Selenium and the San Joaquin Valley

Selenium-topsoilOften lost in the hubbub in the summer of 2009 about the trials and tribulations of western San Joaquin Valley farmers beset by vanishing water allocations from the federal Central Valley Project, is the subject of toxic selenium (Se) haunting their soils.

Although humans can benefit from its role as an antioxidant in the foods that make up our daily nutrition, selenium is the most dose-sensitive of all nutrients. Toxicity occurs via biochemical pathways unable to distinguish selenium from sulfur, thus substituting excess selenium into proteins and altering their structure and function. This can lead to serious deformities in developing embryos, and disease in adult organisms, such as occurred in the early 1980s at the Kesterson National Wildlife Refuge.

Historically, farmers in the Grasslands area of the western San Joaquin Valley have discharged subsurface agricultural drainwater through wetland channels in the San Luis National Wildlife Refuge Complex (Refuges) to the San Joaquin River. This drainwater contains extraordinarily elevated concentrations of selenium, boron, chromium, molybdenum, and extremely high concentrations of various salts that disrupt the normal ionic balance of the aquatic system.

The US Geological Survey succinctly summarizes the pathways by which selenium becomes a pollutant, put into motion by irrigation drainwater, coal mining, or petroleum refining. They also have a thorough scientific library about the geo- and biochemistry of selenium.

There are many USGS open file reports dating from the 1980s and early 1990s that address the geographical distribution of selenium in the San Joaquin Valley. We hope to make them available online in the near future.

Further reading: The “Rainbow Report