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2.2 TEXACO ERVICE STATION 2.2.1 Background ' The Texaco Service Station is located at 440 West Charter Way on the southeast corner of Charter Way and Lincoln Street in Stockton, California (see Figure 1) Based on file review at SJCPHS, there are four underground gasoline tanks in use at the site Three ' tanks have a volume capacity of 10,000 gallons, and the other tank has a capacity of Y 12,000 gallons ' 2.2.2 Site Assessment To evaluate residual hydrocarbons in soil and groundwater, 16 soil borings were drilled on site and 9 soil bonngs were drilled off site, 7 on-site groundwater monitoring wells and 9 off-site monitoring wells were installed, and 4 on-site vapor extraction wells were ' installed i Site Geology and Hydrogeology. Sediments beneath the site consist primarily of ' silty clay to silt from surface grade to approximately 35 feet bgs (see Figures 2 through 4), clayey silt from approximately 35 to 43 feet bgs, and silty sand and sand from 43 to 50 feet bgs (SEACOR, September 29, 1994) Since monitoring began u1 May 1991, the depth to water beneath the site has fluctuated from approximately 34 to 43 feet bgs Based on fourth quarter 1993, first quarter 1994, ' and second quarter 1994 sampling events, the direction of groundwater flow has vaned from northeast to northwest ' Chemical Distribution in Soil and Groundwater Results Soil appears to contain gasoline hydrocarbons beneath and adjacent to the eastern dispensers The vertical extent of gasoline hydrocarbons is from approximately 15 to 35 feet bgs TPHG concentrations ' ranged from below detection limits to 5,300 ppm (sample collected at 15 feet bgs and beneath dispenser island), and benzene concentration ranged from below detection limits to approximately 33 ppm SEACOR estimates approximately 400 cubic yards of soil ' containing gasoline hydrocarbons Analytical results of soil samples are presented in Attachment B Dissolved gasoline hydrocarbons have been detected in samples collected from all wells, except from well MW-16 Concentrations of TPHG have ranged from 65 ppb to 260,000 ppb, and concentrations of benzene have ranged from 0 73 ppb to 51,000 ppb The highest concentrations were deterred in well MW-1 (located near the eastern dispenser island), in which free product✓was observed during 1992 Eight isoconcentration maps (which contain data on concentrations of TPHG, benzene, and volatile organic SAON\PJM8"8051460 2AS-94\pw i Rev 0 515195 0905-14602 2-3