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X. 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 22 August 1993, the 4 groundwater monitoring wells (MW 1, MW8, MW 10, MW 13), three groundwater recovery wells (RW1-RW3), two vapor extraction wells (V2 and V4), and two piezometers (P1 and P2) were sampled The samples were analyzed for petroleum ' hydrocarbons Figure 1 shows the locations of all wells on and off the site (except MW11), and the locations of former site facilities Figure 2 shows the vicinity of RS 7-3942 and locations of all wells off the site 1 2. SUMMARY OF CURRENT STATUS ' Groundwater monitoring wells MW1–MW7 were installed on the site and MW8–MW10 were installed off the site between January 1987 and April 1988 RW1–RW3 were installed on the ' site in May 1990 From January through April 1991, EA installed IW 1, P1, P2, RW4, Vl–V4, MW11, MW12, 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 store that dispense gasoline are located at the respective southwest and northwest corners of the intersection of North Pershing Avenue and Rosemarie Lane Soil and groundwater investigations have been conducted at the Shell site TPH-g at concentrations up to 120,000 pg/L has been found in samples collected from MW8, which is located on the sidewalk adjacent to the eastern boundary of the Shell site, upgradient of Exxon RS 7-3942 I 3. QUARTERLY GROUNDWATER SAMPLING On 22 August, wells were monitored for the presence of liquid-phase hydrocarbons and depths to water were measured with an optical interface probe Selected wells were purged with a vacuum truck and dedicated PVC piping that had been steam-cleaned before being used in the wells At least four casing volumes of water were removed before samples were collected Groundwater samples were collected with disposable polyethylene bailers that had 73942/Q893rrX] I