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Third Quarter 2002 Monitoring and Sampling Report 2 • 7-Eleven, Inc, Store#21756,853 E Yosemite Avenue,Manteca CA October 21, 2002 None of the sod samples contained detectable levels of BTEX, TPH-G, or TPH-D, soil MTBE levels ranged up to 0 18 milligrams per kilogram (mglkg), in the northwest corner of the UST pit at a depth of about 14 feet bg ' Initial site assessment was conducted in June 1999(IT Corporation, 1999)and consisted of the installation of five push-probe borings to total depths of up to 25 feet bg Soil samples collected during drilling contained up to 0 167 mg/kg MTBE. and no detectable benzene or TPH-G Grab groundwater isamples collected at the base of the borings contained up to 607 micrograms per liter(ug/L) benzene, 32,200 ug/L TPH-G, and 46,300 ug/L MTBE (the highest concentrations were observed in boring GP-3, located near the western end of the former dispenser island location) On December 20, 1999, three groundwater monitoring wells (MW-1 to MW-3)were Installed to depths of ' 25 feet bg, and screened from 10 to 25 feet bg (IT Corporation, February 2000) Soil samples were collected from each borehole from depths of 5 feet bg to total depth Soil benzene and TPH-G were not detected Sod MTBE was found only in one location (GP-4) at a concentration of 0 167 mg/kg Groundwater analytical data Indicated dissolved concentrations of benzene (up to 0 32 ug/L), TPH-G (up to 55 ug/L), MTBE(up to 230 ug/L), and TBA(3,710 ug/L) The reported TBA detection appears to be anomalous, as TBA has not been detected in subsequent quarterly samples The MTBE plume was defined downgradient of the hydrocarbon source area (the former LIST and pump island locations) A workplan for additional site assessment and feasibility testing (IT Corporation, October 2001)was reviewed and approved by the San Joaquin County Public Health Services/Environmental Health Department(SJCEHD) On April 1-3, 2002, monitoring well MW-4 was installed to 30 feet bg and screened from 15 to 30 feet bg (Shaw, August 2002) Dual-nested spargelvent wells (SPV-1 and SPV-2) were advanced to 29 feet bg, and screened from 24 to 29 feet bg as a sparge well and 12 to 22 feet bg as a soil vapor extraction (SVE)well A Geoprobe soil boring (GP-6)was advanced to a depth of 40 feet bg Soil samples collected from GP-6 and MW-4 were non-detectable for BTEX, TPH-G, and MTBE/oxygenates A Simulprobe grab groundwater sample from GP-6 was also non-detectable for all analytes tested An SVE test was conducted on April 24, 2002, by application of vacuum to well SPV-1 Magnehelic gauge measurements of wells MW-1, MW-2, MW-3, and SPV-2 Indicated an estimated radius of vapor extraction influence of 25 to 30 feet around the extraction point Test findings will be re-evaluated by inclusion of additional wells in a follow-up test, to be conducted during the fourth quarter 2002 Test methodology will be the same as was conducted previously, but more wells will be tested (i e , SPV-2 and MW-1) i 1