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Report on Additional Site Assessment and Soil Vapor Extraction Testing 4 7-Eleven Store No 20632 4627 Da Vinci Drive Stockton,CA August 14 2002 42 Results of Sod Sample Analyses Table 1 summarizes the laboratory analytical results for soli samples collected on April 8 and 9, 2002 Benzene and TPH-G were detected above reporting limits in only one soil sample (SPV-1 @ 10') at a concentration of 0 035 mglkg and 13 mg/kg, respectively MTBE was detected in nine soil samples (Table 1), with the highest concentration in sample MW-7 @ 15 feet at 2 4 mg/kg TBA was detected in five soil samples (Table 1)with the highest concentration in sample SPV-1 @ 15 ft at 50 mg/kg TBA detections may indicate ongoing intrinsic biodegradation, as TBA is a degradation product of MTBE Other than MTBE and TBA, no oxygenates were detected above reporting limits in the soil samples analyzed Copies of the analytical results and chain-of-custody documents are included in Appendix B 43 Results of Groundwater Sample Analyses Table 1 summarizes the laboratory analytical results for Hydropunch groundwater samples collected from boring SB-1 on April 8, 2002 Benzene and TPH-G were not detected above reporting limits in either of the samples Dissolved MTBE was detected in both samples SB-1 @ 30 ft and SB-1 @ 40 ft at concentrations of 4,300 µg/L and 5,700 µg/L, respectively TBA was detected in one of the groundwater samples (SB-1 @ 30 feet) at 2,800 µg/L TBA detections may indicate intrinsic biodegradation, as TBA is a degradation product of MTBE Other than MTBE and TBA, no oxygenates were detected above reporting limits in the samples analyzed Copies of the analytical results and chain-of-custody documents are included in Appendix B so SVE EVALUATION aOn April 22, 2002, a brief SVE evaluation was conducted at the site, to determine the radius of vapor Y extraction influence and evaluate h drocarbon/oxygenate vapor production (based on field instrument data) within the vadose zone Applying vacuums of up to 50 inches of water column (IWC) using a 250-cubic-feet-per-minute (cfm) mobile (trailer-mounted) thermal oxidizer, wells MW-4R, MW-7, and SPV-2 were tested briefly (approximately 1 hour each), followed by completion of a 3 5-hour test on well SPV-1 (Appendix Q Prior to initiating the test, sampling ports on wells MW-1, MW-2, and test wells not undergoing extraction were connected to calibrated magnehelic gauges to monitor pressure changes Extracted fiowrates from SPV-1 measured during the test ranged up to 62 cfm, with extracted vapor concentrations (measured using field instruments) ranging up to 95 parts per million by volume (ppm[vl) The proximity of well SPV-1 to the UST pit (and expected communication with UST backfill gravel) did not result in higher flow rates from this well Air sparging into the AS segment of the test well did not increase concentrations in the vadose zone (although dissolved oxygen levels increased from less than 0 5 milligrams per liter[mg/Ll to 2 4 mg/L) Based on vacuum responses of up to 0 02 lWC in the nearest observation wells (located 28 feet from the extraction point), the radius of vapor extraction influence is estimated at 10 feet (Figure 2) (Table 3) 20632sardoc