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A34 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA Ground-water pumpage declined sharply when surface water from the Sacramento Valley became available to the western San Joaquin Valley via the California Aque- duct in the late 1960's. Since 1970, there has been very little subsidence except during the 1976-77 drought, when pumpage from wells sharply increased and com- paction briefly resumed (fig. 22). As of 1983, land subsidence in the San Joaquin Valley had either slowed considerably or stopped (Ireland, 1986). Locally in Fresno and Kings Counties, between 1977 and 1982, rebound of the land surface was significant (about 0.5 ft), indicating that subsidence during the 1976-77 drought was elastic. In the future, subsidence will resume only if renewed pumpage is sufficiently heavy to cause ground- water levels to drop below their previous lows. In the Sacramento Valley, subsidence of more than 1 ft is limited to the Davis-Zamora area in the southern part of the valley northwest of Sacramento (fig. 19), where land subsidence of about 2 ft was reported by Lofgren and Ireland (1973). However, some additional leveling suggests that subsidence increased between 1973 and 1979 (Williamson and others, 1989). Subsidence in the Central Valley has created engineer- ing and economic problems, although many are not noticeable by casual observation because land subsided over such large areas at the same time. Subsidence of canals and irrigation and drainage systems has resulted in cracking and the loss of carrying capacity. In areas susceptible to hydrocompaction, it has been necessary to precompact sediments by prolonged wetting before con- struction of canals a costly procedure. Failure of well casings due to compressional stress resulted in the loss of thousands of irrigation wells during the 1950's and 1960's. Frequent surveying to determine elevations of bench marks has been required for construction purposes and revision of topographic maps. QUALITY OF GROUND WATER Historically, ground-water quality of the Central Val- ley has been studied as three units: the Sacramento Valley, the San Joaquin Valley, and the Delta area (fig. 1). Olmsted and Davis (1961) described ground-water quality of the Sacramento Valley, Hull (1984) described ground-water quality of the Sacramento Valley exclusive of the southern part near the Delta, and Davis and others (1959) described ground-water quality of the San Joaquin Valley south of the Delta. Numerous authors of the U.S. Geological Survey described ground-water quality con- ditions in smaller parts of the valley, as indicated in the references. Central Valley ground-water chemistry is influenced by water from streams that enter the valley from the surrounding mountains and provide most of the natural recharge (Davis and others, 1959; Hull, 1984). The quality of water in streams that enter the valley from the east is influenced by the granitic Sierra Nevada and is notably different from the quality of water in streams from the west, which is influenced by the marine sedi- ments of the Coast Ranges. In general, the east side, the axial part, and the west side of the Central Valley are characterized by distinctive ground-water chemistry. The water chemistry is further influenced by an increase in reducing conditions and cation exchange processes as the water moves through the sediments. Davis and others (1959) divided the San Joaquin Valley into three areas of ground-water-quality characteristics: the east side, the axial part, and the west side. In general, ground water on the east side is bicarbonate type and has low to moderate dissolved-solids concentra- tions, ground water of the axial part differs greatly in chemical types and generally contains higher concentra- tions of dissolved solids than does water on the east side, and ground water on the west side is typically a sulfate or bicarbonate type and contains higher concentrations of dissolved solids than does water on the east side (Davis and others, 1959). Davis and others (1959) further divided ground water in the San Joaquin Valley into three vertical zones: unconfined, semiconfined, and confined. Confined waters generally have lower concentrations of dissolved solids and higher percent sodium. The confined waters also differ in chemical types from the unconfined waters owing to cation-exchange reactions, which occur on the clay particles (Davis and others, 1959). Hull (1984), in a detailed study of the Sacramento Valley, delineated six hydrochemical facies: two on the east side, two in the center of the valley, and two on the west side. In general, ground water on the east side is low in dissolved solids and high in silica, reflecting the quality of recharge water from the granitic rock of the Sierra Nevada. Reducing conditions produce high con- centrations of dissolved iron, manganese, and arsenic in the central part of the valley. Ground water on the west side is lower in silica and higher in dissolved-solids concentrations than ground water on the east side. Also, dissolved-solids concentrations tend to increase from north to south along the axis of the Sacramento Valley (fig. 24). BASE OF FRESHWATER The base of freshwater less than 2,000 mg/L (milli- grams per liter) of dissolved solids in the Sacramento