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6.0 GEOLOGY AND HYDROGEOLOGY <br /> 61 Vicinily Qeology <br /> From review of boring logs generated during this and previous subsurface investigations <br /> (Appendix D) and the hydrogeologic cross sections (Figures 3 and 4), the native stratigraphy <br /> from 0 to 10 fbg consists in the north (from MW-4 northward) and east (in the vicinity of <br /> MW-6) predominantly of a relatively homogeneous mixture of silty clay to clay. To the <br /> south (AMW-2) and west (AMW-6 and AMW-5), this unit is underlain by sandy clay to <br /> clayey sand and sandy silt starting at approximately 10 fbg and continuing to at least 20 fbg <br /> Utility trench backfill was encountered in bonngs for MW-5, AMW-2,AMW-5, and AMW-6 <br /> The backfill material generally consists of clayey to sandy gravel, extending from <br /> approximately 5 to 8 fbg <br /> 6 2 Vicrruty Hydrogeolg <br /> Ground water elevations from Wells MW-1 through MW-8 and AN4W-1 through AMW-4, <br /> as measured on September 26, 1991, were used to develop the ground water elevation <br /> contour map shown in Figure 5,while measurements taken on December 10, 1991 (including <br /> AMW-5 and AMW-6),were used to develop the ground water elevation contour map shown <br /> in Figure 6. The average depth to water, as measured in September 1991, was approximate- <br /> ly 6 5 fbg (19 5 feet msl), while in December 1991, the average depth was approximately 5 8 <br /> fbg (20 2 feet msl) The average ground water gradient direction for both events was <br /> determined to be to the north, with average magnitudes of approximately 0.007 and 0 005 <br /> foot per foot across the site Both the magnitude and direction of the ground water gradient <br /> have remained relatively consistent since monitoring was initiated in 1987. <br /> 7.0 RESULTS OF CHEMICAL ANALYSIS <br /> • Neither TPH-G nor BTEX detected in soil samples collected from AMW-5,while no <br /> samples were recovered from AMW-6 (native soil was not encountered). <br /> • During the September 1991 sampling event, concentrations of TPH-G in the ground <br /> water samples ranged from 810 ppb in MW-4 to 20,000 ppb in MW-6, and benzene <br /> from 13 ppb in MW-8 to 510 ppb in MW-6. <br /> • During the December 1992 sampling event, concentrations of TPH-G in ground <br /> water samples ranged from 45 ppb in ANff-1 to 7,500 ppb in MW-6, and benzene <br /> from 13 ppb in MW-8 to 510 ppb in MW-6. <br /> • Consistent with past results,detectable concentrations of TPH-G and BTEX were not <br /> present in ground water samples, collected during either sampling event, from <br /> 6 <br />