Dames & Moore, 1999 - USDA Forest Service

laboratory studies to field exposure estimates incorporates this conservatively biased uncertainty into the
risk estimates.
Assumptions regarding the protectiveness of aquatic life criteria and sediment screening guidance imparts
an unknown degree of conservative bias to the assessment because these criteria are intentionally derived to
incorporate "safety factors" or margins-of-safety. The level of conservative bias introduced by conclusions
based on these criteria is generally not quantifiable because both types of criteria are driven largely by
numbers of studies evaluated rather than a possibly more appropriate evaluation of actual results variability
and differential receptor sensitivities. An attempt has been made to reduce these sources of uncertainty by
incorporating surrogate-specific bioassay results and deriving guild-specific TRVs. In most cases it was
possible to focus on the specific guild to be protected (i.e.. aquatic insects and trout) at Holden Mine. and to
reduce the uncertainty associated with scaling for mammals. However, because of site-specific conditions.
both known and unknown, there is a degree of uncertainty associated with the TRVs, doses, and risk
estimated for all potential exposure pathways.
The use of the Long et al. (1995) sediment quality guidelines is an exception to this rule. They were used in
the present document without change because there are few areas of Railroad Creek where sediments
accumulate and because there is little sediment data in the published literature for most of the COCs. There
are a number of uncertainties associated with the use of the Long et al. (1995) ER-M and ER-L values:
The guideline values were derived from a number of different studies with different species
and different endpoints
The guidelines were derived in marine and estuarine waters where sediment communities
are well developed
The guideline values do not account for sulfide binding to metals
Because many of the different studies used by Long et al. (1995) were field studies, there is no unambiguous
way of knowing which chemicals in these sediments were actually responsible for the apparent adverse
effects. Furthermore, although the SEL guideline values for freshwater developed by Persaud et al. (1993)
are generally lower than the ER-M values of Long et al. (1995), the freshwater PAETs developed by
Ecology (1991) are higher than the ER-Ms for copper (1.3x), lead (2.2x), mercury (2.3~) and zinc (2.4~).
This adds credence to DiToro et al. (1991) conclusion that sediment quality guidelines that do not account
for sulfide binding are indefensible. This is particularly relevant to Holden Mine and Railroad Creek where
the source of the metals are sulfide ores which require strong acid digestion to enable measurement.
Simpson et al. (1998) have shown that pure copper and nickel sulfides are not digested by extraction in 1 M
HCL after 30 minutes, while cadmium, zinc, manganese and iron sulfides are digested by this treatment.
However, Long et al. (1995) rejected all sediment data in which strong acid was not used. Therefore, it is
important to note that pH in Railroad Creek ranges between about 5.5 and 7.8, a range where none of these
sulfide-bound metals would be bioavailable.
To date, no acceptable sediment quality criteria exist against which to compare concentrations of COPCs
to definitively evaluate potential risks (O'Connor et al., 1998; O'Connor, 1999). This is because the
cumulative uncertainties, inherent measurement errors, and differences between laboratory and in situ
conditions in current approaches (e.g., apparent effects thresholds, spiked sediment bioassays, equilibrium
partitioning) make them far too imprecise for regulatory use as more than very generally applicable
screening values. Recent efforts to base sediment quality criteria for nonionic organic chemicals on
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