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June 26, 1992 <br /> . ARCO Service Station No. 549, Stockton, California <br /> Previous Work <br /> Applied GeoSystems conducted a Preliminary Tank Replacement Assessment investigation <br /> in October 1990 to evaluate soil for the presence of hydrocarbons (Applied GeoSystems, <br /> Inc.,December 18, 1990). During this investigation, five soil borings (B-1 through B-5)were <br /> drilled at the site. The locations of the borings are shown on Plate 2 and were selected <br /> using ARCO's guidelines for Preliminary Tank Replacement Assessments. <br /> Reported concentrations of total petroleum hydrocarbons as gasoline (TPHg)in.soil samples l <br /> from the borings ranged up to 1,400 parts per million (ppm), and reported concentrations i <br /> of benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) ranged up to 160 ppm ; <br /> in the soil samples collected from borings B-1 and B-3. TPHg and BTEX concentrations, <br /> were below 1.0 ppm in soil samples collected from borings B-2, B-4, and B-5, except in the <br />` i sample.collected from 10 feet below grade in boring 13=2-which had a TPHg concentration <br /> of 1.6 ppm. Table 2 presents the analytical results of soil samples collected from these, <br /> ',borings� <br /> LOCAL GEOLOGY <br /> The site is underlain by the Modesto Formation, which is composed of alluvial sediments <br /> that range in size from silt to gravel (Marchand and Allwardt, 1977). The Modesto <br /> Formation dips gently to the west (Wagner, et al., 1981). <br /> The site is in the Eastern San Joaquin County Ground-Water Basin (California Department <br /> of Water Resources, 1980). According to personnel of the San Joaquin Public Health <br /> Services, the direction of regional ground-water flow is to the southwest and the depth to <br /> ground water ranges from 45 to 50 feet below surface grade.?The groundwater basin has <br /> been developed for intensive irrigation, domestic, industrial, municipal, and stock use. <br /> 3000a-a 3 <br /> f <br />