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Lj Section 3 • REGIONAL GEOLOGIC AND HYDROGEOLOGIC SETTING The McMullin Gas Dehydrator Station is located in the San Joaquin Valley with the Great Valley Geomorphic Province. The San Joaquin Valley is a northwestward -trending, asymmetric structural basin bounded on the east by the Siena Nevada Range and the west by the Coast Ranges. Locally, this basin has been infilled with up to 6 vertical miles of both marine and non -marine rocks and sediments that range in age from Jurassic (about 160 million years ago) to Holocene (the past 11,000 years). Geologic maps of the San Joaquin Valley prepared by Page (1986) show the site is immediately underlain by flood plain deposits of Holocene age. These deposits consist predominantly of clay and silt with some interbedded sands. Geologic cross sections through the area show that the flood plain deposits are underlain by lacustrine and marsh deposits of Pliocene to Holocene age (i.e., younger than 5 million years). The lacustrine deposits include a relatively thick and regionally extensive clay unit, referred to as the E -clay, which is present beneath the site between the approximate depths of 200 to 300 feet (Page 1986). Older rocks and sediments of continental and marine origin underlie the lacustrine and marsh deposits. This facility is also located in the Eastern San Joaquin County Groundwater Basin (CDWR 1980). A regional groundwater potentiometric surface map prepared by the San Joaquin County Flood Control District for Fall 1973 shows that groundwater occurs at an elevation of about 10 feet above mean sea level and flows to the west-northwest. This is consistent with the groundwater elevation data collected at the site by CH2M Hill in late 1997. Recharge to the basin occurs along the foothills of the Sierra Nevada Range at the eastern boundary of the basin, and from surface infiltration. Discharge from the basin occurs through drainage into the San Joaquin River, a gaining river in the Tracy -Manteca area, and from extensive groundwater withdrawal in the area of Stockton. 402 331-98 63 3-1