Environmental Analysis Tools - Environmental Impact Analysis: Process and Methods - page 291

processing prior to disposal, and the low bioavailability, particularly under
the disposal area environmental conditions.
Similar to the physical exposure assessment, the determination of effects
from the physical stress of tailings deposition was not straightforward or
amenable to standard ecological risk assessment approaches. It was clear
to the environmental analysis team that extreme rates and depth of tailing
deposition would smother and alter benthic habitat, to the point that little
fauna could reside in the sediments. It was also well established that the
indigenous benthic infauna and the epibenthic fauna (e.g., flat fish and crabs)
could tolerate modest rates of tailing deposition and accumulation. The effects
characterization challenge was identifying the break point where the com-
munity associated with the bottom was not adversely affected and where it
was severely degraded. The literature was sparse on the relationship of solids
deposition and its effects on organisms associated with the bottom and at the
time the issue was not addressed as part of most ecological risk assessments.
However, making use of whatever literature was available, some creativity,
and discussions with the TAC, effects levels were established as follows:
r Tailings accumulation less than 4 cm/yr (centimeters per year) would
have no adverse effect on the community associated with the seafloor.
r Accumulation between 4 cm/yr and 10 cm/yr would alter the ben-
thic habitat and shift community composition.
r Deposition rates greater than 10 cm/yr would result in virtually
uninhabitable benthic conditions.
These rates were derived from studies showing that most organisms could
survive instantaneous burial rates greater than 10 cm (Reid and Baumann
1984, Nichols et al. 1978, SAIC 1993) so 10 cm/yr was chosen as a conservative
(i.e., over predicts risk) rate above which there would not be a sustainable level
of benthic activity. Instantaneous rates below 5 cm did not produce significant
mortality (Kranz 1974 and Nichols et al. 1978) so the rate of 5 cm/yr to 10 cm/yr
was conservatively chosen to result in a change in the benthic habitat but sup-
portive of a community that could survive. Based on these studies, mine tail-
ings deposition rates less than 4 cm/yr were predicted to have no measurable
effects on the benthic or epibenthic communities. These rates were consistent
with the natural sedimentation rates (approximately 5 cm/yr) in the adjacent
Taku Inlet, which during summer receives the melt from Taku Glacier. With
the exposure and effects level characterized, the final step of risk character-
ization should have been a simple matter of comparing the two. However as
stated above, little in ecology or risk assessment is quite so simple.
7.2.9.4 RiskCharacterization
Actually, the characterization of risk due to chemical exposure was simple.
The concentrations in the mine tailings, both solid and dissolved phases, were