[A. Pole]

To introduce a chemical agent would involve a large quantity

Why? The nerve agents are effective in small amounts.

[nunya bidness]

It's my understanding that in order to have an effective release in a water supply the amount of any chemical or biological would have to be of a substantial ratio to the overall supply. We're talking thousands of gallons or hundreds of pounds. And the choices are limited because some bugs don't like water. Not to mention the chemicals used in the treatment.