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1 Second Quarter 2002 Monitoring and Sampling Report and Request for Closure 2 7-Eleven Inc Store#17647, 1048 West Yosemite Avenue Manteca CA July 25 2002 ' Soil samples collected during UST removal were analyzed for BTEX, MTBE, TPH-G, other oxygenates, and lead Benzene was not detected in the soil samples Sod TPH-G levels ranged up to 1,300 milligrams per kilogram (mg/kg) at the base of the excavation, and sldewall samples contained no ' detectable TPH-G Soil MTBE levels were less than 0 02 mg/kg, and soil lead levels ranged up to 6 1 mg/kg On the basis of these findings, the SJCPHS requested further assessment ' On June 18, 1999, five push-probe drilling points (GP-1 through GP-5)were advanced to depths of about 15 feet bg (IT Corporation, August 1999), at locations adjacent to and farther from the hydrocarbon source area (former USTslpump islands) Soil samples collected during drilling contained up to 0 026 mg/kg MTBE, and no other detectable constituents Grab groundwater samples ranged up to 19,400 micrograms per liter(pg/L) TPH-G and 126 pg/L MTBE Site closure was requested on the basis of these ' findings The SJCPHS requested groundwater monitoring well installation, and a workplan for this activity was submitted (IT Corporation, October 1999) ' On December 21, 1999, three groundwater monitoring wells (MW-1, MW-2, and MW-3)were installed to 20 feet bg (IT Corporation, February 2000) The wells are screened from 10 to 20 feet bg at each ' location Soil benzene concentrations obtained during well installation were non-detectable Soil TPH-G and MTBE concentrations ranged from 0 005 mg/kg to 0 0233 mg/kg, respectively Highest soil hydrocarbon concentrations were detected at depths of 15 to 20 feet bg Only well MW-3 contained detectable dissolved hydrocarbon concentrations benzene (0 94 pg/L), TPH-G (81 pg/L), MTBE (108 pg/L), and TBA(111 pg/L) Dissolved DIPE, ETBE, and TAME were not detected in any of the samples collected ' Groundwater analytical data obtained since well installation indicate over oneY ear of low to non- detectable MTBE levels (100 pg/L) in all wells at the site, and consistently low(a3 pg/L) benzene ' concentrations as well ' The summary of site soil and groundwater analytical data verify low residual levels and dissolved plume delineation On March 20, 2002, two additional push-probe drilling points (GP-6 and GP-7)were installed to a depth of 20 feet bg (IT Corporation, April 2002), as requested by the SJCPHS to verify lateral plume delineation Groundwater samples were collected using a hydropunch at depths of 13 feet bg Sol] hydrocarbon ' (GP-6 @ 15', GP-7 @ 18') and grab groundwater(GP-6 @ 13', GP-7 @ 13') sample concentrations were non-detectable for gasoline constituents ' Geology/Hydrogeology The site is located in the central part of the San Joaquin Valley The shallow stratigraphy in the area consists of Quaternary alluvium and lake deposits The materials encountered during recent drilling at the . site generally consisted of silty fine-grained sand, grading downward, to medium-grained sand to a depth ' of about 15 to 20 feet bg, with scattered interbedded clay 21756g4 rpt