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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A41
<br />that ground-water quality is degrading as a result of
<br />increasing application of agricultural chemicals and
<br />growth of urban population.
<br />The concentrations of dissolved solids increased signif-
<br />icantly since the 1950's throughout the Sacramento
<br />Valley except for an area south of Sutter Buttes between
<br />the Sacramento and Feather Rivers (Hull, 1984). How-
<br />ever, increases in nitrate concentrations were found only
<br />on the west side and in the southeastern part of the valley
<br />(Hull, 1984). Hull (U.S. Geological Survey, oral com-
<br />mun., 1984) indicated that the rate of nitrate build-up has
<br />been increasing since 1912. He found that in 1912 to 1913,
<br />2.2 percent of the wells had nitrate concentrations
<br />greater than 5.5 mg/L; between 1960 and 1969, 4.9
<br />percent had concentrations exceeding 5.5 mg/L, and from
<br />1974 to 1978, 10.5 percent had concentrations exceeding
<br />5.5 mg/L.
<br />Studies to determine human impact on ground-water
<br />quality in the San Joaquin Valley were not done, largely
<br />because different time periods of sample collection and
<br />the use of different analytical techniques make the
<br />comparability of the existing data questionable. Because
<br />agricultural practices in the San Joaquin Valley are
<br />similar to those in the Sacramento Valley, it is likely that
<br />ground-water quality in the San Joaquin Valley is also
<br />degrading as a result of human activities. However,
<br />differences in geology, soils, and irrigation techniques
<br />could all affect the impact of human activities. Areas of
<br />possible contamination of ground water due to human
<br />activities in the San Joaquin Valley were mapped by
<br />Templin (1984, pi. 8). Sources listed were industrial,
<br />municipal and domestic solid-waste disposal sites, and
<br />agricultural chemical build-up. This map suggests that
<br />human-induced contamination exists throughout much of
<br />the valley.
<br />The use of pesticides is another possible impact on
<br />ground water that has not been studied in detail.
<br />Pesticides have been used intensively in the Central
<br />Valley for many years and because dissolved-solids and
<br />nitrate concentrations are increasing, it is likely that
<br />pesticide concentrations in ground water also are increas-
<br />ing.
<br />The presence of dibromochloropropane (DBCP) in
<br />ground water in the San Joaquin Valley was mapped by
<br />Templin (1984, pi. 7D). The presence of this pesticide at
<br />levels above 0.0005 mg/L near Bakersfield, Fresno,
<br />Modesto, and north of Merced and Stockton coincides
<br />with land-use patterns. More specifically, these areas are
<br />occupied by orchards and vineyards, where DBCP is
<br />commonly used. The presence of DBCP in the San
<br />Joaquin Valley suggests that other pesticides have prob-
<br />ably built up in ground water of the Central Valley.
<br />Selenium, which is toxic to humans and animals at very
<br />low concentrations, occurs naturally in the soils and
<br />ground water on the west side of the San Joaquin Valley
<br />(Deverel and others, 1984). Ecological and health effects
<br />of selenium and other trace elements in agricultural
<br />drainage water on the west side of the San Joaquin Valley
<br />have become subjects of extensive study since high
<br />incidences of mortality and birth defects were observed
<br />in waterfowl nesting in the area where drainage water is
<br />discharged (U.S. Bureau of Reclamation, 1984). In a
<br />preliminary study (Deverel and others, 1984), the areal
<br />distribution of selenium and other inorganic constituents
<br />was examined along the west side of the valley west of
<br />Fresno. Selenium concentrations (median concentrations
<br />of 10-11 mg/L) were highest in the central and southern
<br />parts (south of Los Banos and south of Mendota) of the
<br />area studied. Extensive studies by U.S. Department of
<br />Interior agencies (Bureau of Reclamation, Fish and
<br />Wildlife Service, and the Geological Survey) and also
<br />many studies by universities, State water resources
<br />agencies, and concerned local agencies are still in
<br />progress (1989).
<br />SELECTED REFERENCES
<br />Ayers, R.S., 1977, Quality of water for irrigation: Proceedings of the
<br />American Society of Civil Engineers, Journal of the Irrigation and
<br />Drainage Division, v. 103, no. IR2, p. 135-154.
<br />Back, William, 1961, Techniques for mapping of hydrochemical facies,
<br />in Geological Survey research, 1961: U.S. Geological Survey
<br />Professional Paper 424-D, p. D380-D382.
<br />Bailey, E.H., Blake, M.C., Jr., and Jones, D.L., 1970, On-land
<br />Mesozoic crust in California Coast Ranges: U.S. Geological Survey
<br />Professional Paper 700-C, p. C70-C81.
<br />Bennett, G.D., 1979, Regional ground-water systems analysis: Water
<br />Spectrum, fall 1979, p. 36-42.
<br />Berkstresser, C.F., Jr. 1973, Map showing base of fresh ground-water,
<br />approximately 3,000 micromhos, in the Sacramento Valley and
<br />Sacramento-San Joaquin Delta, California: U.S. Geological Survey
<br />Water-Resources Investigations Report 40-73, 1 sheet.
<br />Berkstresser, C.F., Jr., French, J.J., and Schaal, M.E., 1985, Data for
<br />four geologic test holes in the Sacramento Valley, California: U.S.
<br />Geological Survey Open-File Report 85-488, 110 p.
<br />Bertoldi, G.L., 1971, Chemical quality of ground water in the Dos
<br />Palos-Kettleman City area, San Joaquin Valley, California: U.S.
<br />Geological Survey open-file report, 45 p.
<br /> 1976, Chemical quality of ground water in the Tehama-Colusa
<br />Canal service area, Sacramento Valley, California: U.S. Geological
<br />Survey Water-Resources Investigations Report 76-92, 44 p.
<br /> 1979, A plan to study the aquifer system of the Central Valley
<br />of California: U.S. Geological Survey Open-File Report 79-1480,
<br />48 p.
<br />Blair, T.A., andFite, R.C., 1957, Weather elements: Englewood Cliffs,
<br />New Jersey, Prentice-Hall, 414 p.
<br />Bloyd, R.M., Jr., 1978, Ground-water conditions in the Sacramento
<br />Valley, California, 1912, 1961, and 1971, in Evaluation of ground-
<br />water resources Sacramento Valley: California Department of
<br />Water Resources Bulletin 118-6, p. 97-136.
<br />Brown, Eugene, Skougstad, M.W., and Fishman, M.J., 1970, Methods
<br />for collection and analysis of water samples for dissolved minerals
<br />and gases: U.S. Geological Survey Techniques of Water-Resour-
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