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layer (encountered only at Site #2) is a thin layer of fill ranging from the <br /> • ground surface to two to three feet below ground surface (bgs) The <br /> 1 second layer, semi-consolidated silts and clays, is found below the fill at <br /> Site #2 and directly below the pavement at Site #1, and ranges to a depth <br /> between approximately 10 and 20 feet bgs Below the clay/silt layer and <br /> to the depth explored (approximately 25 feet bgs), the soils consist of <br /> unconsolidated sand with minor amounts of silt and gravel <br /> Ground Water Hydrology <br /> Investigation and tank removal activities indicate the presence of confined <br /> shallow ground water conditions at Former Tank Sites #1 <br />' and #2 During drilling activities performed by ERM and previous <br /> investigators, ground water was typically encountered in borings at <br /> depths ranging from 3 5 to 19 feet bgs In these borings, ground water <br /> was first encountered after drilling through a silt and clay zone into sandy <br /> soils The fine-grained silts and clays appear to confine the ground water <br /> to the sandy soils The confined nature of the ground water was <br /> confirmed after construction and development of monitoring wells at both <br /> sites indicated that stabilized water levels tended to be higher than initial <br /> water levels In general, the ground water potentiometric surface at the <br /> property is at approximately 4 to 6 feet bgs <br /> Review of ground water potentiometric surface maps prepared <br /> individually for Sites #1 and #2 indicates that ground water flow <br /> directions are consistently toward the south-southwest The individual <br /> data from both sites provide mutually supporting data interpretations <br /> The apparent cause of the flow direction is the Stockton Deep Water <br /> Channel that lies immediately adjacent to the northern site property line <br /> Tidally induced water level fluctuations in the channel appear to be the <br /> cause of slight to moderate variations in the ground water flow gradient <br /> Ground water wells are a common source of potable water in the Great <br /> Valley geomorphic province, however, no water supply wells are located <br /> within a 2,000-foot radius of Former Tank Sites #1 and #2 <br /> 2-2 <br />