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A40 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA extreme northern and extreme southern parts of the valley. Fogelman (1983) delineated an area east of Red Bluff where ground water had concentrations of boron exceeding 0.75 mg/L. These high boron concentrations are attributed to the nearby Pliocene Tuscan Formation (Fogelman, 1983). Wood and Dale (1964) reported concentrations of boron generally greater than 3 mg/L in the area southwest of Bakersfield and ranging from 1 to 4 mg/L in an area southeast of Bakersfield. Dale and others (1966) noted boron concentrations as high as 4.2 mg/L near Button- willow Ridge and Buena Vista Slough. The nearby marine sedimentary deposits were the probable source of the high boron concentrations of the west side, whereas the east side concentrations were probably derived from continental sedimentary deposits (Wood and Dale, 1964). A large area of high boron concentrations in the southwestern part of the Sacramento Valley extends from Arbuckle on the north to Rio Vista on the south. There is one band of low boron water extending through the center of this area from Vacaville to West Sacra- mento. High boron concentrations in this area were attributed to marine deposits of the Upper Cretaceous Chico Formation and Lower Cretaceous rocks of the Coast Ranges, from which the recharge water is derived (Fogelman, 1983). Another large area of high boron concentrations, in the northwestern part of the San Joaquin Valley, extends from the northernmost edge of the valley west of the San Joaquin River to the Kings-Fresno County line. Bertoldi (1971) reported high boron concentrations in the lower zone in the southern part of this area. Bertoldi also found high boron concentrations near the Diablo Range, indi- cating that these marine sediments of the range are the likely source of the boron. Sorenson (1981) also attributed high boron concentrations in the northern part of the valley to the marine sediments of the Coast Ranges. NITRATE Nitrate toxicity usually does not affect adults, but it can cause a blood disorder known as methemoglobinemia, which is sometimes fatal in infants and young children. The recommended maximum concentration in drinking water for nitrate (as nitrogen) is 10 mg/L (National Academy of Sciences and National Academy of Engineer- ing, 1973). Nitrate in irrigation water is usually considered an asset because of its value as a fertilizer. However, some crops such as sugar beets, apricots, grapes, citrus, and avocados are sensitive and may be adversely affected by high nitrate concentrations. Problems can result from concentrations as low as 5 mg/L, and severe problems result from concentrations above 30 mg/L (Ayers, 1977). Concentrations of nitrate as nitrogen exceeding 30 mg/L are very rare and extremely localized in the Central Valley. However, there are several areas where concen- trations exceed 10 mg/L. Several potential problem areas were delineated in the Sacramento Valley with respect to nitrates in drinking water (Fogelman, 1983). Hull (1984) estimated the max- imum concentration of nitrate under natural conditions of the Sacramento Valley to be 3 mg/L and considered that areas having 5.5 mg/L or more are those in which nitrate concentrations may be increasing. Forty wells in the Chico-Corning area contained water in which nitrate- nitrogen concentrations exceeded 5.5 mg/L, and 25 percent of these wells contained water with concentra- tions exceeding 10 mg/L. In the Gridley-Marysville area, 21 wells contained water in which concentrations of nitrate-nitrogen exceeded 5.5 mg/L, and 33 percent of these contained water with concentrations exceeding 10 mg/L. In both areas, the wells containing high nitrate concentrations are shallow, and surface contamination from leaching of applied nitrate fertilizers, urban waste- treatment facilities, or septic systems was suggested as the probable cause (Fogelman, 1983). Sorenson (1981) reported nitrate concentrations greater than 5 mg/L over a large part of southern San Joaquin County between Lodi and Stockton. These high concentrations were attributed to agricultural practices. In several other small areas of the San Joaquin Valley, ground water contains concentrations of nitrate exceed- ing 10 mg/L. Such an area south of Bakersfield was identified by Wood and Dale (1964) and 2 years later another such area slightly north of Bakersfield was identified by Dale and others (1966). High concentrations also have been reported around the Fresno metropolitan area. In this area, nitrate concentrations decrease with depth (Page and LeBlanc, 1969), indicating surface contamination. Sporadic high concentrations of nitrate were also found near the foothills of the Sierra Nevada in the Hanford-Visalia area (Croft and Gordon, 1968). Other occurrences of nitrate-nitrogen exceeding 10 mg/L are extremely localized and usually are attributed to localized pollution sources such as septic tanks, dairies, or feed lots (Sorenson, 1981; Bertoldi, 1971). EFFECTS OF HUMAN ACTIVITIES Temporal changes in dissolved-solids and nitrate con- centrations in the Sacramento Valley were studied by Hull (1984). Dissolved-solids concentrations were used as an indicator of changes in the overall water quality, and nitrate concentrations were used as an indicator of human sources such as applied fertilizers or human waste. Significant increases in concentrations of both dissolved solids and nitrates were observed, indicating