The thickness of aquifer saturated with freshwater (water with less than 1,000 milligrams per liter dissolved-solids concentration) in the Central Valley aquifer system varies greatly (fig. 98) and depends, for the most part, on the depth to and permeability of the rocks that underlie continental deposits. In the Sacramento Valley, the base of freshwater generally coincides with the base of continental deposits. The several isolated lenses of saline water that are within the freshwater zone may be evaporation residues or estuarine water that was trapped by subsequent sedimentation. The depth to the base of freshwater is as much as 2,500 feet in the Sacramento Valley.
In the San Joaquin Valley, the pattern is more complex. Freshwater is mainly in continental deposits, but also is in Tertiary marine rocks on the southeastern side of the valley and in pre-Tertiary igneous and metamorphic rocks. However, sediments of continental origin are the primary source of freshwater. The thickness of aquifer saturated with freshwater in the San Joaquin Valley ranges from 100 to more than 4,000 feet (fig. 98).
Freshwater is available throughout most of the Central Valley. The concentration of dissolved solids in the ground water reflects the general chemical character of water in the streams that recharge the aquifer system. Dissolved-solids concentrations in the streams, in turn, are directly related to the type of rocks that form the mountains in which the streams rise. Stream water from the Cascade Range and the Sierra Nevada, which are underlain primarily by igneous rocks, has much smaller dissolved-solids concentrations than water from the Coast Ranges, which are underlain by marine sedimentary rocks. Thus, ground water in the Sacramento Valley and the east side of the San Joaquin Valley has generally smaller dissolved-solids concentrations than water from wells on the west side of the valley (fig. 99).
In general, dissolved-solids concentrations increase with depth in the Central Valley aquifer system. Therefore, the generally deeper wells in the western and southern parts of the San Joaquin Valley are likely to produce water with larger dissolved-solids concentrations than the generally shallower wells in the Sacramento Valley and the eastern part of the San Joaquin Valley.
Ground water in agricultural areas can become excessively saline and damaging to crops because evaporation of sprayed irrigation water and evapotranspiration of soil moisture and shallow ground water leaves behind dissolved salts. As a result, the concentration of salts in the soil and shallow ground water increases and may reach levels detrimental to plant growth. Shallow irrigation wells worsen the problem by recir-culating the saline shallow ground water, thus accelerating the process. The only remedy for this problem is to provide subsurface drainage to remove the shallow saline ground water.
The potential for crop damage due to saline irrigation-return flow is widespread in the Sacramento--San Joaquin Delta and the San Joaquin Valley (fig. 100). Although several individual irrigation return-water drainage systems are operated, no current (1995) valleywide system removes shallow saline ground water. An estimated 400,000 to 700,000 acres of arable land may be lost by 2010 because of increasing soil and water salinity with an accompanying loss of from $32 million to $320 million annually.
Soils on the western side of the San Joaquin Valley are derived primarily from the marine rocks that form the western boundary of the aquifer system and contain relatively large amounts of selenium, which also is in ground water in the marine rocks and is concentrated in the soil by evapotranspiration. Excess irrigation water applied to leach salts from the soil, thus preventing salt buildup, leaches selenium from the soil and the marine rocks and transports it to shallow ground water or surface drains. Water that contains dissolved-selenium concentrations of 1,400 micrograms per liter is in some of the regional surface drains, and a concentration of 4,200 micrograms per liter was reported in water from the drainage system on one farm. Concentrations of dissolved selenium as large as 3,100 micrograms per liter have been detected in shallow ground water in the western part of the San Joaquin Valley; the largest concentrations are west of the San Joaquin River flood plain.
Although large local concentrations of selenium are in the western part of the San Joaquin Valley, evidence that drinking-water supplies in the Central Valley are currently (1995) at serious risk because of selenium contamination is lacking. The large selenium concentrations in the western part of the San Joaquin Valley are known only to be in the shallow ground water and not in the deeper parts of the aquifer system from which most wells that supply municipalities obtain water. A 1986 survey of wells outside of the western part of the San Joaquin Valley indicated selenium concentrations to be larger than 10 micrograms per liter in water from only 3 of 113 wells sampled in the Central Valley (fig. 101).
Boron is an essential micronutrient but may be toxic to sensitive plants in concentrations as low as 0.5 milligram per liter. Currently (1995), the U.S. Environmental Protection Agency has no standards for boron in drinking water. Boron is found in concentrations potentially harmful to plants in the northern and southwestern parts of the Sacramento Valley and in the Tulare Basin in the extreme southern part of the San Joaquin Valley (fig. 101). Large concentrations of boron also have been detected in shallow ground-water in the western part of the San Joaquin Valley. Chromium and mercury have been detected in shallow ground water in the western part of the San Joaquin Valley at levels that exceed U.S. Environmental Protection Agency standards for protection of aquatic life.
Excessive concentrations of nitrate in water are potentially harmful to infants and young children, and the maximum recommended for drinking water by the U.S. Environmental Protection Agency is 10 milligrams per liter. Some crops may be affected by nitrate concentrations as low as 5 milligrams per liter. During a survey conducted in 1983, nitrate levels in ground water were found to exceed 10 milligrams per liter in three areas in the Sacramento Valley (fig. 101). The contaminated wells were shallow, and the source of nitrate pollution was attributed to effluent from waste-treatment facilities, discharge from septic tanks, or leaching of nitrogen fertilizers. Occurrences of nitrate in concentrations of greater than 5 milligrams per liter are sporadic in the San Joaquin Valley and seem to be confined mainly to the shallow parts of the aquifer. The contamination is usually attributable to local sources, such as septic tanks, feed lots, and dairies.
Agricultural use of pesticides is widespread in the Central Valley. Dibromochloropropane (DBCP), which is a potential carcinogenic nematocide, is in ground water in every county in the San Joaquin Valley (fig. 101), and has been detected in 2,522 of 8,190 private and public-supply wells sampled from 1979 through 1984; in California, DBCP has been outlawed from use since August 1977. At least 50 other pesticides, including 1,2--dichloropropane and ethylene dibromide, had been detected in ground water in the Central Valley by 1984.