Aquatic Life Criterion - Selenium

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Draft Aquatic Life Chronic Criterion for Selenium in Freshwater

EPA is updating its recommended aquatic life water quality chronic criterion for selenium in freshwater. EPA released a draft updated criterion in July 2015 and accepted public comment until October 30, 2015. EPA is updating the selenium criterion to reflect the latest scientific information, which indicates that selenium toxicity to aquatic life is primarily driven by organisms consuming selenium-contaminated food rather than by direct exposure to selenium dissolved in water.

Once finalized, EPA's water quality criterion for selenium will provide recommendations to states and tribes authorized to establish water quality standards under the Clean Water Act.

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What is selenium?

Selenium is a naturally occurring element that is nutritionally essential, but it can be toxic to aquatic life (such as fish) where concentrations are elevated. It is also toxic to cormorants and other birds that consume aquatic organisms containing excessive levels of selenium.

Where does selenium come from?

Being a natural element, selenium can be found throughout the environment. Toxic levels of selenium in water bodies have mostly been related to irrigation of western soils that are naturally high in selenium, ash pond discharges from coal-fired power plants using coal that has selenium in it, petroleum refinery effluents, and runoff or discharges from certain mining activities. 

How does selenium affect aquatic life?

Selenium is a bioaccumulative pollutant. Aquatic life is exposed to selenium primarily through their diet. Risks stem primarily from aquatic life eating food that is contaminated with selenium rather than from direct exposure to selenium in the water. Although selenium bioaccumulates, that is, accumulates in tissues of aquatic organisms, it is not significantly biomagnified, unlike mercury or PCBs, except when the food web is primarily mollusk-based (i.e., the fish eat mostly clams or mussels).

For aquatic life, the toxic effects with the lowest thresholds are effects on the growth and survival of juvenile fish and effects on larval offspring of the adult fish that were exposed to excessive selenium. In the latter case, besides reducing survival, selenium causes skeletal deformities.

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