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Subsurface Investigation Report-Tosco(Former BP)Service Station No. 11195 Lathrop,California 41� Aril 11,2002 The static depth to groundwater in the eleven existing monitoring wells at the site was measured at 6.40 to 9.79 feet bgs on February 7, 2002. The groundwater flow direction on February 7, 2002 varied with a gradient range of 0.006 to 0.01 feet/feet(ft/ft). The historical groundwater flow direction generally varies at an average gradient of 0.001 ft/ft. 2.3 Previous Environmental Work A retail gasoline outlet has been located at the subject site since at least 1983. BP Oil Company(BP)acquired the site from Mobil Oil Corporation in 1989 and sold the site to Tosco in 1994 (Alisto, Additional Site . Investigation Report, dated January 25, 1999). Five on-site groundwater monitoring wells (MW-1 through MW-5) were installed in December 1992. Soil samples collected from the borings were reported to contain petroleum hydrocarbons at concentrations of up to 120 parts per million (ppm)of Total Petroleum Hydrocarbons as gasoline(TPHg),and 16 ppm of benzene. The initial groundwater samples collected from MW-3 and MW-4 contained concentrations of up to 10,000 parts per billion(ppb)of TPHg(Alisto, 1999). Two additional wells (MW-6 on-site and MW-7 off-site) were installed in September 1993. Soil samples collected at a depth of 6 feet below ground surface(bgs)were reported as not detected for petroleum hydrocarbons (Alisto, 1999). One 550-gallon fiberglass waste oil UST and associated remote fill line were removed in 1997. One soil sample was collected from the bottom of the waste oil UST excavation at 10 feet bgs. A second soil sample was collected = from beneath the remote fill line at 3 feet bgs. Both samples were reported as not detected for all petroleum hydrocarbons(GeoStrategies, Waste Oil UST and Remote Fill Line Soil Sampling Report, dated August 6, 1997). In July 1998, an abandoned water well, located north of the former UST pit adjacent to Louise Avenue, was properly destroyed. The well was destroyed by perforating the steel casing and grouting the well with neat cement placed by the use of a grout pump and tremie pipe. The well destruction was permitted and observed by the PHS- EHD. Five off-site wells (MW-8 through MW-12) were installed and one off-site well MW-7 was destroyed in September 1998. Soil samples collected during the investigation at depths of 4 to 6 feet bgs contained petroleum hydrocarbon concentrations of up to 1.5 ppm of TPHg and 0.0034 ppm of benzene. Five oxygenate compounds or methyl tert-butyl ether(M(BE)were not detected in any soil samples (Alisto, 1999). A sensitive receptor survey and a'h-mile water well survey were performed by Alisto. No basements or subways were found within a 300-foot radius of the subject site. An off-site subsurface utility survey revealed the presence of a sanitary sewer trench adjacent to the subject site which may influence the transport of petroleum hydrocarbons in groundwater. A search of California Department of Water Resources(DWR)and Banks Information Solutions files determined that seven water wells had been installed within 1/2-mile of the subject site, but only two could be definitively located. A visual reconnaissance of the area did not identify any additional water.wells(Alisto, 1999). 140193.11-2 2