Selenium in the Blackfoot River Watershed

Science Center Objects

The upper Blackfoot River receives runoff from 12 large phosphate mines. Shale waste rock that is a byproduct of mining is highly enriched in naturally occurring selenium. At optimal concentrations, selenium can be a positive nutrient and antioxidant in mammals and fish. At elevated concentrations, however, it can damage fish and animal immune systems. As early as 1996, livestock deaths attributed to selenium contamination were reported in the upper Blackfoot River watershed.

Since 2001, we have collected streamflow and water-quality data at streamgage station 13063000, Blackfoot River above Blackfoot Reservior near Henry, ID in cooperation with the Bureau of Land Management.

In 2014, USGS water-quality specialist Christopher Mebane combined USGS data collected in cooperation with the Bureau of Land Managementwith data collected by the Idaho Department of Environmental Quality (IDEQ) from 21 sites along the Blackfoot River and tributaries. Mebane and his colleagues analyzed the temporal and spatial datasets and found that:

  • During the river’s highest flow in May of each year, concentrations of selenium exceeded Idaho’s aquatic health criterion of 5 micrograms per liter (µg/L) in 80 percent of the samples collected.
  • During April and June, selenium concentrations exceeded the aquatic health criterion less frequently, and concentration never exceeded the limit other than in April, May and June.
  • Throughout the 11-year study period, a consistent upward trend in selenium concentrations was evident when the river’s flow was lowest (August-October).
  • Selenium concentrations remained elevated throughout the study period, but no consistent trend could be identified when the river’s flow was highest (May).
  • IDEQ sampling in the watershed indicates that East Mill Creek contributed the most selenium to the upper Blackfoot River.