The impurities in this sample look organic in nature. Not quite waste, but certainly something toxic. I am no expert on wildlife, but I would guess that there is something upstream causing the water contamination. Some kind of animal, or at least a creature. Follow the rivers as far north as you can, and see if you can find what is responsible for this. If it is an animal, it must be put down. It is for the safety of all here in Farholde.