The mercury is not coming from runoff in most cases. Coal fired power plant emissions are distributing it globally via the atmosphere. China is turning on a new coal fired power plant EVERY THREE DAYS to give you a picture of where we are headed. We don't just get cheaper goods from our explotation of low wage chinese labor. These are not the newer much cleaner type power plants. This is why there is mercury in the lakes in The Northwest Territories for example. It is also why as far as mercury goes the warning is statewide. Of course some water has other toxic elements which just add to the problem. Lower Yakima would be an example. Ever notice that pretty jet black purplish sand on some shores at Banks Lake or on the upper Columbia? That's coming from our friends north of the border and it ain't sand.... It's an interesting game we humans play with ourselves. We spend untold fortunes to solve diseases. We spend tons of money on medications to try to maintain quality of life. Yet we continue to create things that cause these problems in the first place. Reminds me of dog chasing his tail sometimes.