While sulfide-ore mining techniques vary and continue to evolve, in the past some safety records have caused concern. A 2012 study by the nonprofit Earthworks reviewed 14 U.S. sulfide-ore copper mines—­predominately open-pit—which produced 89 percent of the country’s copper in 2010, the most recent data available from the U.S. Geological Survey. All the mines experienced pipeline spills or other accidental releases. Tailings spills occurred at nine operations, and at 13 of the 14 mines, the study says, “water collection and treatment systems have failed to control contaminated mine seepage, resulting in significant water-quality impacts.”

That risk is more worrisome in Minnesota, where 6 percent of the surface area is water—more than any other state in the country. The three-million-acre Superior National Forest, which contains the Boundary Waters, holds 20 percent of the fresh water in the U.S. national-forest system. It also borders Lake Superior, the largest and least polluted of the Great Lakes, which holds 10 percent of the world’s fresh surface water.