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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A39
<br />Two depth zones are recognizable in the axial trough.
<br />In the lower zone, water types are generally sodium
<br />bicarbonate, sodium chloride, and calcium sodium bicar-
<br />bonate. Ground water in the upper zone is more variable
<br />than in the lower zone.
<br />Generally, sodium and calcium are the dominant cat-
<br />ions of the axial trough ground water. In the northern
<br />part of the valley, chloride and bicarbonate are the
<br />dominant anions, and in the southern part of the valley,
<br />sulfate and bicarbonate are the dominant anions.
<br />There is also a large amount of areal and vertical
<br />variability of water types on the west side of the San
<br />Joaquin Valley; here, in places, the Corcoran Clay
<br />Member of the Tulare Formation separates different
<br />types of ground water. Water types in the zone above the
<br />Corcoran Clay Member are less variable than those of the
<br />axial trough. Sodium is the predominant cation in this
<br />area with few exceptions. Sodium and sodium calcium
<br />predominate in the south, and sodium and calcium sodium
<br />predominate in the north. Sulfate is the predominant
<br />anion in the zone above the Corcoran Clay Member in the
<br />south (Davis and Poland, 1957; Wood and Davis, 1959;
<br />Hotchkiss and Balding, 1971), whereas chloride and
<br />bicarbonate predominate in the north. The presence of
<br />bicarbonate in the north is attributed to recharge from
<br />intermittent streams (Hotchkiss and Balding, 1971).
<br />Chemical analyses for wells perforated below the
<br />Corcoran Clay Member north of the Fresno-Merced
<br />County line are limited, but the water type is probably
<br />similar to that above the clay. South of this area to the
<br />Tulare Lake bed, the water type below the Corcoran
<br />Clay Member is generally sodium sulfate (Bertoldi,
<br />1971). The few wells that are perforated below the
<br />Corcoran Clay Member south of the Tulare Lake bed tap
<br />ground water that is generally sodium chloride (Dale and
<br />others, 1966).
<br />PROBLEM COMPOUNDS
<br />Local concentrations of boron, chloride, and nitrate in
<br />the Central Valley are high enough to be a problem either
<br />to crops or to humans. Other constituents, such as
<br />pesticides and trace metals, have been investigated only
<br />on a random basis and, with the exception of selenium in
<br />the western San Joaquin Valley, are not known to be a
<br />problem.
<br />CHLORIDE
<br />High chloride concentrations are generally not consid-
<br />ered a health hazard. On the basis of taste preference, the
<br />U.S. Environmental Protection Agency (1979) recom-
<br />mends a limit of 250 mg/L for chloride in drinking water.
<br />High chloride concentrations can be toxic to plants, but
<br />salinity usually impairs growth before chloride alone
<br />reaches toxic levels. Water with chloride concentrations
<br />up to 700 mg/L can be used on most crops, depending on
<br />soils and irrigation practices, without impairing growth
<br />(National Academy of Sciences and National Academy of
<br />Engineering, 1973).
<br />Chloride concentrations of Central Valley ground wa-
<br />ter generally are less than 250 mg/L; however, several
<br />areas are notable for having higher chloride concentra-
<br />tions.
<br />Two bands of high chloride concentrations are located
<br />in the Sacramento Valley. One band is adjacent to Salt
<br />Creek near Williams; the other is adjacent to Petroleum
<br />and Salt Creeks near Arbuckle. In these areas, high
<br />chloride concentrations are attributed to the recharge
<br />from local streams (Bertoldi, 1976). A third area of high
<br />chloride concentrations is south of Sutter Buttes, in the
<br />southwestern part of the Sutter basin. This area coin-
<br />cides with a shallow saline water body previously de-
<br />scribed by Berkstresser (1973), which is probably the
<br />source of the high chloride concentrations.
<br />The most notable locations of high chloride concentra-
<br />tions in the San Joaquin Valley are in the northwestern
<br />and north-central part of the valley along the course of
<br />the San Joaquin River and adjacent lowlands. Within this
<br />area, depth to the base of freshwater is shallower (500 ft
<br />or less) than elsewhere in the valley. Ground-water flow
<br />was upward prior to development and, currently, flow
<br />remains upward locally in this area (Williamson and
<br />others, 1989). Therefore, the most probable source of
<br />high chloride in the shallow ground water is upward flow
<br />of saline ground water. Sorenson (1981) mapped high
<br />chloride concentrations adjacent to and west of the San
<br />Joaquin River in San Joaquin and Contra Costa Counties.
<br />Others mapped high chloride concentrations all along the
<br />San Joaquin River (Page and LeBlanc, 1969; Mitten and
<br />others, 1970; Hotchkiss and Balding, 1971; Page and
<br />Balding, 1973).
<br />BORON
<br />Boron is a critical element in irrigation water supplies.
<br />In small quantities, boron is an essential micronutrient;
<br />however, boron becomes toxic to sensitive plants at
<br />concentrations as low as 0.75 mg/L and is toxic to most
<br />crops at concentrations exceeding 4.0 mg/L. Within this
<br />range, crops have been classified into three categories of
<br />boron tolerance: sensitive (less than 1 mg/L), semitoler-
<br />ant (1-2 mg/L), and tolerant (more than 2-4 mg/L)
<br />(National Academy of Sciences and National Academy of
<br />Engineering, 1973).
<br />Boron is found in concentrations greater than 0.75
<br />mg/L in several areas of the Central Valley. Small areas
<br />of high boron concentrations have been observed in the
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