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A12 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA limits that an error in estimating their thickness has no effect on the flow-system analysis. WATER-BEARING CHARACTERISTICS OF THE AQUIFER SYSTEM (POST-EOCENE CONTINENTAL DEPOSITS) The post-Eocene continental deposits that constitute the Central Valley aquifer system contain mostly fluvial deposits and interbedded lacustrine deposits but include some volcanic material. The continental deposits consist predominantly of lenses of gravel, sand, silt, and clay. The numerous lenses of fine-grained (silt, sandy silt, sandy clay, and clay) sediments are distributed through- out the valley and in most places constitute over 50 percent of the total thickness penetrated by wells, as determined from electric logs (Page, 1986). Most of these fine-grained lenses are not areally extensive; however, several major ones were mapped, principally near the axis of the San Joaquin Valley. The most notable is the Corcoran Clay Member (Pleistocene) of the Tulare Formation (Pliocene and Pleistocene), which is part of the modified E-clay of Page (1986) and underlies most of the west side of the San Joaquin Valley. This diatomaceous clay unit underlies an area of approx- imately 5,000 mi2 (Page, 1986) and ranges in thickness from near zero to at least 160 ft beneath the present bed of Tulare Lake (Davis and others, 1959; Page, 1986). The northern extent of the Corcoran Clay Member is not known because of the lack of well data north of Stockton, particularly in the Delta area. A diatomaceous clay similar in composition to that of the Corcoran Clay Member was present in a test hole drilled northwest of Sacramento, and drillers have filed reports showing a diatomaceous clay in several deeper wells north of Stockton (Page and Bertoldi, 1983). Laboratory tests of the clay indicate that it is highly susceptible to compac- tion, like the Corcoran Clay Member; however, the clay was not present in six other test holes northwest of Sacramento, so the full extent of it is not known. The Corcoran Clay Member is important to the hy- draulics of the aquifer system in that prior to develop- ment it acted as an effective confining unit. However, the drilling of large-diameter wells through the Corcoran and the practice of perforating wells both above and below it have made the present effectiveness of the Corcoran as a confining unit questionable. In the basis of drillers' logs, electric logs from gas wells and the water wells, plus information from seven U.S. Geological Survey test holes drilled as part of this study, Page (1986) concluded that no extensive fine-grained lenses underlie the Sacramento Valley. However, there are two areas of mostly fine-grained sediments interbed- ded with coarse-grained sediments along the northeast flank of the Sacramento Valley adjacent to and south of Chico (Page, 1986, fig. 8), at depths from 600 to 900 ft, and along the southwest flank of the Sacramento Valley north of Cache Creek in T. 3 N., at depths from about 600 to 2,700 ft (Page, 1986, figs. 8-14). Relating aquifers within the post-Eocene continental deposits to specific formations in the subsurface is difficult. In the valley, investigators use mainly physiog- raphy, weathering characteristics, and soils to map upper Cenozoic formations; however, in the subsurface, espe- cially under saturated conditions, equivalents of surface units cannot be mapped with any certainty because differences in lithology are not apparent. In the Central Valley, then, physical properties of the aquifer materials and the distribution of these properties are more impor- tant than the delineation of formation boundaries to understanding regional and local flow patterns and to quantifying water in storage. The general relations in the Sacramento and San Joaquin Valleys among geologic units, hydrologic units, and layers used in the computer simulation of ground-water flow are shown in figure 9. STORAGE COEFFICIENT Storage coefficient is the amount of water that can be released from or added to the ground-water reservoir. It is usually defined as the volume of water an aquifer system releases from or takes into storage per unit surface area of aquifer per unit change in head (Lohman and others, 1972, p. 8). In the zone of water-table fluctuations, the storage coefficient is virtually equal to the amount of water released from storage by gravity drainage, referred to as specific yield. Below the zone of water-table fluctuations, the storage coefficient is the amount of water released by compression of the sedi- ments and expansion of the water. This amount is usually much less than the amount released by gravity drainage. Laboratory values of specific yield and porosity are shown in table 1. For the purposes of uniformity, only reported values obtained by the "sample saturation and drainage" method described by Johnson (1967, p. D5) were used in table 1. In general, sand yields more water from gravity drainage than fine-grained deposits like silt and clay, even though the porosities are nearly the same. The fine-grained deposits usually have much smaller specific-yield values because the tiny pores do not drain readily. Williamson and others (1989, table 7) used specific- yield values for aquifer materials similar to those shown in table 1 and then estimated an aggregated specific yield for the first few hundred feet of saturated sediment on the basis of lithologic descriptions from about 17,000 well logs. They estimated an average specific yield of 7 percent for the Sacramento Valley, 8 percent for the Delta area, and 10 percent for the San Joaquin Valley and Tulare Basin.