Just call your local environmental protection dept and they will talk to you about it. I have actually done that in the past. They say they are sure a lot of it comes from coal burning power plants. Mercury is by product of coal. Then of course there's manufacturing which uses heavy metals, and PCB's etc etc etc. People like dentists may play a small role. Even the sewgae with all of our chelated junk which gets recycled out and into the water system again...

In the fish though it's accumulative. The bigger the fish, the worse the problem because big fish eat lots and lots of small fish and acquire the heavy metal burdens of each little fish via ingestion. it is not natural in the water in most places. Oil spills don't help either.

The water is pretty filthy these days just about everywhere you go, fresh or salt water near the coast of any populated area.