The Kesterson Reservoir is the name of a former unit of the Kesterson National Wildlife Refuge which is part of the current San Luis National Wildlife Refuge. The site gained national attention during the latter half of the 20th century due to selenium toxicity and rapid die off of migratory waterfowl, fish, insects, plants and algae within the Kesterson Reservoir.
The Kesterson Reservoir is located in the heart of the San Joaquin Valley in central California. The reservoir and San Luis National Wildlife Refuge are located in western Merced County, approximately 18 miles (29 km) north of Los Banos, California. The refuge includes four units, the Kesterson, Freitas, Bear Creek and original San Luis Units. The refuge is 26,609 acres (107.68 km2) and includes a variety of wetland and riparian habitat which supports a large variety of waterfowl, mammals and other wildlife.
The San Joaquin Valley has been considered by historian Kevin Starr as being "the most productive unnatural environment on Earth" as approximately 25% of the United States’ agricultural products originate from the valley. Examples of the agricultural exports from this area include grapes, cotton, nuts, citrus, and vegetables. Cattle and sheep ranching also contribute to the agricultural output of the area.
The San Joaquin Valley is bordered on the west by the Coast Range and on the east by the Sierra Nevada mountains. The Coast Range include Cretaceous and Tertiary marine sedimentary rocks. Weathering and oxidation of the Moreno Formation, a black marine shale, produces Pyrite, FeS2, and Iron Selenide, FeSe2. As the weathered products concentrate in evaporative minerals and salts, selenates (for instance, Na2SeO4 · 10 H2O or Na2Mg(SeO4)2 · 4 H2O) and Sulfates (Na2SO4 · 10 H2O or Na2Mg(SO4)4 · 4 H2O) can form. This results in selenium salts and selenium rich soils that are sloughed off the mountains via debris flows or landslides into the San Joaquin valley. Additionally, the San Joaquin Valley has a shallow aquifer bounded by impermeable clays.
The climate of the San Joaquin Valley does not lend itself well to agricultural production and results in large scale irrigation projects in order to keep fertile farms in operation. According to some estimates, the climate of the San Joaquin Valley has approximately 10 inches (250 mm) of precipitation and over 90 inches (2,300 mm) of evaporation annually. In order to keep the area productive, irrigation is a requirement.
A side effect of irrigation in the San Joaquin Valley was that ground water levels began to rise over time. This led to a condition where excess water was accumulating and starting to harm crops. In 1968, the Bureau of Reclamation created the 134 km long San Luis Drain and the Kesterson Reservoir. Farmers in the San Joaquin valley installed drainage tiles in an effort to maintain water tables at 2 meters. The Kesterson reservoir was completed in 1971 by the Bureau of Reclamation and consisted of 12 evaporation ponds within the Kesterson National Wildlife Refuge.
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 began to become a problem shortly after drainage tiles were installed. Initially (from 1971–78), the reservoir was receiving all fresh water. After 1978, this began to change and by 1981, all water coming into the Kesterson Reservoir was saline drain water. Contributing to the salinity of the drain water was the highly-mobile ion of selenium, selenate, SeO42−. Selenium begin to bioaccumulate in the waterfowl and wildlife that used the reservoir.
Prior to 1981, the Kesterson Reservoir supported a wide variety of life, including several species of fish. After 1981, the reservoir only supported the most saline tolerant mosquito fish. The habitat change occurred quickly and also included algal blooms and disappearing waterfowl. In 1982, the United States Fish and Wildlife Service began a study to determine the cause for declining wildlife use at the reservoir. Selenium concentrations at these locations were found to be greater than 1400 micrograms per liter.
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