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https://en.wikipedia.org/wiki/Inuvik/Shell_Lake_Water_Aerodrome

The selenium cycle is a biological cycle of selenium similar to the cycles of carbon, nitrogen, and sulfur. Within the cycle, there are organisms which reduce the most oxidized form of the element and different organisms complete the cycle by oxidizing the reduced element to the initial state.

In the selenium cycle it has been found that bacteria, fungi, and plants, especially species of Astragalus, metabolize the most oxidized forms of selenium, selenate or selenite, to selenide. It is also thought that microorganisms may be able to oxidize selenium of valence zero to selenium of valence +6.

Evidence for a selenium cycle is found through the study of selenium accumulator plants. These plants are found in semi-arid, seleniferous soils. The plants biosynthesize forms of organic selenium compounds and release the compounds into the soil when they decay. If the compounds were not oxidized, then an increase in organic selenium would be seen, but selenium in these areas is mainly inorganic.

Selenium can be removed from the ecosystem and bound in sediment through natural processes of chemical and microbial reduction of the selenate form to the selenite form. The reduction is followed by adsorption to clay, reaction with iron species, and coprecipitation or settling. After selenium is in the sediment, other chemical and microbial reduction may occur, causing insoluble organic, mineral, elemental, or adsorbed selenium. Some organic forms may be released into the atmosphere from