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S E C O R QUARTERLY SUMMARY REPORT Third Quarter 2004 76 (former BP) Service Station No. 11195 16500 South 1-5, Lathrop, California City/County ID#: Lathrop County: San Joaquin PREVIOUS ASSESSMENT 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 tremie 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). One 10,000-gallon, one 8,000-gallon, and one 6,000-gallon single-wall fiberglass gasoline USTs, two dispenser islands, related product lines, and one canopy were replaced with two 12,000-gallon double-wall gasoline USTs, two dispenser islands, related product lines, and one canopy in December 1998. The new USTs were installed in an excavation located immediately adjacent to the former UST complex. These excavations are hydraulically connected with pea gravel backfill material encountered in the northeast corner of the former UST pit. Soil samples collected from the capillary fringe of the former and new UST excavations at depths of 11 to 12 feet bgs contained petroleum hydrocarbons at concentrations of up to 130 ppm of TPHg, _ 50 ppm of MtBE, 290 ppm of ethanol, and 51 ppm of tertiary butyl alcohol (TBA). All of the soil