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Regional drainage is toward central Stockton, immediately north of the site. The nearest body of ' water is the Mormon Slough approximately 0.6 miles north of the site. Duck Creek and �- Homestead Canal are both south and southeast of the site by approximately 1.14 miles. Depth to groundwater in the monitoring wells on the site has varied from approximately 30 to 48 feet bgs. Groundwater flow direction at the site during the most recent sampling event on 15 July 1998 was toward the southeast, which is consistent with historical groundwater flow directions toward the east to southeast. 2.3 SITE HISTORY AND PREVIOUS INVESTIGATIONS Information regarding the history of station operation at 1605 South El Dorado Street was obtained through a telephone interview of a staff member at the City of Stockton Planning Department(CSPD), conducted by EA in May 1998. Based on the telephone interview,the subject property has been issued only one use permit and was initially a Regal service station with USTs in 1954. From 1954 to 1988, title information for the service station could not be •- released by the CSPD. The site was purchased by Exxon in July 1988 with the existing building, canopy, pump islands, and three 10,000-gallon, single-walled steel USTs containing regular leaded,regular unleaded, and premium unleaded fuels. The service station was inactive between �. 1990 to 1992. During previous investigations, seven groundwater monitoring wells(MW1—MW7) and two vapor extraction wells (VEW1 and VEW2)were installed at the site. Figure 3 shows the location of the station features and the seven groundwater monitoring wells, which are sampled quarterly. Previous investigations at the site were conducted by AEGIS Environmental Inc. and are �. summarized below in chronological order: .. February 1991: Two inclined soil borings (SBI and SB2)were drilled adjacent to and beneath the three 10,000-gallon UST tank basin on 22 February 1991 (Figure 3). The two borings were drilled at an angle of 30°from vertical to an actual vertical depth of approximately 53 feet bgs. Laboratory analysis indicated that soil beneath the tanks was impacted by petroleum hydrocarbons. Most notably, a soil sample collected at approximately 35 feet bgs beneath the central tank location had a TPH-g concentration of +-� 16,000 milligrams per kilogram (mg/kg) and a benzene concentration of 63 mg/kg. Groundwater was not encountered during drilling. Soil analytical results are presented in Table 1 (AEGIS, 5 March 1991). • July 1991: Three groundwater monitoring wells (MWI—MW3)were installed to characterize the site(Figure 3). For the installation of the three 4-inch-diameter monitoring wells,the boreholes were drilled to a total depth of 55 feet bgs. Petroleum hydrocarbons were not detected in the soil samples at concentrations equal to or greater 4. than the laboratory method detection limits,except for a sample collected at MW3 at 20.5 feet bgs which had toluene and total xylenes at concentrations of 0.010 mg/kg. Soil analytical results are presented in Table 1 (AEGIS, 13 September 1991). Analysis of 'o]37.%TW8.Ci.05.wr.wrn 3