San Joaquin River Valley, California

The irrigation systems were constructed in this valley in 1930-1960s. It was found afterward that irrigation drainage water from parts of the San Joaquin Valley contains levels of selenium and other trace elements that have been implicated in bird deformations in the Kesterson Reservoir. Depending on location and season, the drainage water contains 100-1400 pg/L Se, predominantly as selenate (SeO 2, Se-IV), the most soluble form, whereas the California State Water Resources Control Board has recommended an interim maximum mean monthly selenium concentration of 2-5 pg/L in receiving waters and wetlands. 

Irrigation using surface and ground water in this region has substantially altered the physical and chemical nature of the groundwater flow system and Anally the total biogeochemical food web. 

The deposits of the San Joaquin Valley are derived from the Sierra Nevada on the East and the Coast Range on the West. The Sierra Nevada, a fault block that dips 

In the lower fan areas Se concentrations were monitored up to 400 pg/L. Similar historical distribution of soil Se content and shallow groundwater content indicate that dissolved selenium species were leached from saline soils by irrigation water. The drainage discharge of shallow groundwater and subsurface irrigation water was accompanied by increasing accumulation of Se in Kesterson Reservoir. 

Aquatic birds nesting at Kesterson Reservoir in 1983 were found to have high rates of embryo deformities and mortality. Beginning in 1984, adult birds were also found dead in unusually high numbers. Through a series of field and laboratory studies, these effects were attributed to the exceptionally high concentrations of selenium in the biogeochemical food web of the birds. 

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