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I 1• INTRODUCTION Former Exxon Retail Site (RS) 7-3942 is a vacant lot located at 4444 North Pershing Avenue, Stockton, California, on the southeast corner of the intersection of North Pershing Avenue and Rosemarie Lane The lot has been vacant since July 1986, when four underground storage ' tanks, used to store gasoline and used oil, were removed (Figure 1) On I 1 and 12 May 1993, one groundwater monitoring well (MW 13) and two air-sparging wells (SWI, SW2) were constructed on and off the site as shown in Figures 1 and 2 Six soil samples (two from each borehole) were collected and analyzed for petroleum hydrocarbons On 18 May 1993, the groundwater samples were collected from four groundwater monitoring wells (MW1, MW8, MW10, and MW13), four groundwater recovery wells (RW1—RW4), two ' piezometers (Pl, P2), and one vapor extraction well (V4) The samples were analyzed for petroleum hydrocarbons 2. SUMMARY OF CURRENT STATUS ' Groundwater monitoring wells MW 1—MW7 were installed on the site and MW8—MW 10 were installed off the site between January 1987 and April 1988 RWl—RW3 were installed on the site in May 1990 From January through April 1991, EA installed IW1, P1, P2, RW4, V1—V4, MW 11, MW 12, and vapor monitoring cluster wells, on and off the site Water is presently found approximately 28-30 feet below ground surface, in 1987, groundwater was found 18 feet below ground surface The groundwater hydraulic gradient in the area is to the northeast at an approximate magnitude of 0 01 Soils consist largely of clay and silt down to 46-50 feet below ground surface, the clay and silt are interrupted locally by lenses of sand, clayey sand, silty sand, and gravel Some of the lenses of coarse sediment are more than 5 feet thick Below 46-50 feet, the soil is a well-cemented dry, silty clay Liquid-phase hydrocarbons have been found in wells MW3, MW6, RW3, V1, and V4 ' Petroleum hydrocarbons have been found in groundwater samples collected from all wells except MW 11 and MW 12, winch are the most-downgradient wells and are located off the site (Figure 1) A soil vapor and groundwater extraction and treatment system has been designed to remediate ' the soil and groundwater, catalytic oxidation will be used to abate petroleum hydrocarbons in the extracted soil vapors and in the effluent air stream from air-stripping of the groundwater Aur sparging will be implemented with groundwater extraction to initiate the removal of ' hydrocarbons from the groundwater and from soil beneath the water table When groundwater extraction is implemented, the intent is to reinject treated groundwater along the northwest border of the site to prevent hydrocarbons from being drawn across Pershing Avenue (see results from MW8), but the application for this action is awaiting approval 8373942(2yRoi6gyrx 3 1