| |||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||
Introduction Selenium is a naturally occurring elemental metal that is both an essential nutrient and a potential poison. In the past it has generated commercial interest. The electrical conductivity of selenium varies with light intensity. Its photoelectric properties enable it to be used in electronic eyes, light meters, and electrical rectifiers. The steel industry uses the metal to produce stronger, corrosion resistant products. Selenium is used as a rubber vulcanizer, in dandruff control shampoo and as an alloy to improve the machinability of copper. Red, pink and ruby colored glass obtains its tint from Selenium.
Metallic selenium is odorless and tasteless. The gaseous, methylated forms have an odor similar to garlic. Selenium is highly poisonous. It is used in insecticides and paints applied to ship hulls to prevent growth of barnacles.
In Wyoming, the geologic occurrence of selenium is widespread. It is naturally found in volcanic tuff, coal deposits and some shales. Selenium weathered from rocks and found in soils is taken up by plants which consequently may be ingested by animals. Thus, much of the interest in Wyoming has centered around its known or potential toxicity to animals. It has been of concern to residents of Wyoming as its presence in some of the state's vegetation and water has resulted in adverse effects to animals in specific localities.
The concentration at which selenium becomes toxic in a plant depends on the form of selenium, the presence of oxalates, alkalodes, or other natural products, the type of animal ingesting the plant, and the quantity of vegetation consumed. Mildly toxic effects can generally be observed if a quantity of plants containing five part per million of selenium are ingested. One milk vetch from the Powder River Basin contained approximately 15,000 parts per million of selenium. There have been many reports of livestock deaths in the state, some of which may have been associated with selenium poisoning. Trelease and Beath reported that during the summer of 1907 and 1908 more than 15,000 sheep died after grazing in an area north of Medicine Bow with an abundant growth of seleniferous vegetation. In 1919, 275 sheep died overnight after grazing in an area with seleniferous vegetation near Rock River. Selenium may have been a factor in some of those deaths. However, no detailed pathology was reported at the time of death, and other toxic factors may have been prevalent. Very few livestock deaths have been attributed to selenium in the last 40 years, perhaps due to a better understanding of the problem and better management practices.
Selenium is an essential micronutrient and is required for vitamin E metabolism. Nationwide, selenium deficiency in livestock is a far more common occurrence than selenium toxicity. Livestock in the midwest and eastern U.S. are often fed selenium supplements because the element is often lacking in locally raised forage. Selenium deficiency is also a localized problem in Wyoming livestock.
Irrigation drainwater has recently become the focus of studies to determine the extent of adverse effects on fish and wildlife resources in habitats receiving irrigation drainage in the western United States. Concern for the effects of drainwater quality on natural resources was prompted by Fish and Wildlife Service studies in 1983 at the Kesterson National Wildlife Refuge in the Central Valley of California. High concentrations of selenium and other trace elements in impounded drainwater at Kesterson were associated with reproductive failure and embryo deformities and mortality in several waterfowl species. This discovery prompted more detailed investigations of damwater contaminant impacts on fish and wildlife species. Which in turn preceded termination of Federal drainwater service to irrigated croplands in the Western San Joaquin Valley containing high concentrations of selenium in soils and ground water. Drainwater service was discontinued by the Department of the Interior (DOI) to comply with provisions of the Migratory Bird Treaty Act.
Higher than desirable levels of selenium have recently been found in bird eggs, waterfowl and shore birds taken from the Kendrick/Casper-Alcova Irrigation Project and shore birds collected from the Laramie plains. There is sketchy evidence that selenium intoxication has occurred in elk and moose in northwestern Wyoming.
Selenium poisoning in humans is rare. The current Federal drinking water standard for human consumption is 10 parts per billion, however, the concentration at which selenium becomes toxic in water is not well defined. Human selenium intoxication has been reported in the western United States.
Water Resources Publications List
Water Resources Data System Library |
Water Resources Data System Homepage
