BOISE, Idaho (AP) — An 11-year state and federal study of selenium pollution in a southeastern Idaho watershed where some 700 sheep, cattle and horses have died over the last several decades after grazing in contaminated areas has found the toxin is likely moving through groundwater.

The 36-page study on the Upper Blackfoot River Watershed released earlier this month by the U.S. Geological Survey also found that selenium levels spiked in the river during spring thaw. Researchers say the inactive Maybe Canyon Mine is producing the most contamination. The upper watershed has 12 phosphate mines, four of them active.

Phosphate ore is used in farm-based fertilizers and other products. Waste rock contains selenium that has been blamed for killing livestock as well as harming trout populations in some area streams.