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A36 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA Valley was delineated by Berkstresser (1973). With the exception of several localized pods of shallow saline water, ground water in the continental and volcanic deposits of the Sacramento Valley is fresh. Several localized shallow saline zones were described by Berk- stresser. Of these, four are now known to be underlain by freshwater. These shallow saline zones, underlain by freshwater, are located around the northern part of the Delta, along the Sacramento River, adjacent to the Sutter and Yolo bypasses, and around the base of Sutter Buttes. The zone around Sutter Buttes reflects the configuration of the underlying marine deposits (Berk- stresser, 1973). The base of freshwater as delineated by Page (1973b) for the San Joaquin Valley is more complex. In the San Joaquin Valley, the base of freshwater lies within the unconsolidated continental deposits of Pliocene to Holo- cene age, the more consolidated marine and sedimentary deposits of Tertiary age, and the igneous and metamor- phic rocks of pre-Tertiary age. Unlike the Sacramento Valley, the base of freshwater in the San Joaquin Valley is underlain by a saline water body. The depths to the base of freshwater in the San Joaquin Valley range from less than 100 to more than 3,500 ft below land surface. DISSOLVED SOLIDS The areal distribution of dissolved-solids concentra- tions in ground water of the Central Valley is shown in figure 24. The map for the Sacramento Valley was prepared using a combination of data for dissolved solids measured by the residue-on-evaporation method (72 percent of the analyses) (Brown and others, 1970, p. 145) and data derived from specific conductance measure- ments (Fogelman, 1982a). The map for the San Joaquin Valley was produced primarily from dissolved-solids measurements made us- ing the residue-on-evaporation method. Where data were sparse, the map was modified by examining additional dissolved-solids data calculated as the sum of the dis- solved constituents. Because 2,000 mg/L of dissolved solids is considered to be the maximum concentration in freshwater (Olmsted and Davis, 1961, p. 134), only those wells that yielded water with a dissolved-solids concen- tration of less than 2,000 mg/L were used to prepare the map. An exception to this was made along the southwest margin of the San Joaquin Valley, where shallow ground water has high dissolved-solids concentrations. The wa- ter samples were collected between 1974 and 1982 in the Sacramento Valley and between 1934 and 1985 in the San Joaquin Valley. Because figure 24 is a two-dimensional representation of data compiled from existing wells with a wide variety of depths and screen lengths, the map cannot show vertical variations in water quality. It is therefore a generalization of the dissolved-solids concentrations that are likely to be found in a particular area, and it is most representative of the ground-water zones commonly used. About 11 percent of the wells in the Sacramento Valley data set yielded water with dissolved-solids con- centrations that were higher or lower than the mapped interval. This proportion of concentrations not conform- ing to mapped intervals is probably similar for the San Joaquin Valley (Fogelman, 1982b). The U.S. Environmental Protection Agency (1979) secondary drinking-water standards recommend a limit of 500 mg/L for dissolved solids. The California Domestic Water Quality Regulations allow a maximum of 1,000 mg/L if water of better quality is not available. However, because dissolved-solids concentrations only indicate the total amount of dissolved constituents in water, the usability of water that exceeds 500 mg/L needs to be evaluated according to the concentration of each chemical constituent. Dissolved-solids concentrations are lower in the north- ern part and along the east side of the Central Valley. Dissolved solids are higher in the south-central part of the Sacramento Valley and in the western part of the San Joaquin Valley. This distribution reflects the low concen- trations of dissolved solids in recharge water that origi- nates in the Cascade Range and the Sierra Nevada, and the predominant regional ground-water flow pattern. In the Sacramento Valley, dissolved-solids concentra- tions generally do not exceed 500 mg/L. Two large areas of shallow ground water in which concentrations of dissolved solids range from 500-1,500 mg/L are present in the southern part of the Sacramento Valley (fig. 24). One area is south of the Sutter Buttes in the Sutter basin, and the other is west of the Sacramento River extending from West Sacramento on the north to the confluence of the Sacramento and San Joaquin Rivers on the south (fig. 24). In the San Joaquin Valley, dissolved-solids concentra- tions are lower on the east side and higher on the west side of the valley. In the center and on the east side of the valley, dissolved-solids concentrations generally do not exceed 500 mg/L; on the west side, most of the ground water contains concentrations of dissolved solids in excess of 500 mg/L. Concentrations of dissolved solids generally increase to the west, and concentrations in excess of 2,000 mg/L are not uncommon along the west margin of the valley. HYDROCHEMICAL FACIES Where a few ions dominate the dissolved-solids content of ground water, the term "hydrochemical fades" (Back, 1961) is used to describe the dominant ion patterns. The