Years ago I knew someone who worked for the local waterworks and asked what they do to prevent the water supply from being poisoned. He said while they do test

Years ago I knew someone who worked for the local waterworks and asked what they do to prevent the water supply from being poisoned. He said while they do test on a regular basis, the real security is in the dilution factor as you'd need a really large quantity of any poison to effectively contaminate a reservoir. A biological contaminate that reproduces would be an easier attack but I'm guessing that's where the frequent testing comes in to catch it before it gets to a deadly concentration.