Well, pollution has to do with littering in the water? If so, I'm thinking that it would have to do with activities such as swimming, diving, and possibly ice hockey? Not so positive on the hockey part though?
The pollution of the water would create chemicals that would spread throughout the water and if someone went into the water, it could effect their lungs since you are breathing it.
It's basically like when you forget to put the top on your hot tub and you get algae.