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2905 West Benjamin Holt Drive, Stockton, California March 16, 1994 Chevron U.S.A. Products Company Page 3 of assessment are summarized in Table 1. Cross sections A-A' and B-B' (Figure 3) show the shallow stratigraphy beneath the site and the distribution of hydrocarbon-impacted soils. Soil samples collected during removal of the former USTs in January 1989 at 14-16.5 ft BGS ranged from 400 to 3,600 parts per million (ppm) TPH-G with the highest concentrations found beneath the fill ends of the 10,000-gallon tanks located in the northwestern portion of the former UST pit (Blaine Tech Services, Inc., 1989). Additional samples were collected at 17 and 20 feet BGS beneath the two 10,000-gallon tanks and contained 800 and 6.5 ppm TPH-G, respectively. One product-line sample, collected at 6 ft BGS near the northeastern corner of the former UST pit contained 310 ppm TPH-G. The rest of the product-line samples collected were below method detection limits (<MDL) for TPH-G. Soil samples collected from the former used-oil UST pit contained low (1.1 ppm TPH-D) or non detectable concentrations of hydrocarbons (TPH-G, total oil and grease, chlorinated hydrocarbons). Soil samples collected from stockpiled soil (approximately 570 cubic yards) generated from excavation of the new UST pit and new product lines in April and June 1991 were <MDL for TPH-G (Blaine Tech Services, Inc., 1991). In January 1989, 24 soil vapor samples were collected from 5 and 15 feet BGS at 12 locations across the site property (Western Geologic Resources, February 1989). Soil vapor samples collected at 5 feet BGS ranged from <MDL (in eight samples) to 2,500 ppm (VP-11). The highest hydrocarbon concentration vapor sample in the 5 feet BGS zone was located northeast of the UST pit, near the highest concentration product-line soil sample. TPH-G concentrations in soil vapor samples collected at 15 ft. BGS ranged from <MDL in four of the vapor samples to 91,000 ppm (VP-11). The highest hydrocarbon concentrations in soil vapor were found in samples collected within 30-40 feet of the former UST pit. Twenty-eight soil samples have been collected from soil borings drilled for groundwater monitoring well and vadose well Installation between 1989 and 1993 (Western Geologic Resources, April, 1989; Groundwater Technology, Inc, 1992). Of the 28 total soil samples collected, 16 samples were <MDL for TPH-G. The 12 samples containing measurable concentrations of TPH-G were collected from four borings (MW-1, MW-2, MW-5, VW-1) and ranged from 0.3 to 2,700 ppm TPH-G. The highest concentration of TPH-G was noted in samples collected at 16 ft. BGS from each of the borings. TPH-G concentrations in samples collected at 19-21 ft. BGS in the same four borings were at least one order of magnitude lower than those collected at 16 feet BGS. 6.. 5073PAfl.RPT iLr GROUNDWATER E1�1:17 TECHNOLOGY