Areal distribution of selenium and other inorganic constituents in shallow ground water of the San Luis Drain service area, San Joaquin Valley, California: A preliminary study

Abstract

This study is a preliminary assessment of the areal distribution of selenium and other inorganic chemical constituents in shallow ground water of the San Luis Drain service area. In previous studies, high concentrations of selenium were detected in agricultural drainage water in parts of the service area. Shallow ground water was sampled at 130 sites during May 5-21, 1984. The sites were distributed among three different physiographic zones of the service area--the alluvial fan zone along the western edge of the area, the basin trough zone adjacent to the San Joaquin River, and the basin rim zone in between. Most of the shallow ground water throughout the service area was alkaline and slightly saline. Sulfate was the dominant anion in 62 percent of the samples and sodium was the dominant cation in 57 percent of the samples. There were significant (a = 0.05) differences between zones in which ions were dominant. Boron concentrations exceeded the water-quality criterion for irrigation use in 80 percent of the samples. All eight priority pollutants included in the sample analyses-arsenic, cadmium, chromium, copper, lead, mercury, selenium, and zinc were detected at least once, but only chromium, mercury, and selenium occurred in concentrations approaching water-quality criteria for protection of freshwater aquatic life. Selenium was detected in 76 percent of samples, chromium in 77 percent of samples, and mercury in 32 percent of samples. Median selenium concentrations of 10 micrograms per liter in the basin rim zone and 11 micrograms per liter in the alluvial fan zone were not significantly (a = 0.05) different from each other, but both were significantly higher than the median concentration of less than 1 microgram per liter in the basin trough zone. Six of the ten highest selenium concentrations (370 to 3,800 micrograms per liter) were in the basin rim zone and four were in the alluvial fan zone. Overall, selenium concentrations were highest in the central and southern parts of the alluvial fan and basin rim zones, and lowest in the northern parts of all zones and in the entire basin trough zone. 1 Chemists, U.S. Geological Survey, Sacramento, Calif. 2 Hydrologist, U.S. Geological Survey, Sacramento, Calif. 3 Hydrologist, U.S. Geological Survey, Laguna Niguel, Calif. 4 Physical Scientist, U.S. Bureau of Reclamation, Sacramento, Calif. SELENIUM AND OTHER CONSTITUENTS, SAN JOAQUIN VALLEY