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103 West Eleventh Street, Tracy, California January 7, 1993 <br /> Chevron U S A Products Company Page 6 <br /> 4.0 SITE CONDITIONS <br />' 41 Hydrogeology <br /> The site is located in the northwest portion of the San Joaquin Valley, on the distal northern slope of <br />' an alluvial fan constructed of Quaternary sediments shed from the Coast Ranges Subsurface <br /> materials encountered during drilling to the depth of exploration at 21 5 feet BGS are dominantly silty <br />' clays and clayey silts with a lens of pebbly sand penetrated at a depth of approximately 14-18 feet in <br /> MW-3 Groundwater was encountered during drilling at approximately 12 feet BGS, and was gauged <br /> ' at about 10 feet on December 14, 1992 <br /> The water quality in the Tracy area is being degraded by intrusion of saline connate water as a result <br /> ' of overdraft of deeper aquifers Shallow groundwater is generally poor along the western boundary <br /> of the San Joaquin Basin, it is characterized by high concentrations of dissolved-solids D S <br /> ' Geolo ical Survey Professional Pager 1401-A 1991 <br /> 42 Results of Soil Analyses <br /> Results of all sod sample analyses are summarized in Table 1 and the laboratory results are enclosed <br /> ' in Appendix E Samples analyzed from the shallow borings at intervals of 5 and 10 feet contained <br /> concentrations of TPH-G ranging from nondetectable (ND) to 3,400 parts per million (ppm) and <br /> benzene concentrations from ND to 7 4 ppm All samples analyzed from the borings for the <br /> ' monitoring wells (MW-1 through MW-3) were ND for all constituents Total lead concentrations for <br /> the two samples analyzed (MW1-10 and MW3-10) were 6 ppm, barely above the detection limit of 5 <br /> ' ppm <br /> ' 43 Results of Groundwater Analyses <br /> Results of all water sample analyses are summarized in Table 2 and the laboratory reports are <br /> ' enclosed in Appendix E Elevated concentrations of TPH-G were detected in samples from both MW- <br /> 1 and MW-2 at levels of 2,400 and 910 ppb respectively No benzene was detected in the MW-1 and <br /> MW-2 samples, however, traces (less than 20 ppb) of toluene, ethylbenzene, and xylenes were <br /> ' reported The groundwater sample from MW-3 was below laboratory detection limits for all analyzed <br /> constituents <br /> GROUNDWATER <br /> ' TECHNOLOGY <br />