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i i ' Indoor and Outdoor Exposure Currently, only the potential for outdoor exposure exists. Given the proximity of the sidewalk and Stanisiaus St. it is not expected that structures will be erected over the contaminated area. ' The sole exposure pathway evaluated, therefore, was volatilization from soil to outdoor air. Potential Receptor Scenarios ' Exposure to vapors b human receptors can be modeled under two scenarios residential or Po Po Y p commercial. Risk under a residential use scenario is higher than under a commercial use setting. The reasons for this are the extended daily, weekly and lifetime periods of exposure under the residential ' scenario as well as the greater health sensitivity of infants and children. ' Risk was evaluated for both commercial and residential property uses. Parameters Used in the Risk Evaluation ' Potential health risk was evaluated using the computer program RSCA Tool Kit for Chemical Releases by Groundwater Services, Inc., 1998. This program incorporates (with some ' modifications) the default chemical, toxicological, exposure, hydrogeological and physical parameters used in ASTM guidance for risk-based corrective actions (ASTM PS-104, 1998). The program also uses the transport/concentration models and equations from the ASTM document. The default parameters used by the program are intentionally very conservative (maximizing ' indicated health risk) and are designed as a screening measure. In general, more realistic parameters are substituted for default parameters where measured site characteristics are significantly different than default values. ' In our analysis, the parameters below were modified from the default values: ' Cancer Potency Factors ("Slope Factors') Of the chemicals of concern at the site, benzene is the only known carcinogen, and this was the ' controlling risk factor. For non-carcinogens, toxicity as quantified by the reference dose (Rfd) controls the acceptable exposure levels. For carcinogens, cancer potency factors (or "slope factors"), in units of mg/kg-day-' are designed to be multiplied by any particular daily dose of ' a carcinogen in units of mg/kg-day, the product of which multiplication is the incremental increase in cancer risk at that dose. For vapor inhalation, the cancer slope factor can be translated into a "unit risk factor" which describes the cancer risk accorded exposure to a 1 velieyvwlreporislrbal.txt 3 i