Dames & Moore, 1999 - USDA Forest Service

equilibrium partitioning have been relatively more successful, as have partitioning between acid-volatile
sulfides (AVS) and TOC and the sediment pore water.
Models
Only the simplest of models were used in this ERA. They consisted of soil-to-plant uptake factors, pore
water-to-insect extrapolations, and soil-to-small mammal extrapolations. While the soil-to-plant factors are
based upon a number of studies, there are large interspecies differences in the efficiency with which metals
are taken up from soils. Some of the factors that drive these differences are: soil moisture. organic content
metal complex, and weather conditions. In addition, most plants only bioaccumulate metals in the roots and
pass very little on to the leaves, and less on to the seeds. This is not reflected in the soil-to-plant uptake
factors used.
The remaining extrapolations were based upon a single study and must be used with' caution. The
conditions in the original studies from which these were extrapolated may have been very different from the
conditions at Holden Mine. For instance, the Clark Fork River pore-waters used to extrapolate metals
concentrations in Railroad Creek insects may have had higher concentrations of dissolved organic matter
than Railroad Creek. They may also have had relatively high levels of acid-soluble sulfides. Both of these
factors could reduce the apparent bioavailability of metals to insects.
The extrapolation of soil-to small mammals relied on the only known database for this type of information,
but suffers fiom the same kinds of potential uncertainties as the soil-to-plant and pore water-to-insect
extrapolations.
Risk Characterization
It is important to note that no one approach to risk characterization is adequate for all sites and chemicals.
For instance, the toxicity benchmarks used in this assessment are all chemical-specific and, as such.
cannot address the additive, antagonistic, or synergistic effects of the mixtures of chemicals typically
found in the environment. Further, they do not rake into account the nature and constitution of the
ecosystem present at the Site, site-specific conditions regulating chemical contact and bioavailability, the
potential toxicity of other constituents that were not quantified, or the pervasive influence of physical
stressors associated with natural conditions in severe climates.
In the present ERA, risks were characterized using the quotient method. This method relies upon a single
value of dose and TRV and provides only a single estimate of risk. Given the uncertainties and variances
presented above, it is apparent that the hazard quotients could be in error. An attempt was made to reduce
this source of error by deriving site-specific TRVs. and overestimating exposures, but the quotient method
does not yield a measure of central tendency. Probabilistic methods provide such as measure of central
tendency but are only as good as the input data. For the purposes of the RI, the hazard quotients were
determined to provide sufficient conservatism as to protect the receptor guilds identified in this ERA.
In the present ERA, it was assumed that metals were the only and most serious sources of risk to plant and
animal populations. However, there is good evidence that physical factors may also contribute to adverse
effects on both plant and animal populations. For instance, it is known that tailings piles have limited
amounts of organic matter and nutrients and have small water holding capacities. In addition, severe winter
and summer weather conditions and cropping by herbivores make re-establishment of vegetation on these
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