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/ Subsurface agricultural drainage waters from western San Joaquin Valley, California, were found to contain elevated concentrations of the element selenium in the form of selenate. In 1978, these drainage waters began to replace previous input to Kesterson Reservoir, a pond system within Kesterson National Wildlife Refuge; this substitution was completed by 1982. In the 1983 nesting season, unusual rates of deformity and death in embryos and hatchlings of wild aquatic birds (up to 64% of eared grebe and American coot nests) occurred at the refuge and were attributed to selenium toxicosis. Features necessary for contamination to have taken place included geologic setting, climate, soil type, availability of imported irrigation water, type of irrigation, and the unique chemical properties of selenium. The mechanisms of biogeochemical cycling raise questions about other ecosystems and human exposure.California is the leading agricultural state in the United States today mainly as a result of the productivity of its Central Valley (Figure 1), which relies on a program of extensive government-supported irrigation. However, the geologic setting and climate of the southwestern Central Valley have created soil salinization problems that are accentuated by that irrigation. Engineering solutions to save threatened agricultural lands from salinization have themselves created problems that affect the quality of irrigation return waters. These waters have been used in wildlife habitats where serious biological effects from elevated levels of selenium have been documented. The contamination of the ecosystems has developed from naturally occurring selenium, in a form that is highly mobile in the environment. In this article, we summarize the history of the geology and of agricultural water development in the Central Valley, outline geochemical pathways of selenium, and discuss the occurrence, causes, and impacts of water quality problems resulting from agricultural wastewater disposal. Our goal is to describe a case of conflict between maintaining agricultural production in a marginally supportive environment and retaining an ecosystem capable of supporting major environmental resources--

Severe gross and microscopic lesions and other changes were found in adult aquatic birds and in embryos from Kesterson Reservoir (a portion of Kesterson National Wildlife Refuge), Merced County, Calif., during 1984. Adult birds from that area were emaciated, had subacute to extensive chronic hepatic lesions, and had excess fluid and fibrin in the peritoneal cavity. Biochemical changes in their livers included elevated glycogen and non-protein-bound sulfhydryl concentrations and glutathione peroxidase activity but lowered protein, total sulfhydryl, and protein-bound sulfhydryl concentrations. Congenital malformations observed grossly in embryos were often multiple and included anophthalmia, microphthalmia, abnormal beaks, amelia, micromelia, ectrodactyly, and hydrocephaly. Mean concentrations of selenium in livers (94.4 ppm, dry weight) and kidneys (96.6 ppm) of birds collected at the Kesterson ponds were about 10 times those found at a nearby control area (8.3 and 12.2 ppm). We conclude that selenium present in the agricultural drainage water supplied to the Kesterson ponds accumulated in the food chain of aquatic birds to toxic concentrations and caused the lesion and other changes observed.