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1. INTRODUCTION Former Exxon Retail Site (RS) 7-3942 is a vacant site located at 4444 Pershing Avenue, Stockton, California, on the southeast corner of the intersection of North Pershing Avenue and Rosemarie Lane Three grades of gasoline and used oil were stored in four underground ' storage tanks, which were removed in July 1986 On 30 December 1993, groundwater from three monitoring wells (NMI, MW8, MW 13), four groundwater recovery wells (RW1-RW4), two vapor extraction wells (V2 and V4), and two j piezometers (P1 and P2) was sampled. Figure 1 shows the locations of all wells on and off the site except MW 11 and the locations of former site facilities Figure 2 shows the vicinity of RS 7-3942 and locations of all wells off the site Samples were analyzed for petroleum hydrocarbons 2. SUMMARY OF CURRENT STATUS Groundwater monitoring wells MW 1–MW7 were installed on site and MW8–MW 10 were installed off site between January 1987 and April 1988 RWI–RW3 were installed on site in May 1990 From January through April 1991, EA installed IW1, Pl, P2, RW4, V1--V4, MW 11, MW 12, and two vapor monitoring cluster wells on and off the site Two air sparging wells (SWI, SW2) and one off-site monitoring well (MW13) were installed in May 1993 Water is presently found at 27 feet below ground surface at the site (In 1987, groundwater was found at 18 feet below ground surface ) The inferred direction of the groundwater gradient is east to northeast at a magnitude of 0 01 Soils beneath the site consist of clayey and silty alluvial sediments to a depth of 10 to 15 feet below ground surface Between 10 to 15 feet and 35 to 40 feet the soils consist of coarser sediments—clayey sands, sands, and gravels—that are intersected by discontinuous clayey lenses ranging from a few feet to 25 feet in thickness The coarser sediments are underlain by clays and silts to an unknown depth A Shell service station and a 7-11 food and gasoline service store are located at the southwest and northwest corners, respectively, of the intersection of North Pershing Avenue and Rosemarie Lane During the December sampling event a TPH-g concentration of 50,000 gg/L was detected in MW8, which is located adjacent to the Shell site, upgradient of Exxon RS 7-3942 3. Q UARTERLY GROUNDWATER SAMPLING ' On 30 December, wells were monitored for the presence of liquid-phase hydrocarbons Water levels in each well were measured with an optical interface probe Selected wells were purged with a 2-inch pump At least four well casing volumes of purge water were removed prior to sample collection Groundwater samples were collected with disposable polyethylene bailers which had been cleaned at the factory EA field personnel rinsed the bailers three times with deionized water prior to collecting a groundwater sample The water was placed 73942Q1293 2 1