Dames & Moore, 1999 - USDA Forest Service

Finally, the analysis performed for this assessment did not account for site-specific factors such as natural
attenuation of PCOCs over time, adaptive tolerance, reproductive potential, the relatively small size of the
affected area, and recruitment &om similar adjoining areas. Such factors would tend to mitigate the
degree and ecological significance of loss or impairment of a portion of ecological population(s) due to
both chemical and physical stressors in the area. As a result, the approach used in this assessment
necessarily results in overestimation of risk.
Exposure
While aquatic insects and fish are restricted to relatively constant exposures from surface waters and
sediments, those for terrestrial receptors, with the exception of plants and earthworms are highly variable.
Birds and mammals have home ranges and forage ranges which would not restrict them to the worst case
exposures at the site, but it is difficult to estimate actual exposure raies. Although home and forage ranges
are available for each of the selected ROCs, these areas are usually given in square units such as hectares.
In a glacial valley such as the Railroad Creek drainage, animals are restricted to smaller ranges by the
topographical relief. Furthermore, migratory patterns probably also have an effect on the potential doses
received from ingestion within the Holden Mine area, and further up or down the valley. Quantification of
such ranges typically takes years of study and is beyond the scope of the present ERA. An attempt was
made to account for at least a portion of this uncertainty by using median exposure estimates in addition to
the worst case estimates.
An additional source of uncertainty associated with estimated exposures is a lack of knowledge concerning
the bioavailability of metals to either plants or from food items. There is a large literature which shows that
metals incorporated into either soils, minerals, or biological tissues are much less bioavailable than those
present as water-soluble salts. Since nearly all the bioassays used to construct the TRVs and criteria were
based upon the use of water-soluble salts, this source of uncertainty is most likely very conservative.
Animals such as deer may or may not be selective feeders at the Holden Mine site, and may.or may not
consume the plant species present on the tailings piles or Holden Village soils. Similarly, osprey could
possibly catch trout fiom Railroad Creek, but it is much more likely that they forage over the open waters of
Lake Chelan. Mink may or may not be present adjacent to Railroad Creek, but it is likely they would use
trout only as a supplement to the more abundant and more easily acquired small mammals which are
ubiquitous in the edge zone of clearings and forests.
Toxicity Benchmarks
Uncertainties regarding the screening toxicity benchmarks (National Ambient Water Quality Criteria, and
NOAA sediment quality guidelines) derive principally fiom the fact that they are conservative and
generic, protective of all species, including the most sensitive, rather than site-specific indicators of
potential risk to ecological receptors present at the Site. NAWQC are derived from the highest quality
and most applicable data that were available at the time of development. However, the physiology and
toxicology1pharmacology of COPCs, particularly in relation to the fish and wildlife taxa identified as
ROCs, is known only with some certainty. These differences are shown to some degree by the
differences beween the NAWQC and the site-specific TRVs for trout, for instance. This bias has an
unquantifiable, but likely large effect on risk estimates. This conservatively biased uncertainty cannot be
quantified, but incorporates both error and biological variation. The latter is likely the largest component
of uncertainty, because organisms have highly variable responses to toxicants, and extrapolation from
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17693-005019Vuly 27.1999:5:16 PMDRAFT FINAL RI REPORT