Kesterson Reservoir
Kesterson Reservoir is a series of holding ponds located in Merced County, California, designed to collect drainage from irrigated agricultural lands in the San Joaquin Valley. Originally integrated into the National Wildlife Refuge System in the mid-1970s, the reservoir became a focal point for ecological studies after severe environmental issues arose due to high levels of dissolved selenium in the water. Selenium, a trace element necessary for health in small amounts, became toxic at elevated concentrations, leading to alarming rates of deformities and deaths among migratory birds nesting in the area. By the early 1980s, investigations revealed that bird embryos exhibited significant abnormalities, and adult birds were often found dead, primarily due to bioaccumulation of selenium in the food chain. In response to these findings, drainage into the reservoir was halted in 1986, and mitigation efforts were implemented to reduce contamination risks. The area has since been monitored for selenium levels, as studies suggest that contamination may persist for decades. Research into bioremediation methods, such as using microorganisms to reduce selenium concentrations, has also been explored, highlighting ongoing concerns over the environmental impact of agricultural practices.
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Kesterson Reservoir
IDENTIFICATION: Series of holding ponds for drainage from irrigated agricultural lands in California’s San Joaquin Valley
After the transfer of subsurface drainage water from the San Joaquin Valley into the Kesterson Reservoir led to widespread death and deformity of migratory birds at the Kesterson National Wildlife Refuge, owing to toxic levels of dissolved selenium in the water, the U.S. Geological Survey set up programs to study the ecosystem effects of irrigation drainage water.
Selenium is a naturally occurring trace element that is essential for health in concentrations ranging from 0.05 to 0.3 parts per million (ppm) but becomes increasingly toxic at concentrations exceeding these low dietary levels. Selenium is found in igneous rocks, sedimentary rocks, and fossil fuels. Some soils contain naturally high selenium concentrations, and selenium occurs in the drainage from these soils because its ionic forms are readily soluble.
The Kesterson Reservoir in Merced County, California, was a series of holding ponds, spread over more than 486 hectares (1,200 acres), that collected the surface and subsurface drainage from irrigated agricultural land in the western San Joaquin Valley. Because development had eliminated more than 90 percent of California’s and because of the scarcity and high price of water in California, the US Bureau of Reclamation and the US Fish and Wildlife Service mutually agreed to incorporate the and its waters as part of the National Wildlife Refuge System in the mid-1970s.
By 1982 virtually all the water entering the Kesterson Reservoir was from subsurface drainage, and in 1983 scientists found that aquatic and migratory birds nesting in the refuge had grossly deformed embryos and high embryo rates. By 1984 dead adult birds were being discovered in the area in unusually high numbers. Agricultural chemicals were initially suspected to be the cause of the deaths and deformities, but tissue analysis of the birds revealed that they had selenium concentrations one hundred times in excess of normal concentrations because of selenium bioaccumulation in the food chain.
In 1986 of drainage water into the Kesterson Reservoir was halted, and the reservoir was allowed to drain naturally. In 1988, to protect migratory waterfowl from nesting in periodically flooded zones of the reservoir, about 60 percent of the lowest-lying parts of the reservoir were filled with enough selenium-free backfill to ensure that the average seasonal level remained 15 centimeters (6 inches) below the soil surface. These steps were taken because wildlife biologists determined that the potential for selenium contamination of wildlife was much more limited in dryland environments than in wetland environments.
The Kesterson Reservoir has continued to be monitored because studies indicate that selenium concentrations in plants and wildlife in the area will remain elevated for decades. Bioremediation schemes were also investigated, and it was observed that microorganisms, particularly fungi, were able to volatilize up to 50 percent of the selenium in their vicinity within one year.
Bibliography
Byron, Earl R., et al. “Ecological Risk Assessment Example: Waterfowl and Shorebirds Feeding in Ephemeral Pools atKesterson Reservoir, California.” In Handbook of Ecotoxicology, edited by David J. Hoffman et al. 2d ed. Boca Raton, Fla.: CRC Press, 2003.
Fordyce, Fiona. “Selenium Deficiency and Toxicity in the Environment.” In Essentials of Medical Geology: Impacts of the Natural Environment on Public Health, edited by Olle Selinus et al. Burlington, Mass.: Elsevier Academic Press, 2005.