Large concentrations of chloride can make water unusable for drinking and can also be toxic to plants. Drinking-water recommendations of the U.S. Environmental Protection Agency suggest a chloride concentration of less than 250 milligrams per liter. However, chloride in concentrations as low as 106 milligrams per liter may be toxic to some plants; such concentrations have been detected in ground water in the Santa Rosa Basin. Sources of chloride in the north San Francisco Bay area aquifers include seawater intrusion, thermal water, and dissolved minerals from marine and volcanic rocks. The valleys most affected by large chloride concentrations are the Petaluma, the Sonoma, and the Napa, in which seawater intrusion caused by excessive ground-water withdrawals has been the primary source (fig. 110). Reduced withdrawals and increased surface-water imports have helped alleviate the salinity problem.
Excessive sodium in irrigation water can be toxic to plants and can decrease soil permeability. Possible sources of sodium in the north San Francisco Bay area valleys include cation exchange between ground water and clay minerals, upward migration of salty water along faults, dissolved minerals in water from marine sediments, thermal water, and seawater intrusion.
Sodium is often the dominant cation in ground water in the north San Francisco Bay area valleys and has been reported locally in concentrations in excess of 250 milligrams per liter, which is sufficiently large to be of concern. The problem is widespread in the Santa Rosa Basin, where large sodium concentrations are thought to be related primarily to cation exchange. The source of excessive sodium in the other four valleys could be one or all of the sources listed above, but seawater intrusion is the primary source in the alluvial-fan deposits in the southern ends of the Petaluma, the Sonoma, and the Napa Valleys.
Although essential to plant growth in small amounts, boron in excess of 0.5 milligram per liter can be stressful or toxic to many plants, and water with a boron concentration of greater than 2.0 milligrams per liter is toxic to most plants. Boron is usually associated with water that has a large sodium concentration and the sources for boron in the area are generally the same as for sodium. Ground water that has a boron concentration of 0.5 milligram per liter or larger is found in scattered wells throughout the north San Francisco Bay area.
The presence of nitrate in ground water is usually an indication of contamination by septic tanks, fertilizers, or waste from farm animals. Large nitrate concentrations can cause methemoglobinemia (a blood disease) in infants, and State drinking-water standards in California have been set at 45 milligrams per liter of nitrate, or 10 milligrams per liter nitrogen. Nitrate is not a widespread problem in most of the north San Francisco Bay area, except locally, northwest of Petaluma (fig. 110) where nitrate concentrations are as large as three times the maximum allowed for drinking water. The probable sources appear to be septic-tank leachate plus livestock and poultry manure that was placed in unlined pits.