Why is this Russian river blood red?

The residents of the Russian city of Norilsk, inside the Arctic Circle, have been posting photos on social media of an alarming sight: a bright red river. Is it the End Times, a chemical leak, or something else?
The Russian authorities have already started an investigation into the matter, and they’re looking at a local metals plant as the possible culprit. Norilsk is a heavily polluted industrial city and it’s home to Norilsk Nickel, a mining giant that has a nickel smelting plant u