[syriacus]

since it would take an awful lot of bad stuff to effect an entire water system.

http://www.mercola.com/2003/sep/3/water_safety.htm

If terrorists were smart, they'd make sure to contaminate specific parts of the water system.

[creepycrawly]

Yes, of course. But it would still take large quantities of just about any substance (including human sewage or cattle-farm runoff) to be effective.