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David Stavarek, R.G. <br /> June 29, 1998 <br /> Page 6 <br /> of exposure pathways until there is sufficient evidence that it does not significantly contribute <br /> to the bulk of the overall risk(for example, 95%). <br /> 4. The equation for exposure dose is missing a conversion factor, F.= 10-3 kg soil/mg soil. <br /> SECTION 7.0 TOXICITYAssmmENT <br /> 1. This section(page 29) is missing in the Workplan. The authors are expected to use toxicity <br /> criteria as developed by Cal/EPA, and if not available, by the USEPA. If no value is <br /> available from these two sources, then if there is evidence that the chemical poses a health <br /> hazard,then a toxicity criteria may be proposed by the authors of the Workplan. <br /> SECTION 8.0 RISK CHARACTERIZATION <br /> 1. This section describes the estimation of cancer and hazard index. It is recommended that the <br /> explanation be expanded to include the estimation of overall cancer and hazard.index for <br /> each contaminant, exposure pathway, and exposure scenario, as defined in the Conceptual <br /> Site Model section. For example, it may be more informative to present the equation for <br /> Hazard Index as: <br /> Hazard Index = Z Z[HQ a,b <br /> where a=contaminants and b =exposure pathways, so that the Hazard Index is the sum of <br /> all HQs for all U contaminants contributing from all m exposure pathways, for a given <br /> exposure scenario. <br /> Results should be presented for each contaminant and exposure pathway, within each <br /> exposure scenario, in a tabular form. Thus presented, COCs that exceed target risk levels and <br /> may require risk management decisions are easily identifiable. <br /> SECTION 9.0 METHODS FOR DEVELOPING CLEANUP LEVELS <br /> 1. The output of a probabilistic (or"stochastic')risk assessment is a probability distribution <br /> function (PDF) of overall risk for the human receptor and exposure scenario under <br /> consideration. At present, there is no probabilistic method available for conducting a <br /> "backwards" risk assessment using this output PDF of risk, as an input value to generate <br /> cleanup or remediation levels of contaminants. One reason is because the output PDF of risk <br /> is a highly complex function that incorporates multiple covariates. These covariates cannot <br />