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Interim Corrective Action Plan—Tosco(76)Service Station No. 11195,Lathrop, California August 28,2002 2.2 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, then sold the site to Tosco in 1994 (Alisto, Additional Site Investigation Report, dated January 25, 1999). Five onsite groundwater monitoring wells(MW-1 through MW-5)were installed in December 1992. Soil samples collected from the well 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 wells 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 onsite and MW-7 offsite)were installed in September 1993. Soil samples collected at a depth of 6 feet below ground surface (bgs) were non-detect 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 non-detect for 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 trey ie pipe. The well destruction was permitted and observed by the PHS-EHD. Five offsite wells(MW-8 through MW-12)were installed and one offsite 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. Oxygenating compounds including methyl tert-butyl ether(MtBE)were not detected (Alisto, 1999). A sensitive receptor survey and a 1/2-mile radius water well survey were performed by Alisto in 1999. No basements or subways were found within a 300-foot radius of the subject site. An offsite subsurface utility survey revealed the presence of a sanitary sewer trench adjacent to the subject site that may influence the transport of petroleum hydrocarbons in groundwater. A search of California Department of Water Resources (DWR) and Banks Information Solutions files indicated that seven water wells had been installed within a r/2-mile radius of the subject site,however,the wells could not be located based upon the information provided by DWR. A visual reconnaissance of the area did not identify the presence of any additional water wells (Alisto, 1999). 140193.13 2