Benzene Vapor Transport Measurement and Modeling to Evaluate Remedial System Performance and Predict Ambient Air Exposure

At a hydrocarbon remediation site in the southeastern United States, direct measurements of vapor flux and ambient air monitoring were used successfully to evaluate performance of an air sparging system and predict potential exposure to benzene in the surrounding neighborhood.  A civil action filed against the owner of the facility for exposure to benzene allegedly resulting from off-gassing from contaminated groundwater and surface water necessitated air modeling of benzene from various potential sources in the area.  Source terms for the original model were developed by calculations from fixed sources (such as SVE exhaust, tanks, pipes and fittings) and area sources from surface water concentrations.  Three rounds of ambient air sampling were conducted over a period of several months and predicted values were compared to measured values at receptor sites.  The model provided excellent correlation and conservative predictions based on these results.  Furthermore, measured benzene values compared favorably with EPA data or rural background levels.

            Vapor flux monitoring was conducted using EPA’s approved flux chamber technology to directly measure benzene emission in areas overlying contaminated groundwater and surface water.  These values were substituted for calculated source values used in the initial air dispersion model and improved the agreement with measured values.  Furthermore, flux measurements were used to evaluate the changes in air flow rates in the air sparging system to optimize remedial system performance and to address complexities introduced by complex hydrogeology.

            The use of these tools resulted in a successful settlement of the civil action and improvement in the characterization of the groundwater’s response to changes in remedial system modifications.  Results were unambiguous in confirming that the site did not result in above-background exposure to benzene or other hydrocarbons off gassing from the groundwater plume or operation of the remedial system at the site.