National Drinking Water Database

Selenium

Selenium is a naturally occurring mineral element that is distributed widely in nature and is found in most rocks and soils. In the environment selenium typically combines with sulfide minerals or with silver, copper, lead and nickel metals. Selenium is an essential nutrient, but high doses are toxic to humans and wildlife. Selenium and related compounds are used in photographic devices, plastics, paints, anti-dandruff shampoos, vitamin and mineral supplements, fungicides and some types of glass (Agency for Toxic Substances and Disease Registry (ATSDR) 2003c).

Selenium pollution is a worldwide phenomenon and is associated with a broad spectrum of human activities, ranging from the most basic agricultural practices to the most high-tech industrial processes (Lemly 2003). Industrial emissions of selenium frequently come from burning coal; fly-ash landfills with residues from coal-fired power plants act as sources of selenium contamination in local soils, waters, and food (Gutenmann 1976).

Selenium is also released as a byproduct of gold, silver, copper, nickel and phosphorus mining and from petroleum refinery effluents (Mars 2003; USEPA 2008a; Miller 2009). An example of selenium-releasing mining activities is West Virginia mountaintop mining, where selenium-bearing minerals are exposed to weathering (USEPA 2008a). Selenium enters water supplies by washing from soil at hazardous waste sites and abandoned uranium or coal mines (Muscatello 2009). Selenium pollution of lakes and streams near copper and nickel mines has also been reported (Nriagu 1983).

Toxic levels of selenium in water bodies may be also caused by irrigation of farmland soils that are naturally high in selenium. In western United States agricultural areas such as the the San Joaquin Valley of California, irrigation results in soil build-up of salt and selenium that are transported by irrigation drainage to wetlands and other water bodies (Presser 2004). Selenium toxicity problems associated with irrigated agriculture on arid farmland have yet to be adequately resolved (U.S. Geological Survey (USGS) 1998, 2008; Presser 2007).

People are exposed to selenium through air, food and water, with most exposure coming from food followed by water (WHO 2004a). Depending upon the level of intake, selenium can have nutritional or possibly toxic effects. As an essential trace element in humans and animals, selenium is a biologically active part of a number of important proteins, particularly enzymes involved in antioxidant defense mechanisms and thyroid hormone metabolism. Selenium is essential for converting precursor thyroid hormone T4 into its active form, T3; yet, high doses of selenium can cause decreased thyroid hormone production, a condition called hypothyroidism (ATSDR 2003c). Long-term exposure to higher levels of selenium causes brittle hair as well as loss of nails and hair (ATSDR 2003c; WHO 2004a).

Aquatic selenium pollution poses great threats to aquatic life (Lemly 2003). Selenium bioaccumulates in water-living organisms as diverse as bacteria, fungi, algae, muscles, fish, snakes, alligators, birds and mammals (Ogle 1989; Lemly 1996; Hoffman 2003). Selenium toxicosis in many of these organisms is associated with impaired health and reproduction (Hoffman 2003). Eggs and larvae of fish and birds are very sensitive to lethal or teratogenic effects (developmental abnormalities and deformities) due to selenium transferred to the eggs from the body of the female parent (Hoffman 2003). Selenium has been identified as the agent responsible for mortality and reproductive failure in birds at a number of contaminated sites in the United States (USEPA 2008a; USGS 2008).

Millions of pounds of selenium and selenium compounds are released into the environment every year. Solvent recovery and electric utility industries account for the majority of selenium pollution with highest releases in Alabama (almost 700,000 pounds) and Nevada (nearly 500,000 pounds) (USEPA 2009i).