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 41 4.0 Conclusions and Recommendations 4.1 summary of Findings Soil vapor sampling evaluated the distribution of petroleum hydrocarbon impacts in soil vapor at the Site during May, September and October 2010 sampling events.A summary of the sample analysis results is provided below. Benzene was detected in two vapor samples(SV-1 and SV-3)during the May event at concentrations of 49 and 14 pg/m3, respectively. Benzene was detected in vapor sample SV-1 and SV-1 (Dup)during the September event at concentrations of 62 Pg/M3 and 67 pg/m3, respectively.These concentrations are below commercial/industrial CHHSLs. No benzene was detected during fl1e October event. Toluene was ddected in one vapor sample(SV-3)during the May event at a concentration of • 38.0 pg/m3. This concentration is below commercial /industrial CHHSLs for this compound. No toluene was detected during the September and October events. • Total xylenes were detected in one vapor ample(SV-3)during the May event at a concentration of 11.7 pg/m3 This concentration is below commercial/industrial CHHSLs for this compound. No xylenes were detected during the September and October events. • Ethylbenzene was not detected at or above reporting limits in any of the collected vapor samples from the May, September, and October events. • Naphthalene was not detected at or above reporting limits in any of the collected vapor samples from the May, September, and October events. However,the laboratory reporting detection limits for SV-1 were above the associated CHHSLs during the May and September events due to the vacuum remaining in the Summa canister. TPHg was detected at concentrations of 470,000 pg/m3, 1,000,000 pg/m3, and 17,000 pg/m3 at • SV-1 during the May, September, and October events, respectively.Analysis of TICS(for May, September, and October events)and APH (October event)revealed that detected constituents were less toxic than gasoline and indicated that vapors from these detections were not a human health concern. • Atmospheric gas analytical results were consistent with those typical of soil vapor samples. • Helium was not detected at significant levels in any of the samples from the May and October events, indicating that there was no significant leakage of ambient air into the samples and that the vapor samples were representative of the soil vapor conditions. Due to laboratory error, atmospheric gases and helium were not analyzed during the September event. 4.2 Conclusions Based on the results of the additional evaluation of the May and September data, the uncertainty associated with the October evaluation was eliminated. Risk evaluation results for each of the three events shows no elevated risk to human health.The vapor intrusion risk evaluation indicates that the potential ELCR and HI estimated for a residential scenario and a commercial/industrial worker scenario are below the CalEPA and USEPAs target risk range and target HI level. These results represent a worst-case scenario evaluation. Additionally,the commercial use of the property represents an additional safety factor, as no risk was determined for a residential scenario. Based on December 15,2010 6014657 -M16