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.

USGS streamgage station 13063000

USGS streamgage station 13063000, Blackfoot River above Reservoir near Henry, Idaho

(Credit: Jay Bateman, Idaho Water Science Center. Public domain.)

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

Over the same period, the Idaho Department of Environmental Quality (IDEQ) has collected streamflow and water-quality samples from the Blackfoot River and tributaries during spring runoff. Data collected from these independent monitoring efforts were analyzed in two reports listed on the Publications tab above.

In June 2016, the USGS and IDEQ conducted a seepage study in the Blackfoot River near Dry Valley. Although limited in scope, the seepage study explored the hypothesis that unaccounted selenium loading (i.e., loading in excess of tributary inputs) in this reach could be caused by groundwater inflow. The seepage study results showed consistent albeit small unaccounted increases in streamflow and dissolved selenium load in the Blackfoot River near Dry Valley. Field observation of a spring to the north of the river and independent groundwater monitoring data from Dry Valley to the south of the river suggest that alluvial groundwater may discharge to the river from both sides. However, the small unaccounted selenium load measured in the June 2016 study relative to loads measured during runoff suggest that groundwater loading in this reach may occur primarily during runoff.