Basically, you can pollute water by dropping items inside that (except for clay and some other items) and by using smelters, kilns and so on near a lake or river. These actions lower the purity of the water. Depending on the grade of pollution, the purity slowly regenerates after a while (but pollutions also spread in the nearby waters).
This doesn't apply for toxic water. These waters should be visible (maybe with a different color) and provoke shp damage when drinked. Foods made with this water should be poisonous, too. And fishes don't live there.
You can slightly increase purity by boiling water, by the way.
What's the reason of this idea, you could ask?
Basically, i don't find much sense in a quality of water. Pure water is pure water, after all. Nothing can be better than pure water.
On the other side, this should be a more realistic approach. If many people live near a river, with a large and industrious town, the area will be surely polluted. Whileas in the wild, where it lives no one, the water will be pure and clear.
Well, i've finished. Let's begin trolling, people.
