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Report on Additional Site Assessment and Sod Vapor Extraction Test 4 7-Eleven Store No 17334,4501 North Pershing Avenue Stockton, CA August 21,2002 ' 40 RESULTS 41 Geology ' The relatively level site is located at an elevation of approximately 8 feet above mean sea level According to the California Division of Mmes and Geology, the site is underlain by Quaternary alluvial ' deposits assigned to the Modesto Formation (Wagner et at , 1991) ' Recent drilling activities indicate subsurface soils are mostly interbedded silty clay, and clayey silt with minor fine-grained sand, to at least 32 feet bg Calcareous layers (caliche?)were observed at depths of about 24 feet bg in both of the SVE/AS wells Depth to groundwater as first encountered was about ' 23 feet bg 42 Results of Soil Sample Analyses Table 1 summarizes the laboratory analytical results for soil samples collected on April 3 and 4, 2002 Benzene was not detected above reporting limits in any of the samples TPH-G was detected in one of ' the soil samples (SPV-1 @ 30') at a concentration of 3 9 mg/kg MTBE was not detected in any of the soil samples Copies of the analytical results and chain-of-custody documents are included in Appendix B 43 Results of Groundwater Sample Analyses Table 2 summarizes the laboratory analytical results for groundwater sample SB-1 @ 50 feet Dissolved benzene was detected at concentrations of 28 pg/L, TPH-G at 7,100 pg/L, and MTBE was detected at ' 67 pg/L Copies of the analytical results and chain-of-custody documents are included in Appendix B 50 SVE TESTING ' On May 16, 2002, SVE testing was conducted at the site to evaluate the radius of vapor extraction influence and vadose-zone volatile organic vapor generation potential Extraction was conducted for a ' period of 3 hours on well SPV-2, 1 hour on well MW-5A, and about 3 hours on well SPV-1 Vapors were extracted from the test wells and treated prior to discharge to the atmosphere using a ' 250-cubic-foot-per-minute (cfm) thermal oxidizer, mounted on a trailer During the test, applied vacuums ranged between 30 and 50 inches of water column (IWC) Extracted flowrates ranged up to 15 cfm, and extracted vapor levels ranged up to 300 parts per million by volume (ppm[v)) as measured by a flame ' ionization detector(FID) Wells not undergoing extraction were fitted with calibrated Magnehelic gauges to check for vacuum response, results indicate the radius of vapor extraction influence is about 30 to ' 35 feet based on observed readings of 0 42 IWC in well MW-5A during extraction from well SPV-1 (located about 22 feet away) (Figure 2) (Table 3) 17334sardoc I