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17 June 2008 AGE-NC Project No. 99-0637 Page 6 of 7 The only other fuel additive to be detected in the ground water samples collected from the site between April 2001 and March 2008 is TBA. TBA was detected on 12 August 2003 at a concentration of 10 µg/l in well MW9.Analytical results of ground water samples are summarized in Tables 9 and 10. TPH-g was detected in two of the stockpile soil samples collected during UST removal activities, at a maximum concentration of 4.8 mg/kg. TPH-g and benzene were not detected in any of the additional soil samples collected and analyzed at the site since assessment activities began. Only toluene and MTBE in very low concentrations were detected in a total of fourteen soil samples collected at the site.The lateral and vertical extent of impacted soil has been defined.Cross-sections depicting the general lithology in the soil beneath the 'site are illustrated on Figures 5, 6, and 7. Boring logs and well construction diagrams are presented in Appendix C. i i 6.0. ENVIRONMENTAL RISK ASSESSMENT Toluene was detected in the soil samples collected at 15 feet bsg from wells MW-1 and MW-3, at j concentrations as high as 0.025 mg/kg; Toluene was also detected from the soil sample collected at 10 feet bsg in well MW-4, at a concentration 0.01 mg/kg. No other analytes were detected above laboratory reporting limits in the remaining soil samples(collected from the site at 10 and 15 feet bsg (Table 3). I Environmental risk assessment of potential soil vapor intrusion concern to indoor air from volatile organic compound(VOC)-impacted soil was conducted following the San Francisco Bay Regional Water Quality Control Board (SFBRWQCB)-prepared Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater,dated November 2007,the SFBRWQCB guidance document presents environmental screening levels (ESLs). Analytical data for soil collected from 10 and 15 feet bsg during the various phases of site investigation were compared to the ESLs to assess the potential soil vapor intrusion concern. The laboratory reported detection limit was used for soil samples that were non-detect.ESL values were not exceeded when compared to the soil data collected from 10 and 15 feet bsg at the site. i 7.0. CONCLUSIONS AND RECOMMENDATIONS Based on data collected during site assessment,remediation activities;and ground water monitoring events, AGE concludes that the site is a candidate for closure. AGE recommends the following: I • No further subsurface investigation or mitigation at this time. • Consideration of the UST investigation for closure and concurrence with the CVRWQCB. Advanced GeoEnvironmental,Inc. I {