and true ecological risk assessment techniques. The water-quality standards
and criteria for the contaminants found in the mine tailings were the start-
ing point for the chemical effects characterization. These were modified by
review of recent literature reporting toxicity information on the species rep-
resenting the ecological endpoints selected for the AJ Mine risk assessment/
EIS. There were also adjustments made to account for the chemical behavior
of some contaminants under the conditions found in the tailings deposition
area (e.g., pH, salinity, and depth). This characterization yielded effects levels
or concentration levels above which adverse affects would be expected for all
the chemical-related measurement endpoints for both short- and long-term
exposures.