Environmental Analysis Tools - Environmental Impact Analysis: Process and Methods - page 292

below the effects levels. Also the toxicity testing showed no adverse effects from
chemical exposure; therefore the risk assessment concluded marine disposal of
mine tailings would not result in any elevated risk due to chemical exposure.
The evaluation of risk from physical stressors was a different story alto-
gether. The characterization of exposure demonstrated that tailings deposition
would be above the effects levels for the altered and lost habitat in a portion of
the study area (Figure 7.7). Approximately 7.5% of the study area habitat would
be affected (3.4% altered plus 3.9% lost, Table 7.6); however, risk characteriza-
tion must go beyond determining how much area or even what percentage of
total area would be affected. The risk assessment must address the question of:
does the change affect the achievement of ecological endpoints?
Each endpoint was addressed based on the loss of a resource critical to
an ecological endpoint, starting with the density of benthic fauna. This was
the first endpoint addressed because: (1) the benthic assemblage is the most
directly impacted by the deposition of mine tailings and (2) the benthic fauna
are near the base of the food chain and virtually every other ecological end-
point is dependent on secondary food production by the benthos. As it turns
out, the benthic fauna affected is only about half (1.6% altered and 2.0% lost)
of the habitat area affected (Table 7.6). The relatively small effect on the ben-
thic fauna is no coincidence or a random result of the risk characterization.
Extensive investigations clearly demonstrated that the slope habitat supported
almost five times the faunal density of the basin (5980 animals per square
meter on the slope vs. 1350 per square meters in the basin, Table 7.6), and the
mine tailings discharge method and location were configured to minimize
impact on the slope habitat. The risk characterization demonstrated (and the
TAC accepted) that this level of risk to the benthic animals was acceptable and
the sustainability of the benthic assemblage would not be threatened.
However, it still left many other ecological endpoints (Tables 7.4 and 7.5), to
be addressed for full characterization of the risk. The lack of chemical expo-
sure, bioaccumulation of contaminants, and water column effects resulting
from the tailings discharge demonstrated that there would be no risk to the
other endpoints (e.g., pelagic fish, sea birds, and marine mammals) related
to toxicity. But, nontoxicity related endpoints were dependent to some

TABLE 7.6
Benthic Fauna Risk Characterization of AJ Mine Tailings Discharge

Habitat Area (km 2 ) Benthic Fauna Density (Number/m 2 )

Slope: total for study area 25.4 5980

Basin: total for study area 50.6 1350

Other: total for study area 19.3 2550

Total: total for study area 95.3 2827

Total altered 3.4% 1.6%

Total lost 3.9% 2.0%