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r Additional Subsurface Investigation And Well Installation Report 8 1,6 The Southland C Store#19976, 1399 N Main St Manteca California November 15 1998 ' Approximately 28 gallons of purge water generated during this sampling event will be transported from the site during the fourth quarter 1998 Waste documentation will be forwarded once received ' 40 RESULTS ' 4.1 Geology and Hydrogeology The site is located in the middle of the northern San Joaquin Valley Shallow stratigraphic units in the vicinity of Manteca are mapped as Quaternary dune sand overlying the Quaternary Modesto Formation which consists dominantly of arkosic alluvium (California Division of Mines and Geology, 1991, Geologic Map of the San Francisco-San Jose Quadrangle, 1250,00) The materials encountered dunng recent drilling at the site consisted of sand and silty sand to approximately 10 feet BGS underlain by clayey silty and silt to approximately 25 feet BGS and fine to coarse sand, clayey silt and clayey silty sand to approximately 50 feet BGS (Appendix A) Static groundwater level in the bonngs was approximately 14 feet BGS Groundwater flow, as mapped from data collected on September 16, 1998,is to the northeast at a hydraulic gradient of 0 02 foot per foot(Figure 3) Capillary fringe soils are generally low permeabildy,while the vadose-zone is moderately transmissive 4.2 Results of Sod Sample Analyses ' Table 1 summarizes the laboratory analytical results for soil samples collected on September 2 and Septernber 3, 1998 The highest TPH-G concentration was detected in sample MW-1@45'at 17,000 mg/kg The highest benzene concentration was detected In sample MW-2@25' at 0 0708 mg/kg and highest MTBE concentration was detected in sample MW-1@20'at 0 0829 mg/kg While TPH-G concentrations in samples from MW-1,which Is located directly downgradient of the former USTs, are high at 45 feet BGS, BTEX and MTBE concentrations are below detection limits and TPH-G, BTEX and MTBE concentrations at 40 feet BGS frorn GP-1 (upgradienttcrossgradient), 45 feet BGS from GP-2 r (downgradient) and GP-3 (center of plume) and 30 feet BGS in MW-2 (upgradienttcrossgradlent) and MW-3(crossgradient) are non-detectable to very low Based on the BTEX,TPH-G and MTBE ' concentrations in soil samples the lateral and vertical limits of the hydrocarbon plume have been delineated and do not appear to pose a threat to local utilized groundwater Soil impacted with very low ' 19876asrrpt _ FLUOR DANIEL GTI