BOISE - A government study of selenium pollution in a southeastern Idaho watershed where hundreds of grazing animals have died has found that the toxin is likely moving through groundwater.

The 11-year study on the Upper Blackfoot River Watershed was released earlier this month by the U.S. Geological Survey. It found that selenium levels spiked in the river during spring thaw and exceeded healthy limits for aquatic species.

Selenium is an essential nutrient, but it can be toxic in larger quantities.

"That's where the Blackfoot is at," said Chris Mebane, a water quality specialist with the USGS and one of the study's authors. "It's over that threshold of concern."

The upper watershed has 12 phosphate mines, four of them active, and four more mines are being considered. Phosphate ore is used in farm-based fertilizers and other products.

Waste rock from the mines contains selenium that has been blamed for killing livestock and harming trout populations in some area streams. The watershed is a key area for Yellowstone cutthroat trout and wildlife, and it's also a popular recreation area for humans.

Researchers discovered the inactive Maybe Canyon Mine to be contributing the most selenium during the study, which concluded in 2012. An upward selenium trend in fall months couldn't be explained by measurements in tributaries. That, Mebane said, means selenium is entering the upper Blackfoot through groundwater and could mean years of contamination.

"It could be decades or even centuries," he said.

Phosphate mining in the area just southwest of Yellowstone National Park started by 1920. Selenium contamination first attained recognition as a problem in the area, the USGS report said, in 1996 when the deaths of some 700 sheep, cattle and horses started occurring.

The most recent livestock losses happened in October 2012 when 95 sheep grazed at a re-vegetated inactive mine site. Elk and deer, the report said, appear to be more tolerant of elevated selenium.

"The plants get it - the roots pick up moisture from the soil," Mebane said.

The mines' owners are J.R. Simplot Co., Monsanto Co. and Agrium Inc. The mines are located on lands administered by the U.S. Bureau of Land Management, U.S. Forest Service, the state of Idaho, and the Fort Hall Indian Reservation. There is also private land.

Twelve sites in the upper watershed are under Superfund authorities for cleanup. The USGS study, conducted with the Idaho Department of Environmental Quality, provides insights on results of remediation efforts.

"I think the overall data set would suggest that we haven't started to successfully manage our selenium source yet," said Lynn Van Every, a water quality manager with the Idaho agency and a co-author of the study.

The U.S. Environmental Protection Agency lists two segments of the Blackfoot River and 15 tributary segments as impaired. If the watershed consistently fails to meet safe standards, opening new phosphate mines could be more difficult.

"The expectation for mitigation and control and better engineering (of mines), those expectations will remain very high," said Dave Tomten, a Boise-based geologist with the EPA whose duties include compliance and enforcement.

Simplot has been working to reduce selenium discharge from one its mines in the watershed by capping waste rock.

"This cover and new drainage systems are expected to significantly reduce the potential for selenium releases from this overburden area," company spokesman Ken Dey said in an email to The Associated Press.

The most recent mine in the watershed, the Blackfoot Bridge, is run by Monsanto, maker of Roundup weed killer. Authorities said it's designed to avoid selenium problems of the kind attributed to older mines.

The Blackfoot Bridge mine had a setback in early 2013 when an earthen holding pond sprung a leak, sending sediment and millions of gallons of water into an adjacent wetland. Testing showed it was free of contaminants.

"The good news is that the new mines that have come on line, these are a lot tighter operating than what happened back in the 70s and 80s," Mebane said. "The proof will be when we come back and look in a few years."