Selenium is a naturally occurring element in the environment, and is an essential dietary nutrient. However, in high concentrations, selenium bioaccumulates to toxic levels in tissues and can cause reproductive failure and birth defects, or deposit in sediments. Due to the concentration of selenium in the Coast Range west of the San Joaquin Valley, selenium was transported into the valley and naturally accumulated on the valley floor. Selenium toxicity became a problem shortly after drainage tiles were installed at Kesterson. Initially (from 1971–1978), the reservoir received only fresh water. In 1978 this began to change and by 1981, all water coming into the Kesterson Reservoir was saline drainage water. Contributing to the salinity of the drainage water was the highly-mobile ion of selenium, selenate, SeO42−. In 1983 selenium concentrations, primarily in the selenate form, were about 300 µg/L. From 1981 to 1986 about 9000 kg of Se were delivered to Kesterson. Concentrations of selenium in the reservoir increased steadily as the drainage water evaporated from the 12 evaporation ponds. Selenium began to bioaccumulate in the waterfowl and wildlife that used the reservoir.