Evaluation of Uncertainty in a Site-Specific Risk Assessment

An evaluation was performed of the uncertainty in predicted carcinogenic risk resulting from uncertainty in site properties and fate and transport predictions for a simple contaminated soil site. SoilRisk, a risk model for organic contaminants in soil, was applied to a case-study site consisting of sandy soil and a thin, near surface, unconfined aquifer. The site-related parameters found to affect predicted risk most significantly were the soil-water volumetric flux rate (J{sub w}) in the unsaturated zone, the longitudinal dispersivity ({alpha}{sub L}) and the Darcy velocity (V{sub d}) in the saturated zone, and the soil organic carbon fraction in both zones (foc, foc{sub sat}). Model runs using probability distribution functions (PDFs) for these input parameters yielded cumulative distribution functions (CDFs) for the total risk estimates, the shape and location of which depended on the chemical and exposure scenario. In general, uncertainty in risk at the case-study site was found to be greater for the more mobile and less degradable of the chemicals studied [trichloroethylene (TCE) and chlordane] than for benzene, which is highly degradable, and benzo[a]pyrene (BaP), which is very immobile in the subsurface.