Selenium in soil, water, sediment, and biota of the lower Sun River area, West-Central Montana

A U.S. Department of the Interior study started in 1990 examined the source, movement, fate, and possible biological effects of selenium associated with irrigation drainage from the Sun River Irrigation Project in west-central Montana. Concentrations of total selenium in soil samples ranged from 0.1 to 8.5 micrograms per gram; the maximum concentrations were measured in nonirrigated areas overlying geologic formations containing seleniferous shale. In irrigated areas, concentrations of dissolved selenium in ground water flowing toward Freezeout Lake ranged from less than 1 to 18 micrograms per liter (??g/L) in terrace gravel and from 1 to 190 ??g/L in glacial deposits derived from seleniferous shale. Concentrations of total selenium ranged from less than 1 to 180 ??g/L in surface irrigation drainage, and from less than 1 to 1,000 ??g/L in natural flows from nonirrigated land. Selenium concentrations in water from lakes generally were less than the aquatic-life criterion for chronic toxicity. The range of selenium concentrations in bottom sediment of lakes was similar to that of local soils. However, biological samples indicate that selenium is accumulating through the aquatic food chain. Selenium concentrations indicative of biological risk were exceeded in at least 80 percent of the freshwater-invertebrate, bird-egg, and bird-liver samples collected from all wetland sites.