GROUND-WATER MODELING FOR RISK ASSESSMENT PURPOSES - USE OF A GUASSIAN-DISTRIBUTED TRANSPORT MODEL AND A BATCH FLUSH MODEL

Several state regulatory agencies require that risk assessments be performed at contaminated sites to quantify the contaminant levels that may reach the nearest receptors and to evaluate the health risks associated with exposure to such contaminant levels. Petroleum retail facilities with leaking underground storage tanks are prime examples of sites where risk assessments may be required. Ground water modeling is often necessary when evaluating future completed exposure pathways associated with dissolved contaminants. If the aquifer conditions are relatively homogeneous, analytical contaminant transport models may be used in place of complex numerical models. A two-dimensional, Gaussian-distributed boundary condition transport model and a batch flush model may be combined to simulate contaminant transport with linear equilibrium adsorption/desorption and biodegradation effects. This enables a relatively rapid and cost-effective prediction of contaminant levels at sensitive downgradient receptors. The predicted contaminant levels may then be evaluated through an exposure assessment, toxicity assessment, and risk characterization in accordance with U.S. EPA risk assessment protocols. A case study was presented for a gasoline station with a leaking underground storage tank system. The two-dimensional, Gaussian-distributed transport model and batch flush model were used to predict dissolved phase contaminant concentration profiles at a downgradient drinking water well. The predicted contaminant concentrations were subsequently converted to chronic daily intakes which were evaluated in terms of toxicity. Reasonable maximum exposures were developed for both carcinogens and non-carcinogens. The results of the risk assessment indicated that there were no unacceptable risks posed by the residual ground water contamination at the site. This led to the recommendation that the project be closed out in the absence of a demonstrated need for active ground water remediation. The State Water Control Board concurred with the recommendation.