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Hydrologic Setting The Central Valley is drained by two large river systems: the Sacramento River (northern part of the Valley) and the San Joaquin River(most of the southern part of the Valley). The two rivers meet at u deltaic area (San Joaquin-Sacramento Delta) about 20 Ian northwest of the Exxon station. Before agricultural development of the San Joaquin Valley, flow in the San Joaquin River was sustained by runoff of precipitation. Subsequent to this agricultural growth, flow in the river has been sustained by a combination of runoff and agricultural wastewater derived from drained fields that use irrigation water imported from outside the San Joaquin drainage area (Farrar and Bertoldi, 1988). Hydrogeology The chief source of fresh groundwater throughout the Central Valley is the upper 300m of unconsolidated continental deposits and sedimentary rocks of.post-Eocene age S (Page, 1986). Fresh groundwater is known to exist at depths greater than 1000m in parts of the Valley. Below the freshwater zone is saline water, Primarily connate,contained in _ ' the thick, marine, Cretaceous to Eocene sedimentary rocks. Basement in the western part of the Valley consists of relatively impermeable rocks which are an extension of the Coast Manges. Considerable variation exists in the hydraulic properties of aquifers from place to place in the Valley, and thas variabilityis related to differences in lithology and depositional processes (Farrar hnd Bertoldi, 1988). The most productive aquifers are within the coarse-grained deposits of fluvial origin. Fresh groundwater occurs under confined and unconfined conditions in the Valley. Before intensive groundwater development, groundwater flow in bouli confined and unconfined aquifers was generally from the valley sides toward the axis, and from the north and south toward the Sacramento-San Joaquin Delta {Davis et al., 1959, Olmsted and Davis, 1961). Large scale groundwater development has modified the natural now pattern by creating cones of depression in major pumping centers. More recently, importation of surface water has decreased the magnitude of the man-induced changes by decreasing the need for pumping and also by increasing recharge in these areas (Farrar and Bertoldi, 1988). 2-6 175-07.R5 4/24/91