If one does the calcs, it is very difficult to conduct a massive poisoning by water supplies unless it is done directly at local treatment facilities. Poisoning a reservoir takes dump trucks worth of contaminant and then dispersing it.Which is why that wouldn't be there target. These people are not stupid, they are evil sob's but they aren't stupid.
Check this out for example.
I wasn't going to write about interruption of delivery for reasons of security. Poisoning other than by very localized means is still a lot harder to do than most people realize.