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1 2. SITE BACKGROUND 2.1 SITE LOCATION AND LAND USE Former Exxon Retail Site 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 (Figure 1) Three grades of gasoline and used oil were stored in four underground storage tanks, which were removed in July 1986 2.2 SITE HISTORY ' From January 1987 to April 1988, 10 groundwater monitoring wells (MW 1—MW 10) were installed on and off the site, monitoring wells MW 1—MW7 were installed on the site and ' MWS—MW 10 were installed off the site Groundwater recovery wells RW 1—RW3 were installed on the site in May 1990 From January through April 1991, EA installed IW 1, P1, P2, RW4, VI—V4, MW 11, MW 12, on and off the site The locations of all wells except offsite well MW 11 are shown in Figure 2 In February 1997, water was found approximately 17-22 feet below ground surface (bgs) The ' approximate groundwater flow direction at the site in February 1997 was to the northeast, with a gradient of approximately 0 007 Soils consist largely of clay and silt down to 46-50 feet bgs, 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 In the past, liquid-phase hydrocarbons (LPH) have been found in wells MW3, MW6, RW3, V1, V2, and V4, as documented in quarterly groundwater monitoring reports In December 1992, passive skimmers were installed in wells RW3 and VI to recover LPH During past quarterly ' groundwater sampling events, petroleum hydrocarbons have been detected in groundwater samples collected from all wells except MW 11 and MW 12, which are the most-downgradient wells and are located off the site Historical gauging data and analytical results are provided in ' Table 1 (EA 1997) ' A Shell service station and a 7-11 food and gasoline service store are located on the southwest and northwest corners, respectively, of the intersection of North Pershing Avenue and Rosemarie Lane Previous laboratory analyses have consistently identified Total Petroleum Hydrocarbons ' as gasoline (TPH-g) and benzene, toluene, ethylbenzene, and total xylenes (BTEX) in groundwater samples collected from MW8 and MW9, which are located adjacent to the Shell site and the 7-11 site, upgradient and crossgradient of former Exxon RS 7-3942 Soil and groundwater investigations have been conducted at the Shell site, which is upgradient of former Exxon RS 7--3942 In October 1990, EA submitted a Combined Work Plan to Remediate Soil and Groundwater (EA 1990) In this plan, EA proposed to remediate groundwater using conventional technology available at the time a groundwater extraction, treatment, and reinfection system (GETS) Pursuant to the 1990 plan, EA installed the required wells for this remedial approach, but the WP61 73942 R01397 3 3