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Groundwater Monitoring Well Installation and Ozone Microsparge System installation and Startup Report— s-s> t ; ConocoPhillips(76)Service Station No. 11195,Lathrop,California May 20,2003 2.0 SITE DESCRIPTION The subject site is situated immediately east of the Louise Avenue off-ramp of Interstate 5 at the southwest corner of East Louise Avenue and Harlan Road in Lathrop, California (Figure 1). Current station facilities include two 12,000-gallon double-wall gasoline USTs, two dispenser islands, related product lines, one canopy, and a station building. Former station facilities removed during construction activities performed in December 1998 and January 1999 included 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 two canopies. Seven onsite groundwater monitoring wells (MW-1 through MW-6 and MW-10) and thirteen offsite groundwater monitoring wells (MW-8 and MW-9 plus MW-11 through MW-21) are present at and in the site vicinity. Monitoring well MW-7 was properly destroyed in 1998. In addition, three temporary groundwater extraction conductor casings were installed at the site, one in the former UST pit and two in the current UST pit, prior to backfilling. The locations of pertinent site features are shown on Figure 2. As an interim remedial measure, ConocoPhillips has been routinely extracting groundwater from the conductor casing in the former UST pit and selected monitoring wells since January of 1999. =a 3.0 GEOLOGY AND HYDROLOGY The site is situated on generally flat topography at an elevation of approximately 10 feet above mean sea level (MSL). The San Joaquin River is located approximately 1.1 miles southwest of the site and Weatherbee Lake is located approximately 2.7 miles south of the site. The static depth to groundwater in the existing monitoring wells at the site was measured at 8.40 to 11.63 feet below ground surface (bgs) on February 13, 2003. The groundwater flow direction on February 13, 2003, was toward the south at a gradient of approximately 0.001 ft/$ (Figure 3). The historical groundwater flow direction has typically been toward the south-southwest with an average gradient range of 0.001 to 0.04 ft/ft(GR 1 st Quarter, 2003). 4.0 PREVIOUS ENVIRONMENTAL INVESTIGATIONS A retail gasoline service station 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 Tosca 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 wells MW-3 and MW-4 contained concentrations of up to 10,000 parts per billion(ppb) of TPHg(Alisto, 1999). 140193.14-2 2