California's Central Valley, one of the world's most fertile regions, accounted for 60 percent of the state's $29.4 billion agricultural output in 2003. But now 500,000 acres -- about 4 percent of the valley's total farmland -- are threatened by what one scientist calls a "sleeping monster." Selenium, an element common in soils that once lay beneath ancient oceans, is harmless when left undisturbed. But changes in soil chemistry caused by agriculture can turn it into an environmental hazard.

"Once you start to introduce plants and increase the cycles of wetting and drying, once you cultivate the soil and increase microbial activity, you begin to break down the selenium into soluble form," explains Gary Bañuelos, a researcher with the U.S. Department of Agriculture. As it enters surface water, selenium moves up the food chain with devastating effects. The most dramatic results of selenium poisoning are embryonic deformities and death among wetlands birds.

In July 2003 the state of California gave Central Valley farmers a stark choice: Either lower the selenium content of their irrigation and drainage waters or face fines and closure of their water-storage impoundments.

John Diener, a second-generation farmer in the Central Valley, didn't relish the prospect of having to idle hundreds of acres of farmland. So he began to capture and recycle his drainage water, directing it to a designated area of his farm. Then, while searching for a crop that could tolerate the elevated concentrations of selenium and dissolved salts on this acreage, Diener came across Bañuelos's work on phytoremediation -- the use of plants to extract contaminants from soil and water. He contacted Bañuelos and together they decided that Diener should try planting canola, which absorbs soluble selenium into its roots and transfers it to its stems, leaves, and seed.

Diener's 4,000 acres of vegetables, grains, forage crops, almonds, and wine grapes are now graced by more than 100 acres of brilliant yellow canola -- Brassica napus. Diener and Bañuelos extract oil from the canola seed to produce biofuel -- 30 percent canola oil, 70 percent diesel -- to run the tractors on Diener's farm. The men also learned that selenium, while toxic for wetlands birds, is an essential trace mineral for many other animals, including humans. It helps protect cells from the damaging effects of free radicals (naturally occurring oxygen molecules that can disrupt normal cell function) and promotes the functioning of the immune system and the thyroid gland. Dairy cows are at particular risk of selenium deficiency, which dairymen try to prevent by adding the element to feed rations or salt licks or by injecting a slow-release capsule of pure selenium. Bañuelos found that feeding canola seed meal to dairy cows was another safe way to increase the selenium content of their blood.

In Diener's view, phytoremediation is a more sustainable response to the changes in soil chemistry wrought by farmers than simply idling the land. His canola crop yields some 14,000 gallons of seed oil, converting an environmental hazard into an economically viable resource. Diener and Bañuelos are hopeful that other Central Valley farmers will emulate their methods, which could eventually be applied to selenium-tainted farmlands as far away as India and China.
-- Deborah K. Rich