No.

But…

The adage that “the dose makes the poison” is working in your favor here. A large city supply delivers millions of liters of water per day; by the time you dilute your poison into millions of liters of water you’ll either be adding absurd amounts of poison (someone is going to notice massive line of tanker trucks queued up outside the treatment plant), or you are dealing with large - but not unweildly - volumes of something so horrendously toxic that it’s still deadly when diluted that much. There are very few substances that toxic, and someone is going to notice if you start procuring hundreds of liters of botulism toxin or Vx because at that point you are dealing with outlawed chemical warfare agents