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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A37 classification of ground water into hydrochemical facies or chemical water types is based on the relative concen- tration, in chemical equivalents, of cations and anions in the water. The cation and anion that represent at least 50 percent of the total ions are used to designate the chemical water type, such as magnesium bicarbonate. If no one cation or anion amounts to 50 percent, the water is designated by the two ions that make up the largest percentages, such as calcium magnesium bicarbonate. The distribution of chemical water types in the Central Valley aquifer system is shown in figure 25. This map shows only the general distribution pattern of water types. Because the map does not show vertical varia- tions, these variations are discussed herein, where ap- propriate. Areas where data are insufficient to define the water type were left blank. Throughout the Sacramento Valley, with the exception of part of the Sutter basin, bicarbonate is the predomi- nant anion in the ground water. Ground water in the northern and eastern parts of the Sacramento Valley has fairly homogeneous chemical character, with calcium and magnesium being the predominant cations. Two areas, one along Stony Creek and one along the Feather River, stand out as being exclusively calcium or magnesium bicarbonate, respectively, reflecting the recharge waters from the streams that drain into the valley at these locations (Fogelman, 1983). South of the Sutter Buttes, water types are more complex and sodium is the predominant cation. Sodium bicarbonate type water is predominant along Salt Creek and downstream from the confluence of Salt Creek and the Sacramento River. The sodium ion in this area is probably derived from saline water in the Cretaceous formations that are drained by Salt Creek (Fogelman, 1983). Downgradient on the west side of the valley, and extending in a belt across the valley, is an area where the chemical type of ground water is predominantly magne- sium sodium or sodium magnesium bicarbonate. The most notable exceptions to these water types are the areas adjacent to Cache and Putah Creeks and a small area around Dunnigan. The chemical water types in these areas are magnesium, calcium magnesium, or magnesium calcium bicarbonate. The water chemistry in the Dunni- gan area is influenced largely by the Pliocene Tehama Formation, of which the Dunnigan Hills are largely composed (Fogelman, 1983). Recharge from Cache and Putah Creeks is the likely source of water in these areas. South of Putah Creek, sodium is again the predominant cation, although small pockets of calcium sodium and magnesium sodium ground water are also present. Magnesium and calcium are the predominant cations in most of the southeastern part of the Sacramento Valley, and in particular, that area underlying the drainages of the American and Cosumnes Rivers. Smaller areas of sodium, sodium calcium, and sodium magnesium bicar- bonate water types are also present in the southeastern Sacramento Valley. The most notable exception to the predominance of bicarbonate as the major anion is the area in the southern part of Sutter basin just south of the Sutter Buttes. In this area, calcium, magnesium, and sodium, as well as chloride, bicarbonate, and sulfate may be found in any combination. Sutter Buttes may be the source of the high sodium and chloride concentrations (Fogelman, 1983), or the source may be a shallow layer of saline water surrounding the base of the Sutter Buttes (Berkstresser, 1973). SAN JOAQUIN VALLEY The distribution of water types or hydrochemical facies in the San Joaquin Valley is more complex than in the Sacramento Valley. The most important difference is the presence of chloride and sulfate as well as bicarbonate as the dominant anions. Generally, chloride predominates in the northwest, sulfate predominates in the southwest, and bicarbonate predominates in the east. The major exceptions to this are the Hanford-Visalia area, where chloride and bicarbonate predominate, and the extreme southeastern part of the valley, where bicarbonate, sulfate, and chloride are all present in varying concen- trations. On the basis of water types, three areas of the San Joaquin Valley can be delineated: the east side, predom- inantly bicarbonate; the axial trough, variable anion composition; and the west side, predominantly sulfate and chloride. Water types of the east San Joaquin Valley are fairly uniform, most commonly resembling the chemical types of the local surface water that recharges the ground water (Dale and others, 1966; Croft and Gordon, 1968; Page and LeBlanc, 1969; Mitten and others, 1970; Soren- son, 1981). Calcium bicarbonate, calcium sodium bicar- bonate, sodium calcium bicarbonate, and calcium magnesium bicarbonate are the predominant water types of ground water of the east side. Because the axial trough has been the discharge area in the past for the San Joaquin Valley, ground water in this area is a combination of water from the east side and the west side (Croft and Gordon, 1968; Bertoldi, 1971). Local recharge from streams and surface water imported via canals that infiltrates from irrigated fields to the water table (Hotchkiss and Balding, 1971) also affects the ground-water chemistry of the axial trough.