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 pollutingenterprises. 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.