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A second assessment of the site was conducted by Woodward-Clyde Consultants (WCC) <br /> in August, 1988 The details of this investigation are presented in the WCC report titled <br /> " Subsurface Assessment Report, Thrifty Oil Service Station 171, Stockton, California", <br /> dated October 6, 1988 The investigation consisted of advancing three exploratory <br /> borings at the locations shown on Figure 2. Each boring was converted to maintain <br /> 4-inch diameter PVC monitoring well to depths of about 50 to 60 feet Soil sample <br /> MW2 at 36 feet bgs indicated the only significant TPH and benzene, toluene, <br /> ethylbenzene and xylene (BTEX) concentrations (2,200 mg/kg TPH and less than <br /> detectable to 235 mg/kg BTEX). <br /> GEOLOGY <br /> The site is located in the San Joaquin Valley of the Central Valley geomorphic province <br /> Recently published geologic maps indicate that Holocene age flood-basin deposits extend from <br /> ground surface to a maximum depth of approximately 100 feet below the site Oligocene to <br /> Holocene age alluvial deposits lie below the younger flood-basin deposits to a maximum depth <br /> of approximately 3,500 feet below ground surface Continental and marine rocks of pre-Tertiary <br /> to Oligocene age underlie the alluvial deposits, and are composed of shale, sandstone and <br /> conglomerate Crystalline bedrock underlies the sedimentary formations <br /> Based on the drilling logs, sediments beneath the site consist almost entirely of silty clay and <br /> clayey silt from the surface to approximately 65 to 70 feet bgs At this depth a sand layer <br /> occurs which is approximately 5 to 10 feet thick and contains various fractions of silt and clay <br /> Underlying the sand layer at approximately 75 feet, the section is characterized by the silty <br /> clay/clayey silt sediments typical of the shallower areas, to the total depth of drilling, 90 feet <br /> bgs Figure 3 is a geologic cross section which shows the generalized lithologic relationships <br /> The drilling logs from this investigation contain descriptions of the sediments encountered and <br /> are presented in Appendix A <br /> Page 2 <br />