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SITE CLOSURE REQUEST REPORT Former Exxon Service Station No.7-3330 1 3128 West Benjamin Holt Drive 1 Stockton,California Delta Project No. D093-810 Page 3 The Central Valley is drained by two large river systems, the Sacramento River (located in the northern part of the valley) and the San Joaquin River (located in the southern part of the valley). The two rivers meet at a deltaic area (San Joaquin-Sacramento Delta) approximately 12 miles northwest of the former Exxon service station. The main source of potable ground water throughout the Central Valley occurs in the upper 990 feet of unconsolidated continental deposits and sedimentary rocks of post-Eocene age (Page, 1986). Potable ground water is known to exist at depths greater than 3,000 feet in locations of the valley. Below the potable zone is saline water, primarily connate, contained in the thick, marine, Cretaceous to Eocene sedimentary rocks. Local variations exist in the hydraulic properties of aquifers in the valley. These variations are related to differences in lithology and depositional processes (Farr and Bertoldi, 1988). The most productive aquifers are within the coarse-grained deposits of fluvial origin. Potable ground water occurs under confined and unconfined conditions within the valley. Prior to intensive ground water development, ground water flow in both confined and unconfined aquifers was generally from the valley sides toward the center, and from the north and south toward the Sacramento- San Joaquin Delta (Davis et al., 1959; Olmsted and Davis, 1961). Intensive ground water development has modified the natural flow pattern by creating cones of depressions in major pumping areas. Recently, importation of surface water has decreased the magnitude of the depressions by decreasing the need for pumping and also by increasing recharge in these areas (Farrar and Bertoldi, 1988). The intensive ground water development has been largely due to increased agricultural activity sustained by a combination of ground water pumping and importing of surface water. The water importation has largely replaced the ground water pumping in the western portion of the valley. The main source of recharge to the water table is infiltration and percolation of imported surface water applied to irrigated lands. Recharge was formerly supplied primarily by streams and rivers entering the valley. Most aquifer . discharge, previously occurring as evapotranspiration and seepage to streams, is now from pumping wells RPT001.81D