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Groundwater Monitoring Well Installation and Ozone Micros ar e System Installation and Startup Report— ConocoPhillips(76)Service Station No. 11195,Lathrop,California May 20,2003 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 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 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. Oxygenating compounds including methyl tert-butyl ether (MtBE) were not detected(Alisto, 1999). A sensitive receptor survey and a '/2-mile radius 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 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 '/2-mile radius 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). 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 s. TPHg, 50 ppm of MtBE, 290 ppm of ethanol, and 51 ppm of tertiary butyl alcohol (TBA). All of 140193.14-2 3