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Dames & Moore, 1999 - USDA Forest Service

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7.1.4 Site-specific Human Health Risk Assessment<br />

The site-specific HHRA was conducted for those IHSs and exposure pathways (Table 7.1-36. Figure 7.1-2)<br />

which significantly contributed to the risk at the site as determined in the screening level human health<br />

assessment. The methodology for the site-specific HHRA followed the following steps:<br />

Characterize the relevant exposure pathways<br />

Develop exposure concentrations based on statistical evaluation of the data<br />

Characterize the toxicity of the IHSs<br />

Develop site-specific (MTCA Method C) levels<br />

Characterize risks associated with the site<br />

Discuss uncertainties in the risk characterization<br />

Each step of the site-specific HHRA is discussed in detail in below.<br />

7.1.4.1 Exposure Assessment for Refined Exposure Pathways<br />

Exposure Pathways<br />

Exposure pathways evaluated in the site-specific HHRA are shown conceptually in the exposure pathway<br />

model (Figure 7.1-2) and detailed, along with IHSs, in Table 7.1-36. Exposure to IHSs in soil, air,<br />

sediments, surface water, seeps, and mine portal drainage is assumed to represent potential, exposure routes<br />

of concern for the site.<br />

Development of Exposure Point Concentrations<br />

The potential magnitude of exposure is determined by measuring or estimating the exposure point<br />

concentrations of IHSs available in various media at "exchange boundariesn (e.g., the lungs, gastrointestinal<br />

tract, or skin). For the purposes of the site-specific HHRA, the exposure concentration was estimated to be<br />

the 95 percent upper confidence limit (UCL) concentration for the data set. When there was an insufficient<br />

number of samples or a high percentage of uncensored data (data reported as below the detection limit). 95<br />

percent UCL concentrations could not be calculated. For these data sets, the maximum detected<br />

concentration was used for the exposure concentration. The method for calculating the 95 percent UCL was<br />

dependent on the distribution of the data (i.e., normal, lognormal, or neither). The distribution of the data<br />

and the 95 percent UCL concentrations were calculated using the Ecology MTCAStat V2.1 Excel Macro.<br />

The statistical tables included in this section (tables 7.1A through 7.1K) show the distributions and 95<br />

percent UCL concentrations, when calculated, for each media. Table 7.1-37 details the exposure<br />

concentrations for each IHS used in the site-specific HHRA.<br />

7.1.4.2 Toxicity Characterization<br />

The toxicity assessment determines the relationship between the magnitude of exposure to an IHS and the<br />

nature and magnitude of adverse health effects that may result from such exposure. Chemical toxicity is<br />

divided into two categories, carcinogenic and non-carcinogenic, based on the type of adverse health effect<br />

exerted. Health risks are calculated differently for these two types of effects because their toxicity criteria<br />

G:\vpstl~~UloIdcn-2\ni700doc 7-24<br />

17693-005-019Uuly 27.<strong>1999</strong>,5:16 PM;DRAff FINAL RI REPORT

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