Vapor Intrusion Modeling, Midwest Shopping Center
The migration of chemical vapors from contaminated soil and groundwater to indoor air has been one of EPA’s top concerns since 2002. Dry cleaning establishments are a major concern for vapor intrusion (VI), because they are nearly always associated with the release of tetrachloroethene (PCE) and its breakdown products trichloroethene (TCE), dichloroethene (DCE), and often, vinyl chloride. But because of the complications in measuring indoor air directly, VI is usually investigated by measuring groundwater, soil, or soil-gas contamination, and estimating the levels in indoor air with mathematical models.
Cox-Colvin investigated a large Midwestern retail establishment that overlies a former dry cleaner. Soil contaminated with PCE, TCE, and DCE was removed prior to the store’s construction, but lower level contamination remained in surrounding soil, and the regulatory agency with jurisdiction over the site expressed concerns about the risk of VI to store occupants. The agency modeled the VI risk using the Johnson & Ettinger model, and determined that the VI risk was unacceptably high. Cox-Colvin reviewed the model, and determined that it contained inappropriate inputs. Model inputs included a default building dimensions, which would have far greater vapor intrusion than the actual building. The agency’s model used clay loam soil type but conflicting inputs to their model made it calculate vapor risk for sand, which is much higher. These and several other corrections in Cox-Colvin’s revised VI model lowered the estimated risk by more than 100 times resulting in an acceptable VI risk.