ARCHIVED REPORTS XR0000629 - Laserfiche WebLink

Home Browse Search

Hydrologic ydrologie 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 tl:e southern part of the Valley). The two rivers meet at a deltaic area (San Joaquin-Sacramento Delta) about 20 km northwest of the Exxon station. Before agricultural development of the San Joaquin Valle flow aQ y, win the San Joaquin River was sustained by runoff of precipitation. Subsequent tothis 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). 1Flydrogeology s The chief source of fresh groundwater throughout the Central Valley is the upper 300m of unconsolidated continental deposits and sedimentaryrocks of post-Eocene age (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 Ranges. Considerable variation exists in the hydraulic properties of aquifers from place to place in the Valley, and this variability is related to differences in lithology and o depositional processes (Farrar and Bertoldi, 1988). The most productive aquifers are within the coarse-grained deposits of fluvial origin. c Fresh groundwater occurs under confined and unconfined conditions in the Valley. Before intensive groundwater development, groundwater flow in both confined and f' 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 flow 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). i2-6 E 173-07.R5 4R4191 �aRf