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2905 W Benjamin Holt Drive, Stockton, California August 12, 1994 Chevron USA Products Company Page 6 4 0 RESULTS OF ADDITIONAL ASSESSMENT 41 Hydrogeology The site is located In the northern part of the San Joaquin Valley, California The valley fill consists of late Mesozoic through Eocene dominantly marine deposits overlain by late Tertiary and Quaternary non-marine deposits In the Stockton area, the late Cenozoic deposits are approximately 3,000 feet thick and generally consist of alluvial gravel, sand, silt, and clay The non-marine sedimentary units are generally fresh-water-bearing In the area of the valley east and southeast of Stockton, but brackish or saline water occurs in these sediments in the delta area west of Stockton The saline-water/fresh-water boundary within these sedimentary units trends north to south across the western portion of Stockton The saline groundwater west of this boundary is usually overlain by a layer of fresh groundwater to a depth of approximately 50 feet BGS, therefore, shallow monitoring wells usually encounter fresh groundwater on both sides of the boundary The site is underlain by interbedded clays, silts, and silty fine sands from the surface to 6 feet BGS, and fine to coarse sand from 6 to 30 feet BGS During drilling, groundwater was encountered at 17-18 feet BGS During quarterly monitoring (June 27, 1994), depth to water was at approximately 16 to 17 5 feet BGS Groundwater flow, as mapped from data collected on June 27, 1994, is to the east at a hydraulic gradient of 0 008 (Figure 3) 42 Results of Soil Sample Analyses Results of the soil sample analyses are summarized in Table 1 Samples submitted for analysis include the 10, 15 and 20 feet BGS samples from SBs-1 and 2 and MW-9, and the 10, 15, and 17 feet BGS samples from SBs-3 and 4 BTEX and TPH-G were detected in at least one of the samples from each borehole The highest concentrations reported were 11 parts per million (ppm) benzene and 2,600 ppm TPH-G measured in the 15 feet BGS sample from SB-2 The highest concentration of BTEX and TPH-G in each borehole were detected in samples collected between 15 and 20 feet BGS in the zone of seasonal water-table fluctuation All samples collected at 10 feet BGS were below laboratory detection limits for BTEX and TPH-G, except the 10 feet BGS sample from SB-2 Copies of laboratory results and chain-of-custody manifests are included in Appendix E GROUNDWATER 0017aear rpt TECHNOLOGY