Selenium (Se) is a naturally occurring element that is found in shales underlying the central and western USA, as well as other areas throughout the world. It is found in the environment where there are outcrops of this shale, or where the Se containing sediments are disturbed. The Se present in the shale can be moved by plants, by leaching in agricultural runoff, by mining certain materials such as uranium and from other natural and industrial sources (Hossner et al. 1992, Lemly 1999, Sors et al. 2005a, b).
Changes in the presence and availability of Se can lead to chronic and acute Se poisoning through biomagnification in ecosystems, and endangering virtually all inhabitants (Rosenfeld and Beath 1964, Beath 1982, Bañuelos et al. 2002). One of the most infamous examples of Se poisoning is from Kesterson National Wildlife Refuge in Las Baños, California. Agricultural runoff from fields overlaying Se-rich shale that collected in evaporation ponds led to extremely high concentrations of Se in the sediment. Bioaccumulation and biomagnification of the Se resulted in deformities and death in a variety of wildlife and resulted in a severe reduction in local bird population numbers (Ohlendorf et al. 1986). Similar biomagnification was observed as the result of irrigation with In-situ leach mining waste water from the Highland Uranium Mine in WY (Ramirez and Rogers, 2002) leading to deformities in bird populations. Poisoning also occurs in aquatic ecosystems throughout the US and is considered an ongoing, potentially devastating threat to organisms in these habitats (Lemly 1999, Hamilton 2004). Historically, Se poisoning is thought to have affected the horses and pack mules involved in bringing reinforcements and supplies to Custer's army, thus contributing to the outcome of the Battle of Little Big Horn (Hintz and Thompson 2000).
The most common method of poisoning (acute and chronic) of livestock in Colorado and Wyoming is from consumption of plants that are found where ever Se is present in the soil. (Figure 1, Rosenfeld and Beath 1964).
Figure 1. Selenium accumulator plants from left to right: Prince's plume (Stanleya pinnata, yellow); Two-grooved milk vetch (Astragalus bisulcatus, white and purple); Woody aster (Xylorhiza glabriuscula, white):
They typically accumulate around 10,000ppm Se in their tissue, and can accumulate up to 20,000 ppm. They are commonly referred to as locoweed as they can cause animals to act strange due to a disorder called "blind staggers" from acute (short term) poisoning from consumption of these plants. These plants have caused severe losses throughout CO and WY. (See Figure 2 below).
|Figure 2. Historic Se poisoning events with losses of livestock |
- 1907 -- Medicine Bow- 15,000 sheep died
- 1919 -- Rock River- 325 sheep died, 275 over night
- 1930 -- Elk Mountain- 342 sheep died in 24 hours
- 1931 -- Pueblo- 197 of 200 animals died overnight- one year later in the same area, 71 of 157 animals were lost
Livestock suffering from chronic (long term) poisoning will often slough their hooves and lose their manes and tails suffering from selenosis. Other plants, including some crops and natural forage, can accumulate Se to a lesser extent when grown where Se is concentrated in the soil, potentially threatening any organism that consumes them. After Se compounds have been mobilized into the ecosystem, regardless of method, they typically will remain in a bioavailable form due to the arid/semi-arid climates in the Western US that prevent the complete reduction of Se to its less toxic, non-soluble elemental form (Beath 1982).
For a great easy to read overview of the seriousness of the Se problem, see the book Death in the Marsh by Tom Harris, who was a reporter for the Sacramento Bee when he authored the book and became interested after the Kesterson National Wildlife Refuge disaster.
Bañuelos GS, Vickerman DB, Trumble JT, Shannon MC, Davis CD, Finley JW, Mayland HF. 2002. Biotransfer possibilities of selenium from plants used in phytoremediation. International Journal of Phytoremediation, 4: 315-331.