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Al2 REGIONAL AQUIFE"YSTEM ANALYSIS—CENTRAL VALLEY,CALIFORNIA limits that an Error in estimating their thickness has no Chico (Page, 1986, fig. 8), at depths from 640 to 900 €t, effect on the flow-system analysis. I 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. 844). WATER-SEARING CHARACTERISTICS OF Relating aquifers within the post-Eocene continental THE AQUIFER SYSTEM deposits to pecific formations in the subsurface is (POST-EOCENE CONTINENTAL DEPOSITS) difficult. In the valley, investigators use mainly physiog- raphy,weathering characteristics,and soils to map upper The post-Eocene continental deposits that constitute Cenozoic formations; however, in the subsurface, espe- the Central Valley aquifer system contain mostly fluvial cially under saturated conditions, equivalents of surface deposits and interbedded la.custrine deposits but include units cannot be mapped with any certainty because some volcanic material. The continental deposits consist differences in lithology are not apparent. In the Central predominantly of lenses of gravel, sand, silt, and clay. Valley, then,physical properties of the aquifer materials The numerous lenses of fine-grained (silt, sandy silt, and the distribution of these properties are more impor- sandy clay, and clay) sediments are distributed through- tant than the delineation of formation boundaries to out the valley and in most places constitute over 54 understanding regional and local flow patterns and to percent of the total thickness penetrated by wells, as quantifying water in storage.The general relations in the determined from electric logy; (Page, 1986). Sacramento and San Joaquin Valleys among geologic Most of these fine-grained lenses are not areally i units, hydrologic units, and layers used in the computer extensive; however, several major ones were mapped, simulation of ground-water flow are shown in figure 9. 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), STORAGE COEFFICIENT which is part of the modified E-clay of Page (1986) and underlies most of the west side of the San Joaquin Valley. Storage coefficient is the amount of water that can be This diatomaceous clay unit underlies an area of approx- released from or added to the ground-water reservoir. It imately 5,000 min (Page, 1986) and ranges in thickness is usually defined as the volume of water an aquifer from near sero to at least 160 ft beneath the present bed system releases from or takes into storage per unit of Tulare Lake(Davis and others, 1959;Page, 1986). The surface area of aquifer per unit change in head(Lohman northern extent of the Corcoran Clay Member is not and others, 1972, p. 8). In the zone of water-table known because of the lack of well data north of Stockton, fluctuations, the storage coefficient is virtually equal to particularly in the Delta area, A diatomaceous clay the amount of water released from storage by gravity similar in composition to that of the Corcoran Claydrainage, referred to as specific yield. Below the zone of Member was present in a test hole drilled northwest of water-table fluctuations, the storage coefficient is the Sacramento, and drillers have filed reports showing a amount of water released by compression of the sedi- diatomaceous clay in several deeper wells north of ments and expansion of the water.This amount is usually Stockton (Page and Bertoldi, 1983). Laboratory tests of much less than the amount released by gravity drainage. the clay indicate that it is highly susceptible to compac- Laboratory values of specific yield and porosity are tion, like the Corcoran Clay Member; however, the clay shown in table 1- For the purposes of uniformity, only was not present in six other test holes northwest of reported values obtained by the "sample saturation and Sacramento, so the full extent of it is not known. drainage" method described by Johnson (1967, P. D5) The Corcoran Clay Member is important to the hy- were used in table 1. In general, sand yields more water draulics of the aquifer system in that prior to develop- from gravity drainage than fine-grained deposits like silt ment it acted as an effective confining unit. However, the and clay, even though the porosities are nearly the same. drilling of large-diameter wells through the Corcoran and The fine-grained deposits usually have much smaller the practice of perforating wells both above and below it specific-,yield values because the tiny pores do not drain have made the present effectiveness of the Corcoran as a readily. confining unit questionable. Williamson and others (1989, table 7) used specific- In the basis of drillers'logs,electric logs from gas wells yield values for aquifer materials similar to those shown and the water wells, plus information from seven U.S. in table 1 and then estimated an aggregated specific yield Geological Survey test holes drilled as part of this study, for the first few hundred feet of saturated sediment on Page (1986) concluded that no extensive fine-grained the basis of lithologic descriptions from about 17,000 well lenses underlie the Sacramento Valley. However, there logs. They estimated an average specific yield of 7 are two areas of mostly fire-grained sediments interbed- percent for the Sacramento Valley, 8 percent for the ded with coarse-grained sediments along the northeast Delta area,and 10 percent for the San Joaquin Valley and flank of the Sacramento Valley adjacent to and south of Tulare Basin.