NOVEL APPROACH TO THE USE OF GEOLOGIC AND SOIL-GAS SAMPLING DATA IN RISK ASSESSMENT—A CASE STUDY USING AN ADVECTIVE-DIFFUSIVE EMISSION MODEL

A former manufacturing facility in northeast Ohio reportedly used a fence line area as a temporary drum accumulation area that is located proximal to a residential area. The compounds detected above groundwater cleanup levels include cis-1,2-dichloroethene and vinyl chloride. The horizontal extent of these two constituents was considered to be limited, based on knowledge of previous groundwater sampling results, known low water yields, low groundwater seepage rate, and no known users of the shallow water. The groundwater direct exposure pathway was considered to be incomplete and did not represent a risk to human health; however, potential vapor migration from the groundwater into a basement was determined to be of concern due to the proximity to a house.

An advective-diffusive emission model, the Johnson and Ettinger (1991) Infinite Source Model, was selected for use as a groundwater risk-screening tool because the model is capable of predicting indoor air concentrations resulting from subsurface emissions. Although the one-dimensional model was developed for steady-state sources with negligible chemical and biological transformations only, the model makes reasonably conservative predictions of the indoor air concentrations if appropriate site-specific surface and subsurface information is used. Direct measurements of the soil-gas and groundwater vapor concentrations using soil-borings and flow regulated summa canisters were conducted to address the limitations of the model, and were considered to be a measure of the aggregate attenuation affects of the in-situ processes occurring at the site.

Using the site-specific information for a hypothetical residence at the fenceline of the manufacturing facility, the Johnson and Ettinger Infinite Source Model predicts that the worst-case exposure concentrations of cis-1,2-dichloroethene and vinyl chloride in a residence would not exceed the Ambient Air Risk-Based Screening Concentrations set forth by the U.S. EPA Region 3 and Region 9.