Selenium in Water
Selenium is analogous to sulfur in many of its chemical combinations. Selenium occurs in some soils as basic ferric selenite, as calcium selenate, as elemental selenium and in organic compounds derived from decayed plant tissue.
Biologically, selenium is an essential beneficial element recognized as a metabolic requirement in trace amounts for animals. Selenium may be introduced into the environment through the burning of coal and application of phosphate fertilizers and selenium containing insecticides.
The concentration of selenium in surface waters is proportional to the concentrations in the ambient soil. Selenium concentrations in natural waters may be as high as 1 mg/L, but generally concentrations are below 100 micrograms/Liter.
It is believed that selenium is highly toxic to man, but information on human injury by selenium is scanty and definite symptoms of selenium poisoning have not been identified.
Minute concentrations of selenium appear not to be harmful to fish during an exposure period of several days. However, constant exposure to traces of selenium has caused disturbances of appetite and equilibrium, pathological changes and even death of fish after several weeks. Concentrations that are considered safe for human beings have been toxic to fish. It is believed that selenium is passed up through the food chain to the fish which accumulate this element in the liver in lethal concentrations.