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0 0 <br /> FORMER UNOCAL STATION #0187 <br /> 437 East Miner Avenue <br /> Stockton, California <br /> BACKGROUND <br /> The subject site is a vacant lot situated on the northwest corner of East Miner Avenue and California <br /> Street in the City of Stockton, California. The subject site is situated in an area of retail stores and <br /> hotels. Topography in the site vicinity is flat-lying, at an elevation of approximately 15 feet above mean <br /> sea level (MSL). McLeod Lake and the Stockton Deep Water Channel are situated approximately <br /> one-half mile to the west. <br /> The site was an operating Unocal Service Station from 1922 until 1992. Two 1,000-gallon gasoline and <br /> one 550-gallon waste oil underground storage tanks (USTs) were removed from the southeast corner of <br /> the subject site in 1967. Data related to soil testing at the time of UST removal is not available. These <br /> USTs appear to have been replaced with two 10,000-gallon gasoline and one 280-gallon waste oil USTs <br /> situated in a common excavation in front of the station building. <br /> The most recent service station consisted of a three-bay service station building, three product dispenser <br /> islands (D-1 through D-3), and two gasoline and one waste oil USTs in a common excavation. The site <br /> is currently an unpaved vacant lot. Pertinent features of the former service station are shown on the Site <br /> Plan (Figure 2). <br /> PREVIOUS ENVIRONMENTAL WORK <br /> The two 10,000-gallon gasoline and one 280-gallon waste oil USTs, associated product piping, hydraulic <br /> lifts, and oil/water separator were removed and the station facilities demolished in January, 1992. <br /> Petroleum hydrocarbons were not detected in soil samples collected beneath the USTs, hydraulic lifts, <br /> or oil/water separator. Total Petroleum Hydrocarbons as Gasoline (TPHg) and benzene were detected <br /> in one soil sample collected from the product piping trench between the dispenser islands at <br /> concentrations of 150 and 0.81 parts per million (ppm). This trench area was overexcavated, and a <br /> confirming soil sample collected from the base of the excavation. Petroleum hydrocarbons were not <br /> detected in this soil sample. A second 130-gallon waste oil UST was removed from the eastern portion <br /> of the property in May 1992. Petroleum hydrocarbons were not detected in the soil sample collected <br /> from beneath this UST. <br /> In October 1993, GSI drilled eight on-site exploratory soil borings (U-A through U-H) in the locations <br /> of the former USTs, the former product dispenser islands, and the former hydraulic lifts in anticipation <br /> of site closure. During this investigation, TPHg were detected in soil samples collected at 16.5 and 31.5 <br /> feet below ground surface (bgs) from boring U-C, drilled within the former gasoline UST pit in the <br /> southeastern corner of the site, and from boring U-D, drilled adjacent to former dispenser island D-4. <br /> TPHg concentrations ranged from 1,600 to 2,900 ppm. Benzene was not detected in these soil samples. <br /> TPHg or benzene were not detected in soil samples collected from the other borings. <br /> In July 1994, GSI drilled five additional on-site soil borings (U-I through U-M) and installed three on-site <br /> groundwater monitoring wells (U-1 through U-3) at the locations shown on Figure 2. This work was <br /> performed to assess the extent of impacted soil in the southeast corner of the site and evaluate whether <br /> groundwater beneath the site had been impacted by petroleum hydrocarbons. TPHg were detected in soil <br />