Dames & Moore, 1999 - USDA Forest Service

child's daily ingestion rate (1 liter) came h m the mine-impacted area Summing risks for surface water
risks results in an added conservative assumption that a child is ingesting 1.5 liters of mine-impacted surface
water. Obviously, this assumption is unrealistic and overly conservative. Cumulative risks for mineimpacted
surface water ingestion are therefore adjusted downwards by a factor of three to result in an
ingestion rate of 0.5 liters, which is still unrealistic.
Table 7.1-43 shows the cumulative risks, summed across all MSs and exposure routes, for Holden Village
residents and for USFS workers. Table 7.1-43 also shows the adjusted cumulative hazard indices for each
toxic effect endpoint for Holden Village residentsirecreational users and for USFS workers. Cancer risks
and cumulative hazard indices are less than the acceptable level of one.
7.1.4.5 Uncertainty Analysis
Like all modeling efforts, the results of a health risk assessment rely on a set of assumptions and estimates
with varying degrees of certainty and variability. Major sources of uncertainty in risk assessment include:
(1) natural variability (e.g., differences in body weight in a population), (2) lack of knowledge about basic
physical, chemical, and biological properties and processes (e.g., the aflinity of a chemical for soil and its
solubility in water), (3) assumptions in the models used to estimate key inputs (e.g., dose-response
models), and (4) measurement error. Perhaps the greatest single source of uncertainty in risk-based
assessment is the chemicals' dose-response relationships, phcularly carcinogenic potency factors.
Additional uncertainty may also be associated with analytical data, which are subject to both systematic
error (bias) and random error (imprecision). Other major sources of uncertainty include computation of
representative concentrations using conservative fate and transport assumptions, and estimation of dose rate
via default exposure assumptions. These and other sources of uncertainty and their anticipated effect in
estimated risks associated with the site are summarized below.
It has been assumed for the purposes of this risk assessment that constituents detected in
various media are related to operations at the Holden Mine unless adequate data was
available to show that concentrations were below naturally occurring background -
concentrations. However, this assumption was not valid for all media. Site-specific
background data for sediments, air, seeps, and groundwater were insufficient for
determining area background concentrations. Other mining-related activities occurring in
the vicinity of the Holden Mine may have impacted these media. Including non-site-
related constituents as IHSs overestimates the risks associated with Holden Mine.
Estimation of the exposure point concentrations was conservatively based on 95 percent
UCL concentrations or maximum detected values in the media of interest. Use of these
exposure concentrations is likely to overestimate the chronic intake of a chemical, and
may not be realistic for long-term exposures.
Exposure assumptions utilized in the calculation of site-specific Method C criteria were
b&ed on a reasonable maximum exposure scenario, and included assumptions regarding
the types of exposure that may occur, the frequency and duration of those exposures, and
the concentration of chemicals at the point of exposure. Even the use of estimated site-
specific activity patterns are meant to be conservative worst-case exposure assumptions
and as such, are intended to provide a conservative estimate of intake, more likely to
overestimate than to underestimate exposure and risk.
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17693-00M19Uuly 27.1999,5:16 PWDRAFTFINAL RI REPORT