Dames & Moore, 1999 - USDA Forest Service

7.2.4.1 Risk Cbaracterization for Worst-case Current Conditions
For most ROCs, worst case exposures using the UCL or highest value were used as a screening technique to
determine if risk was feasible under more ecologically realistic median exposure conditions. However, for
sessile ROCs such as benthic invertebrates and terrestrial plants, reasonable worst case exposures may be
representative of the risks found at particular localized areas of the site. Since all the remaining ROCs are
free roaming and integrate their respective exposures through their home range, exposures to median
concentrations are more ecologically realistic and r&presentstive. Even for benthic invertebrates and plants.
the median concenuations are more indicative of conditions experienced by plant and animal populations in
the local environment. In fact, since these data were collected only from areas expected to be contaminated.
the median probably exceeds that which would be contacted by foraging animals, as well as much of the
vegetation. Therefore, median exposures were utilized and are likely to be conservative as well as more
ecologically relevant than UCLs. This is certainly true at the population level of organization.
The tiered risk characterization for aquatic and terrestria exposure pathways are described below.
Aquatic Exposure Pathways
Risks to aquatic ROCs were evaluated in tiers of analysis, proceeding from wont case exposures to more
reasonable exposures. In the fvst tier of analysis, only the UCL of the data collected from the South Bank
of Railroad Creek was used. This data ensures a worst case estimation of risk since the water samples were
collected from the creek bank where metals from seeps fmm the tailings piles have just begun to mix with
the main stream of Railroad Creek. If risk was found under these conditions. the UCL data for the main
stream of Railroad Creek were evaluated. The main stream samples were collected across the span of the
creek and represent a more accurate "worst case" scenario of well mixed waters of Railroad Creek. If risk
was still apparent, the median concentrations of the main stream were evaluated. This is a more accurate
picture of the exposures encountered by pelagic invertebrates and fish as they move about in the creek.
- Fish
From Table 7.2.4-]A, it is apparent that trout may be at risk in the highest concentrations of copper (and
possibly of zinc) along the South Bank of Railroad Creek. Therefore, since trout are not restricted to the
worst case exposure conditions, further risk analysis is justified for the UCL of the main stream of Railroad
Creek.
From Table 7.2.4-IB, it may be seen that trout could be at risk from copper in the main stream of Railroad
Creek. Further analysis of median water concentrations (Table 7.2.4-IC), showed that trout could be at risk
from copper in the area of the mine site, and downstream.
Benthic Invertebrates - Surface Water
Hazard quotients (Table 7.2.4-2A and 7.2.4-28) for benthic invertebrates exposed to metals in surface
waters were estimated by dividing the UCL of surface water concentrations along the South Bank of
Railroad Creek found in Table 7.2.2-IA, below, by the chronic TRVs found in Table 7.2.3-28.
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17693405419Uuly 21. 1999.5:53 PM;DR.UT FWAL RI REPORT