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4. SITE CONDITIONS 4.1 HYDROGEOLOGY Undisturbed soil samples collected during borehole drilling indicate a fairly uniform geologic sequence beneath former Chevron 9-7780. In general it consists mostly of silty clays with inter- bedded layers or lenses of sandy clay. Beginning at 69 feet is a water-bearing zone of poorly sorted sand to a depth of about 85 feet. The sand is underlain by a plastic brown sandy clay. The boreholes were terminated in this clayey sediment, and the wells were completed about 2 feet above it. Figure 5 is a geologic cross-section across the site,based on the individual boring logs of EA5, EA6, and MW2, which was installed by Groundwater Technology Inc. in 1987. A drop in the groundwater table of about 30 feet over a period of three years is shown in this cross-section. The cuffent water level was about 69-70 feet below the surface of the-site in.August 1990. Gauging done on 13 and 117 August 1990 indicated a dre,,in water elevation of 0.2 feet over only a few days. The direction of,groundwaterflow cannot be determined,because only two meas- urement points are available. Judging from previous gauging results, the flow direction is very unstable beneath the site, possibly influenced by pumping of water from numerous domestic wells in the vicinity(see Figure 4). Regional flow dirction in this area is northwest,as indicated • in dire Flood Control and Water Conservation District Report,Fall 1989. 4.2 SOIL SAMPLE ANALYSES Soil samples collected during borehole drilling were chemically analyzed for TPH as gasoline and the aromatics benzene, toluene, xylenes, and ethylbenzene by EPA Methods 8015 and 8020, as appropriate. The laboratory analytical results for the soil samples are summarized in Table 2 The laboratory analytical report is included as Appendix G. No concentrations of petroleum hydrocarbons above detection limits were found in any of the soil samples. 4.3. GROUNDWATER SAMPLE ANALYSES Groundwater samples were collected from the two new wells on the site(all of the other monitor- ing wells were dry). The samples were analyzed for TPH as gasoline and BTXE by EPA Methods 8015 and 8020. The laboratory analytical results are summarized in Table 3; the labo- ratory reports are in Appendix G. No concentrations of fuel hydrocarbons greater than method detection limits were found in samples of groundwater(0.5 µgiL for aromatics, 50µg/L for tots - -_--_. petroleum hydrocarbons). i c67J97780{tx 6