ARCHIVED REPORTS_REPORT ON SOIL VAPOR SAMPLING AND HUMAN HEALTH RISK ASSESSMENT DEC 2010

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AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment 3-1 3.0 Vapor Intrusion Modeling and Risk Analysis Soil vapor data from the October 2010 sampling event was evaluated to determine the potential risk associated with the vapor intrusion pathway. As discussed in Section 2.0,the October 2010 data were determined to be of sufficient quality for use in the risk assessment,while the May and September 2010 data were not. However, as a conservative worst-case evaluation, soil vapor data from all three sampling events at the site (May, September, and October 2010)were used to evaluate the worst-case potential risk associated with the vapor intrusion inhalation pathway as part of an uncertainty assessment There are currently two existing on-site buildings, a restaurant(Macaroni Grill [former Wards])and a bank(former Unocal). These buildings are used for commercial purposes and land uses are expected to remain commercial in the foreseeable future.Therefore, a commercial/industrial worker exposure scenario was evaluated.Although it is not expected that the Site will be developed for residential use, a residential exposure scenario was also evaluated as a hypothetical future land use scenario. Figure 2 shows the site plan with soil vapor sampling locations SV-1, SV-2, and SV-3.These locations correspond to areas with the highest potential residual soil hydrocarbon impacts, based on locations with respect to former fuel storage and delivery systems.The analytical soil vapor data is provided in tables presented in Section 2.0. The potential vapor intrusion pathway was evaluated using a tiered or stepwise approach, in accordance with Chevron's Best Practices approach (Chevron, 2006), DTSC guidance (DTSC, 2005), and USEPA guidance (USEPA,2002). For scenarios with sufficient risk,the approach consists of first comparing site concentrations to screening levels in order to select compounds of potential concern (COPCs),followed by indoor air modeling, and an estimate of potential risk, as needed. A preliminary risk evaluation of the May 2010 results was conducted and submitted to the RWQCB on July 15, 2010. As a result of that evaluation, apparent detected concentrations of TPHg and benzene, and reporting detection limits for naphthalene, were above the associated CHHSLs and/or ESLs. Therefore,these compounds were selected as COPCs for further evaluation and estimation of potential risk/hazard using the USEPA Johnson and Ettinger(JE)Model spreadsheets. Concentrations of naphthalene (conservatively evaluated at the reporting detection limit)and benzene did not result in a potential risk to residential or commercial industrial building occupants above target risk thresholds. Concentrations of TPHg did not result in a potential risk to commercialfindustrial building occupants above target risk thresholds. However, concentrations of TPHg resulted in a potential hazard above target levels for the hypothetical future residential exposure scenario. [It should be noted that current and likely future land use conditions will be commercial/industrial.] As discussed in Section 2.0,further analysis of the soil vapor samples show that the majority of the components identified as TPHg are instead relatively short-chain aliphatic compounds, and that the more toxic aromatic compounds are not present. Therefore,the preliminary risk evaluation of the May 2010 soil vapor results,submitted in July 2010,was overly conservative because it assumed the presence of some toxic compounds that were not detected. Additional data from the September and October 2010 sampling events, including the TICS and carbon ranges analyzed by APH methods, are December 15,2010 60146576 M10