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Section 3 <br />' REGIONAL GEOLOGIC AND HYDROGEOLOGIC SETTING <br />' The McMullin Gas Dehydrator Station is located in the San Joaquin Valley within the Great Valley <br />Geomorphic Province. The San Joaquin Valley is a northwestward -trending, asymmetric structural basin <br />bounded on the east by the Sierra Nevada Range and the west by the Coast Ranges. Locally, this basin has <br />' been infilled with up to 6 vertical miles of both marine and non -marine rocks and sediments that range in <br />age from Jurassic (about 160 million years ago) to Holocene (the past 11,000 years). Geologic maps of the <br />' San Joaquin Valley prepared by Page (1986) show the site is immediately underlain by flood plain deposits <br />of Holocene age. These deposits consist predominantly of clay and silt with some interbedded sands. <br />Geologic cross sections through the area show that the flood plain deposits are underlain by lacustrine and <br />marsh deposits of Pliocene to Holocene age (i.e., younger than 5 million years). The lacustrine deposits <br />include a relatively thick and regionally extensive clay unit, referred to as the E -clay, which is present <br />' beneath the site between the approximate depths of 200 to 300 feet (Page, 1986). Older rocks and <br />sediments of continental and marine origin underlie the lacustrine and marsh deposits. <br />1 <br />1 <br />II <br />This facility is also located in the Eastern San Joaquin County Groundwater Basin (CDWR 1980). A <br />regional groundwater potentiometric surface map prepared by the San Joaquin County Flood Control <br />District for Fall 1973 shows that groundwater occurs at an elevation of about 10 feet above mean sea level <br />and flows to the west-northwest. Recharge to the basin occurs along the foothills of the Sierra Nevada <br />Range at the eastern boundary of the basin, and from surface infiltration. Discharge from the basin occurs <br />through drainage into the San Joaquin River, a gaining river in the Tracy -Manteca area, and from extensive <br />groundwater withdrawal in the area of Stockton. <br />cca08/19/93(9660a.doc%p43) 3-1 <br />