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0 0 Work Plan -Former Unocal Station No. 0187 October 9, 1997 of site closure(Figure 2). During this investigation, TPHg were detected in soil samples collected at 16.5 and 31.5 feet below ground surface (bgs) from boring U-C, drilled within the former gasoline UST pit in the southeastern corner of the site, and from boring U-D, drilled adjacent to former dispenser island D-4. TPHg concentrations ranged from 1,600 to 2,900 ppm. Benzene was not detected in these soil samples. TPHg or benzene were not detected in soil samples collected from the other borings. In July 1994, GSI drilled five additional on-site soil borings (U-1 through U-M)and installed three on-site groundwater monitoring wells (U-1 through U-3) at the locations shown on Figure 2. This work was performed to assess the extent of impacted soil in the southeast corner of the site and evaluate whether groundwater beneath the site had been impacted by petroleum hydrocarbons. TPHg were detected in soil samples analyzed from borings U-J (1.7 ppm at 26.5 feet bgs) and U-K (130 ppm at 41.5 feet bgs). TPHg were detected in soil samples analyzed from borings U-3 (330 ppm at 46.5 feet bgs), U-L (19,000 ppm at 45.5 feet bgs), and U-M (5 10 ppm at 45.5 feet bgs). GSI performed a limited environmental site assessment (ESA) of the properties in the vicinity of the subject site. The ESA identified a former station located at 448 East Miner Avenue, south of the subject site. Historical groundwater data indicate that this former station is upgradient of the subject site. Environmental information related to the soil and groundwater conditions beneath this former service station were not obtained during the ESA. To further assess on-site and off-site subsurface conditions, GSI installed three additional on-site groundwater monitoring wells(U-4 through U-6)and two off-site groundwater monitoring wells (U-7 and U-8), drilled two additional on-site exploratory soil borings (U-N and U-O), and installed one on-site vapor extraction well (UV-1) in August 1995 (Figures 2 and 3). TPHg or benzene were not detected in soil samples collected from borings U-5 through U-7 or U-O. TPHg was detected in the soil sample collected from boring U-N at 25 feet bgs (1.7 ppm). TPHg and benzene were detected in the soil sample collected from well boring U-4 at 10 feet bgs (41 and 0.011 ppm, respectively). TPHg was detected in the three soil samples collected from well boring UV-1 at 25, 35, and 40 feet bgs at concentrations ranging from 3,400 to 20,000 ppm. Benzene was detected in the soil samples collected at 35 feet (99 ppm) and 40 feet bgs (15 ppm) in boring UV-1. In August 1997, GSI installed off-site groundwater monitoring well U-9 in an attempt to delineate the downgradient edge of the dissolved plume (Figure 3). Petroleum hydrocarbons were not detected in the three soil samples from the well boring that were submitted for analysis. The groundwater sample from the well contained TPHg (990 ppb), benzene (130 ppb), and MTBE (12 ppb). Quarterly monitoring and sampling of the groundwater monitoring wells began in August 1994. At the time wells U-1 through U-3 were installed in August 1994, groundwater was measured at approximately 48 feet below ground surface (bgs). Groundwater in well U-9 is currently measured at approximately 32 to 33 feet bgs (August 31, 1997), a rise of 15 to 16 feet. Groundwater flow during this period has been to the northeast. Dissolved concentrations of TPHg or benzene are detected in wells U-1 through U-4 and U-7 through U-9. Historical analytical data indicate that dissolved hydrocarbons are undefined to the northeast of wells U-3, U-4, and U-9. 4827.07-1 2