E coli is not necessarily bad. It makes up a good portion of the bacteria in your intestines right now. It is harmless, except for one particular strain, O157:H7. O157:H7 produces a toxin that causes internal hemorrhaging and diarrhea.
Dumping e coli into water supplies may even be beneficial, since e coli produces vitamin k and b-complex vitamins.
Getting e coli is easy, scoop up some feces. Getting O157:H7 requires alot more work. I would be more worried about someone dropping a dead european cow into the water.
Guess what? The Europeans already make all our vaccines and many other things we use daily (out of their dead cows).