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1117 Lone Palm Avenue,Suite 201. <br /> v Modesto,California 95351 <br /> ATC 209-579-2221 <br /> Shaping the Future Fax:209-579-2225 <br /> V <br /> TPHg, ethylbenzene, toluene, xylenes, MTBE, and 1,2-DCA were detected at the site and exceeded the <br /> stated ESL values for soil as listed in Table C-2 of the Regional Water Quality Control Board San <br /> Francisco Bay Region ESLs. Concentration of MTBE exceeded the stated ESL value listed in Table C-2. <br /> Ethylbenzene was the only analyte detected at the site that exceeded its respective RSLs as established by <br /> the EPA for Region 9. None of the remaining analytes of interest were detected in soil at the site. A copy <br /> of Table C-2 from the Tier 1 ESLs and a copy of the Regional Screening Level Summary Table April <br /> 2012 is provided in Appendix A. <br /> 4.2 Tier 2 Modeliny- <br /> Cardno ATC performed modeling using site-specific data and the Johnson and Ettinger Vapor Intrusion <br /> Model for Soil Vapor Emissions to Indoor Air Risk Assessment as appropriate for Residential Exposure <br /> Scenario for High Permeability (Sandy) Soils, version 3.0 (2003) as appropriate for indoor vapor <br /> intrusion exposure scenerio for silty clay with a slab on grade foundation. The vapor intrusion modeling <br /> was performed to estimate the incremental risk from vapor intrusion to indoor air (carcinogen) and the <br /> hazard index quotient from vapor intrusion to indoor air(non-carcinogen). <br /> To provide a conservative estimate, the highest detected soil concentration of ethylbenzene, toluene, <br /> xylenes (as m-xylene), MTBE, and 1,2-DCA in the soil samples collected since 1997 was used in the <br /> Johnson and Ettinger model. From these data, the chemical specific incremental risk to vapor <br /> (carcinogen) and hazard index (non-carcinogen) values were determined. The estimated cumulative <br /> values at the site for each parameter were then determined as shown below: <br /> Incremental risk to vapor,carcinogen(soil)- Site Cumulative 5.0E-01 <br /> Hazard Index,non-carcinogen soil —Site Cumulative 8.42E+01 <br /> To estimate the risk of vapor intrusion from groundwater, the most recent concentrations of 1,2- <br /> Dichloroethane and MTBE were used in the Johnson and Ettinger model. These constituents were the <br /> only concentrations that exceeded their respective MCLs. TPHg and T-butanol were detected but it is not <br /> modeled by Johnson and Ettinger model. <br /> v Incremental risk to vapor,carcinogen(groundwater)-Site Cumulative 4.1E-07 <br /> Hazard Index,non-carcinogen(groundwater)—Site Cumulative 2.97E-04 <br /> A copy of the Johnson and Ettinger Vapor Intrusion Model output summary is provided in Appendix B. <br /> 5.0 ESTIMATED HYDROCARBON MASS VOLUME <br /> 5.1 Contaminant Mass Distribution in Soil <br /> A large percentage of the petroleum-impacted soil resides beneath the former UST system and within an <br /> area immediately surrounding. Chemicals compounds that account for the adsorbed phase contaminants <br /> include BTEX, TPHg, and TPHd. TPHg and TPHd account for the greater percentage of mass compared <br /> to the BTEX compounds. Based on available site data, it appears that residual petroleum constituents at <br /> the site resulted from the UST system. The 1987 UST closure activity and subsequent reporting <br /> S:Enviromental\22493\Reports\Eggiman Closure Report.doc 6 <br /> r <br /> V <br />