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1 <br /> ' 15 March 1996 <br /> AGE-NC Project No 95-0105 <br /> ' Page 5 of 7 <br /> 4.0. FINDINGS <br /> 41 GROUND WATER ELEVATION AND GRADIENT <br /> The elevation of ground water in each well was calculated by subtracting the depth to ground water <br /> from the surveyed casing elevation (Table I) Monitoring well casing elevations were surveyed <br /> ' relative to a City of Tracy bench mark Ground water rose an average of 0 5 feet between October <br /> 1995 and January 1996 <br /> ' Ground water gradient on 18 January 1996 was calculated to be 0 005 ft/ft directed N 151 W The <br /> relative ground water elevation at the site is illustrated in Figure 3 -Relative Ground Water Elevation <br /> Map The flow direction indicates a departure from the previous gradient by 25°to the west, while <br /> ' the hydraulic gradient was greater than previous measurements at the site <br /> ' 4 2 LABORATORY RESULTS OF GROUND WATER SAMPLES <br /> M Benzene was detected in MW-3 at a concentration of 3 8 ,ugll (micrograms per liter, equivalent to <br /> parts per billion, ppb) Toluene, ethyl benzene, xylenes, TPH-g and TPH-d were not detected during <br /> the January 1996 sampling Analytical results of ground water samples are summarized on Table 2 <br /> The laboratory report (Alpha Analytical Laboratory Batch No 96-0119-014) and chain-of-custody <br /> are presented in Appendix C <br /> 43 STRATIGRAPHY <br /> ' Tan clay layers were encountered from the surface to depths of approximately 7 5 feet The upper <br /> clay layer has the qualities of an aquitard (a serrupermeable lithology) Saturated, fine sand beds were <br /> encountered below approximately 7 5 feet in depth A dry, brown clay was also encountered at a <br /> ' depth of approximately 10 feet bsg (see Figure 4 -Cross Section) Ground water in the sand layers <br /> is abundant, however it appears that the clay immediately above the sands, restricts the vertical <br /> ' migration of ground water <br /> The tank excavations at the site have been backfilled with sand, a permeable hthology, which may be <br /> creating a vertical conduit for ground water to the upper clay layer Currently, static ground water <br /> level is at four feet bsg <br /> 1 <br /> 1 <br />