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