QUANTIFICATION OF UNCERTAINTY IN EXPOSURE ASSESSMENTS AT HAZARDOUS-WASTE SITES

Many of the risk assessment approaches being applied at hazardous wast e sites fail to adequately account for the ground-water flow and solut e transport processes (and their uncertainties) that move the contamin ant from the source to the receptor. For example, one recommended mode l assumes the contaminant concentration at the source is equal to the contaminant concentration at the receptor. A more realistic way to est imate receptor concentrations for risk assessment would be to use tech nically accepted ground-water flow equations coupled with the advectio n-dispersion equation and use Monte Carlo techniques to address parame ter uncertainty. A complex numerical model and a simple analytical mod el were used to simulate ground-water flow and solute transport from a hypothetical waste site to a subsurface drain using both small and la rger scale hydraulic properties. Results indicate that the predicted r eceptor concentration is considerably lower than the source concentrat ion when knowledge of the local hydrogeology is considered in the tran sport simulations.