Why not? The things can be removed from the water, and even if they weren't, there could still be a net benefit in having one toxin over another.

History is filled with examples of well-intentioned solutions like this going horribly awry.

Talk to an Australian about cane toads, for example. (http://en.wikipedia.org/wiki/Cane_toads_in_Australia).

Maybe so, however that is a fully general counterargument that applies to any attempt to solve any problem ever.

Let's say you run a barber shop. You could always set the hair on the floor on fire, right? That'll clean it up! What could possibly go wrong?

...or maybe you could just sweep it up.