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r Section 6 RESULTS SUBSURFACE CONDITIONS The soils underlying the Lathrop Gas Dehydrator Station include interbedded layers of silt and sand(see the Boring and Well Logs in Appendix A and the Geologic Cross Sections, Figure 5). The interbedded nature of these soils is typical of fluvial depositional systems. During the drilling of the boreholes,groundwater was noted at depths ranging from 9 to 10'/2 feet. After about 12 hours, the groundwater potentiometric surface stabilized at a depth of about 9 feet below the ground surface. Depth to water measurements and groundwater elevations are shown in Table 4. Figure 6 shows the groundwater potentiometric surface which was measured from the temporary wells (i.e., excluding MW-1) on November 25, 1992. The groundwater elevations measured on this date ranged from 0.15 ft to 0.21 ft MSL with groundwater flow directed to the south at a very slight hydraulic gradient of less than 0.001 ft per ft. CHEMICAL ANALYTICAL RESULTS Soil Samples Table 5 summarizes the results of the laboratory analyses performed on the soil samples collected from the boreholes. The formal laboratory reports and chain-of-custody records are presented in Appendix B. Petroleum hydrocarbons were reported in soil samples collected from several of the boreholes at the site (MW-1, B-2, B-3, B-7 and B-8)primarily from the capillary fringe at a depth of 7'V2 to 9 feet. The highest levels of petroleum hydrocarbons were reported from the 9-ft sample collected from MW-1. This soil sample contained TPH-G(800 mg/kg), TPH-K (4,300 mg/kg) and 1,000 mg/kg of total oil and grease. Discussions with the analytical laboratory revealed that the oil and grease results probably represent the heavier hydrocarbon compounds reported as TPH-K because the oil and grease analysis is sensitive to C 15+hydrocarbons and the TPH-K analysis is sensitive to C 11 to C 17 hydrocarbons. Detectable levels of BTEX compounds were only present in three soil samples (MW-1 at 9 ft, B-2 at 8.5 ft, and B-3 at 7.5 ft). The highest levels of BTEX compounds were also reported from the 9-ft sample collected from MW-1. This soil sample contained benzene at 0.23 mg/kg,toluene at 0.21 mg/kg, ethylbenzene at 1.3 mg/kg and xylenes at 34 mg/kg. cca/08/23/93/9449arpt.doc/mkn4 6-1