Technical Reviews

Water Quality in the San Joaquin-Tulare Basins: California, 1992-1995.
Neil M. Dubrovsky, Charles R. Kratzer, Larry R. Brown, JoAnn M. Gronberg
and Karen R. Burrow

U.S. Department of the Interior, U.S. Geological Survey, Circular 1159. 1998

This 38-page report contains full references, a glossary, multiple full-color maps, figures, photos, and data tables.

The National Water Quality Assessment Program

United States Geological Survey (USGS) Circular 1159 is part of the National Water Quality Assessment (NAWQA) Program, the USGS’ large scale environmental water monitoring. NAWQA’s mission is to “provide a strong and unbiased basis for better decision making by… the United States Congress, Federal, State, and local agencies, environmental groups, and industry. Information from the NAWQA Program also will be useful for guiding research, monitoring, and regulatory activities in cost effective ways.”

NAWQA studies look at both groundwater and surface water in 60 watersheds or groundwater basin areas, which represent drinking water sources for 70 percent of the U.S. population. Generally, the studies monitor the presence, source, and destination of pesticides, nutrients, volatile organic chemicals, and trace elements. Aquatic ecosystem health is also assessed through water chemistry and the presence or absence of native and introduced species.

NAWQA and the San Joaquin-Tulare Basins Study Unit

Circular 1159 describes the San Joaquin-Tulare Basin Study Unit which covers 31,200 square miles in the San Joaquin Valley, the eastern slopes of the coast range, and the western slopes of the Sierra Nevada Mountains. Larger cities in the basins include Bakersfield, Visalia, Fresno, Merced, Modesto, and Stockton. Environmental measurements for this study were conducted from 1992-1995. Although Circular 1159 is an integral part of a national study, it also is intended to serve as a stand-alone resource for those interested in water quality in California.

Definitions of Criteria and Levels of Detection

When the NAWQA researchers measure pollutants in the water, they compare their findings to previously defined criteria (also called standards or guidelines). The United State Environmental Protection Agency (US-EPA), state, and other agencies have previously published these criteria as the maximum allowable concentrations of pollutants in water that are safe for a certain use. The two common uses are drinking water for humans or supporting aquatic life. Criteria can also be defined as “acute” concentrations, possibly causing immediate harm, or much lower “chronic” concentrations, possibly causing harm through continued exposure over long periods of time.

The analytical methods used to detect chemical contaminants have dramatically improved in recent years. Very small amounts of chemicals can be detected. The level of detection for some chemicals is so low that most scientists agree these low concentrations pose no immediate threat to human or aquatic health. Concentrations of contaminants need to be compared to criteria and monitored over time to make sure the concentrations are not increasing.

Selected Major Issues and Findings for Surface Water

• A wide variety of pesticides occur in the San Joaquin River and its tributaries, some at concentrations high enough to adversely impact aquatic life.
• Fish communities in the lower San Joaquin River were highly degraded compared with other NAWQA Study Units, or regions, as was stream habitat at some sites.
• Some nitrate and ammonia concentrations exceed criteria in some small tributaries, but generally do not limit beneficial uses in the main stem of the San Joaquin River.
• Long-banned organochlorine insecticides continue to be transported to streams by soil erosion of contaminated agricultural fields, resulting in contamination of suspended sediment, bed sediment, and aquatic organisms. During measurements made in June 1994, and during the winter storms of January 1995, most whole-water concentrations of p,p´-DDT, chlordane, dieldrin, and toxaphene exceeded chronic criteria for the protection of freshwater aquatic life.
• Forty-nine pesticides were detected in the San Joaquin River and three subbasins, 22 of which were detected in more than 20 percent of the samples. No drinking-water standards were exceeded, but the concentrations of seven pesticides exceeded the criteria for the protection of aquatic life.

Selected Major Issues and Findings for Ground Water

• Nitrate concentrations in ground water frequently exceeded drinking water standards; however, pesticide concentrations rarely exceeded drinking water standards, with the notable exception of 1,2,-dibromo-3-chloropropane (DBCP). DBCP is a soil fumigant banned since 1977.
• Nitrate concentrations in ground water in the eastern San Joaquin Valley exceeded the US-EPA drinking water standard in about one-fourth of the domestic water supply wells sampled.
• Nitrate concentrations in ground water have increased since the 1950s. From 1950 to 1980, the largest source of nitrate–nitrogen fertilizer–also increased from 114 to 745 million pounds per year.
• Pesticides were detected in about two-thirds of the ground water samples collected from domestic water supply wells, but concentrations of most pesticides were low–less than 0.1 microgram per liter (µg/L).
• Pesticide concentrations in ground water generally have not increased in the last decade on the basis of a small number of wells sampled (19) during 1986-87 and again in 1995. Direct comparison of the data is difficult because of changes in detection limits.

This report is free upon request by contacting, U.S. Geological Survey Information Services, Box 25286 Federal Center, Denver, CO 80225. A complete color version is available on line at http://water.usgs.gov/pubs/circ1159

DEC. 604

Contributed by Max Stevenson