MOSCOW — A river in the far north of Siberia turned bright red this week, residents said, leading Russians to nickname the tributary the “blood river.”

A government ministry said it was investigating a possible leak of industrial waste, but had not determined what caused the discoloration. One hint at the possible cause is the path the river, the Daldykan, takes past the Norilsk Nickel mine and metallurgical plant, by many measures one of the world’s most polluting enterprises. The plant belches so much acid rain-producing sulfur dioxide — two million tons a year, more than is produced in all of France — that it is surrounded by a dead zone of tree trunks and mud about twice the size of Rhode Island.

The metal smelters in this ore-rich region produce copious amounts of copper, one-fifth of the world’s nickel — a key alloy in stainless steel — and half of the global supply of palladium, a precious metal nearly as valuable as platinum.

The ore also contains iron, but that red-hued element is far less valuable than the precious metals extracted along with it, and is generally discarded in slurry ponds.