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4.0 VAPOR INTRUSION ASSESSMENT Ground Zero evaluated health risk due to vapor intrusion under potential future site conditions on the assumption that a commercial/industrial structure could be erected over the area of contamination. Due to zoning restrictions and area-wide property use it was felt that a future residential development was highly unlikely. Only volatile contaminants present at the site were evaluated (TPHg, benzene, and MTBE). Vapor intrusion from TPHd as well as Oil and Grease were not evaluated since this carbon fraction range is non-volatile and inhalation will not be a relevant exposure pathway (TPHCWG, 1997). Contaminant concentrations were initially compared with ESLs. ESLs are concentrations expected to result in an insignificant health risk (cancer risk 10-6, cumulative hazard quotient less than 1) under nearly any site condition. The RWQCB derived the published ESL concentrations by modeling vapor intrusion using the Johnson and Ettinger (J&E) model. Physical parameters entered into the model such as building size, air exchange rate, soil permeability, etc. were intentionally very conservative which tends to maximize the calculated vapor flux and the resulting indoor air concentration. One of the most conservative features of the J&E modeling is the assumption that the soil or groundwater beneath the entire building footprint is contaminated. Environmental screening levels, then, commonly overstate risk and contaminant concentrations that do not exceed ESLs can, in almost all circumstances, be regarded as posing no significant health risk. Where ESLs were not available for the exposure pathway or if the ESLs were exceeded, the J&E model was used to predict vapor flux, indoor air concentration and health risk. 4.1 Groundwater Sourced Vapor Intrusion 4.1.1 Groundwater Sourced Vapor Intrusion —UST A No constituent has been detected above ESLs in MW-A. There is no risk to human health or the environment from groundwater sourced vapor intrusion. 4.1.2 Groundwater Sourced Vapor Intrusion —UST B TPHg and Benzene were previously detected in MW-B. Significant soil mass has been removed and no constituent has been detected above ESLs in MW-BR since soil excavation. There is no risk to human health or the environment from groundwater sourced vapor intrusion. 4.1.3 Groundwater Sourced Vapor Intrusion —Bunker Benzene was not detected in any soil sample collected from the Bunker locations. A groundwater sample collected on October 21, 2003 indicated that benzene was below ESLs (Minney, 2010). There is no risk to human health or the environment from groundwater sourced vapor intrusion. - 15 -