Selenium is a naturally occurring substance found in the earth's crust. The element appears in rocks and soil as a gray to black crystal. It is released as a by-product during mining and can also be released during the weathering of rocks and soil. Much of the latter release becomes soluble in water and is taken up by plants. In general, Idaho has selenium deficient soils except in the southern part of the state, where extensive mining has caused toxicity near mines.
Selenium is an essential nutrient. The intake of food is enough to meet the daily requirements. Excess amounts of selenium can cause selenosis, which may be accompanied by brittle hair and nails, or loss of feeling and control in the extremities. Selenium toxicity is rare and only occurs in those who take supplements or work with the element. Horses and grazing animals have gotten sick with selenosis after drinking water downstream from mining debris.
Normal Amounts of Selenium
Selenium is a rare element and only present in the Earth's crust at 0.09 parts per million. In average soils it is 0.2 to 0.4 parts per million. Drinking water can safely contain 0.05 parts per million. Selenium is a metalloid, having properties of metals and non-metals. It can be present in the air, water and soil. It is not a necessary element for plants but plants do uptake it during water intake.
Selenium in Idaho Soil
Idaho has a large phosphate mining industry. The debris removed in the form of rock, shale and soil is high in selenium content. This is not a health hazard as long as the debris is buried back where it came from, but this is not always possible in ongoing mining operations. The form of selenium present in the debris is selenide, which is insoluble. The piles of rock and soil are exposed to weather and oxidation, which makes it into a soluble form. The piles then leach out selenium which gets swept into waterways. The soils are not as contaminated as the water but some high deposits do occur in riparian areas.
Because of the economic importance of the phosphate mining, the operations cannot be shut down to prevent selenium leaching. Management practices are in place to minimize the spread of selenium. These include careful road building practices, ditch liners to prevent runoff from spreading to waterways and redirecting natural water around the mines so it doesn't pick up contamination. Interceptor ditches collect contaminated water and filter it through geo-fabric, rock and shale before allowing it to mix with uncontaminated water.
The toxic levels of selenium first became of concern because of some grazing animals but it can cause more harm. The substance can build up to toxic levels in fish quickly, which then pass it on to predators like bears, eagles and humans. Plants may contain higher levels than are safe due to the uptake of moisture in soil. People in China were sickened by selenium poisoning due to leaching elements in the water. High selenium soils which leach into ponds and streams have also been shown to cause birth defects in waterfowl.