Vapor Intrusion Risk Assessment, Proposed Residential Development, San Dimas, California —
Tetra Tech performed a vapor intrusion human health risk assessment for a six acre former commercial property that was associated with a former dry cleaner and had a former gas station located in close proximity. The results of a screening level soil gas evaluation of soil gas data collected from five to 55 feet below ground surface (bgs), tetrachloroethene, benzene, ethylbenzene, naphthalene, and TPH-gas were well above residential soil gas screening levels at depth ranging from 11 to 55 feet bgs. Similarly, risk estimates based on vapor intrusion modeling using DTSC recommended soil parameters for the soil type at the site exceeded target risk and hazard levels. However, based on a review of the soil gas data, Mr. Horrigan was able to identify the presence of a low permeability soil layer within a depth range of 5 to 10 feet bgs. Based on this observation, Mr. Horrigan worked closely with the client and regulators (Los Angeles County Fire Department and Cal/EPA) to obtain approval collecting site-specific soil parameter data. Vapor intrusion modeling based on the site-specific soil parameter data resulted in in risk estimates that were below residential target risk and hazard levels and the associated risk assessment was approved by the regulators. This eliminated the need for costly mitigation/remediation efforts that would have delayed completion of the development.