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( Applied Geotechnology Inc. <br /> 6.0 SUBSURFACE CONDITIONS <br /> r- <br /> 6.1 =nvironmental Setting <br /> 6.1.1 Regional Geology <br /> The site is situated near the middle of the San Joaquin Valley, a broad <br /> north-south trending structural depression bordered by the Sierra Nevada <br /> Mountains on the east and the Coast Ranges on the west. Several thousand <br /> feet of sediment fill the valley, resting on granitic and sedimentary <br /> j. bedrock. The valley sediment consists predominantly of interbedded silts <br /> K=. and sands derived from erosion of the granitic Sierra Nevadas. <br /> 6.1.2 Geologic Setting <br /> I- <br /> Site geology was characterized by interpreting conditions encountered <br /> during drilling in context with our understanding of the local geology. <br /> Subsurface soils encountered were similar in all four borings and included <br /> approximately 18 to 23 feet of stiff brown sandy silt and silt overlying <br /> gray silty sand to an approximate depth of 19 to 24 feet below ground <br /> surface. <br /> 6.1.3 Hydrogeologic Setting <br /> Groundwater was encountered during drilling at about 12 to 16 feet below <br /> l_ ground surface (bgs) in each boring. Subsurface deposits of sandy silt and <br /> silt yielded groundwater slowly, therefore borings were advanced until <br /> 4 saturated silty sand was encountered. <br /> Groundwater was measured in each well installation after development at <br /> about 9 to 12 feet below ground surface. Groundwater elevations were used <br /> to construct a potentiometric surface map as shown on Figure 3. This ;nap <br /> indicates groundwater flow toward the northeast, based on June 14, 1990 <br /> water level measurements. Groundwater levels may fluctuate seasonally, or <br /> with tidal or other local influences, such as groundwater pumping. Water <br /> level measurements taken on-site at three separate times, morning, after- <br /> noon, and evening, had little variation suggesting tidal influences are <br /> minimal, if present. <br /> I . 5.2 Groundwater Quality <br /> 6.2.1 Field Parameters <br /> Table 8 lists field parameters including pH, specific conductance, and <br /> temperature, taken during the December 1987 and June 1990 samplings. In <br /> general, pH values were near neutral and ranged from 5.5 to 8.2. <br /> -24- <br />