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MAY 1U1. i 8 1 : ?8Al 1J366568i H . 4 <br /> Section 3 <br /> REGIONAL GEOLOGIC AND HYDROGEOLOGIC SETTING <br /> The McMullin Gas Dehycrator Station is located in the San Joaquin Valley with the Great Valley <br /> Geomorphic Province, The San Joaquin Valley is a northwestward-trending, asymmetric structural basin <br /> ' bounded an the east by the Sierra Nevada Range and the west by the Coast Ranges. Locally, this basin <br /> has been infilled with up to 6 vertical miles of both marine and non-marine rocks and sediments that <br /> range in age from Jurassic:(about 160 million years ago)to Holocene(the past 11,000 years). Geologic <br /> i maps of the San Joaquin galley prepared by Page(1986)show the site is immediately underlain by flood <br /> plain deposits of Holocene:age. These deposits consist predominantly of clay and silt with some <br /> interbedded sands. Geologic cross sections through the area show that the flood plain deposits are <br /> underlain by lacustrine and marsh deposits of Pliocene to Holocene age(i.e.,younger than 5 million <br /> years). The lacustrine deposits include a relatively thick and regionally extensive clay unit, referred to as <br /> the E-clay,which is prese it beneath the site between the approximate depths of 200 to 300 feet <br /> (Page 1986)_ Older rocks•and sediments of continental and marine origin underlie the lacustrine and <br /> } marsh deposits. <br /> 1 <br /> This facility is also located in the Eastern San Joaquin County Groundwater Basin(CDVVR 1980). A <br /> regional groundwater powntiometric 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 <br /> level and flows to the west-northwest. This is consistent with the groundwater elevation data collected at <br /> the site by CH2M Hill in i ate 1997. Recharge to the basin occurs along the foothills of the Sierra Nevada <br /> jRange at the eastern boundary of the basin, and from surface infiltration. Discharge from the basin <br /> occurs through drainage into the San Joaquin River,a gaining river in the Tracy-Manteca area, and from <br /> extensive groundwater withdrawal in the area of Stockton. <br /> 402331-9863 3-1 <br />