BOISE, Idaho-Federal regulators are concerned that a dam built by Monsanto Co. earlier this year to trap phosphate mine runoff may be stopping more than just pollution.

They say the dam has also halted millions of gallons of water in Sheep Creek that would otherwise help fill the Blackfoot River.

The Environmental Protection Agency now wants the maker of Roundup herbicide to begin a costly treatment to remove selenium and heavy metals, then discharge clean water downstream, instead of capturing it in a 50-million-gallon lake behind the dam and using it for dust control on its mining roads.

The situation shows the predicament that companies like St. Louis-based Monsanto and the government face in Idaho’s rich-but-polluted phosphate mining country not far from Yellowtone National Park: They must work to contain naturally occurring poisons unearthed during a century of digging, while protecting water supplies in an agricultural state hit hard by drought over the last decade.

The aim is to avoid killing streams just to save them.

“We support efforts to reduce selenium discharges to the creek, but we have serious concerns about the methods Monsanto is using, which is drying up the creek,” said Mark Ryan, a federal Clean Water Act attorney for the EPA in Boise, on Wednesday. “We want to see it (the water) treated and put back into the creek where it belongs.”