Dames & Moore, 1999 - USDA Forest Service

Use of toxicity criteria (CPFs and RfDs) intentionally designed to be conservative is
likely to overestimate the IHSs' toxic potency. For example, the extrapolation of animal
carcinogen bioassay results to human risk at much lower levels of exposure involves a
number of assumptions regarding effect threshold. interspecies extrapolation. high- to
low-dose extrapolation, and route-to-route extrapolation. The scientific validity of these
assumptions is uncertain; because each of the individual extrapolations are designed to
prevent underestimation of risk, in concert they result in unquantifiable but potentially
very significant overestimation of risk. Specifically, the extrapolation of cancer potency
fiom laboratory animals to humans, which forms the basis for the cancer risk estimates,
may be associated with uncertainties ranging from as much as three to five orders of
magnitude (1,000 to 100,000-fold) for selected chemicals.
The risk analysis does not include a likelihood evaluation. For example, it is unlikely '
that the same recreational users will chronically utilize the lagoon area, Railroad Creek.
Copper Creek, seeps, and portal drainage for recreational purposes. Thus. the actual risk
for recreational users is likely to be significantly lower than estimated in this HHRA.
In summary, because the majority of assumptions regarding representative concentrations and contact rates
made in this assessment are conservative, tending to overestimate exposure and risk, the incremental risks to
the defined receptor populations fiom exposure to IHSs at the site are likely to be significantly
overestimated.
7.1.5 Conclusions of Baseline Human Health Risk Assessment
Human health risks were evaluated through a conservative screening process and a site-specific risk
assessment for those constituents exceeding the screening criteria. Site-specific risk assessment results were
evaluated in light of acceptable risk levels defined by MTCA (i.e., a cumulative excess cancer risk of less
than 1 x 10'' and a cumulative hazard index of less than 1.0 for constituents with effects on the same target
organ). Results for each media are summarized below.
7.1.5.1 Surface Soil and Tailings
Surface soil sample results were divided into seven areas based on exposure characteristics, population
exposed, and/or physical location, as follows: 1) Holden Village, 2) vegetable garden, 3) baseball field, 4)
wilderness boundary, 5) maintenance yard, 6) lagoon area, and 7) USFS guard station. Screening of
constituent concentrations resulted in the elimination of the vegetable garden, baseball field, and wilderness
area as areas of concern for ingestion of surface soil. In addition, the screening resulted in the elimination of
the tailings piles as an area of concern for ingestion. Constituents exceeding screening criteria in the
remaining four surface soil areas were selected as indicator hazardous substances (IHSs) and evaluated in
the site-specific risk assessment.
Cancer risks and noncancer hazard quotients for ingestion of all IHSs in all exposure areas were below the
MTCA cancer risk and hazard quotient guidelines. The cancer risks ranged from 8.91 x 10.'~ for beryllium
at the USFS guard station to 5.66 x 10" for arsenic in the maintenance yard. The hazard quotients ranged
from 4.1 5 x 10" for beryllium at the USFS guard station soil to 2.04 x 10.' for copper in the lagoon area soil.
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176936OS-019Uuly 27.1999;5:16 PWRAFT FINAL IU REPORT