Estimation of contaminant subslab concentration in petroleum vapor intrusion

In this study, the development and partial validation are presented for an analytical approximation method for prediction of subslab contaminant concentrations in PVI. The method involves combining an analytic approximation to soil vapor transport with a piecewise first-order biodegradation model (together called the Analytic Approximation Method, including Biodegradation, AAMB), the result of which calculation provides an estimate of contaminant subslab concentrations, independent of building operation conditions. Comparisons with three-dimensional (3-D) simulations and another PVI screening tool, BioVapor, show that the AAMB is suitable for application in a scenario involving a building with an impermeable foundation surrounded by open ground surface, where the atmosphere is regarded as the primary oxygen source. Predictions from the AAMB can be used to determine the required vertical source-building separation, given a subslab screening concentration, allowing identification of buildings at risk for PVI. This equation shows that the "vertical screening distance" suggested by U.S. EPA is sufficient in most cases, as long as the total petroleum hydrocarbon (TPH) soil gas concentration at the vapor source does not exceed 50-100mg/L. When the TPH soil gas concentration of the vapor source approaches a typical limit, i.e. 400mg/L, the "vertical screening distance" required would be much greater.