Poisoning water supplies ( in a Water Reservoir) is a poor way to commit terrorism. There are much better ways to deliver your poisoned water to point of use.
Actually, it's practically impossible. Not to forget that these days, any arabs that get hinky on an airflight will be beaten to death in the aisleways (and they know this), and the rest are being watched by everybody (who ought to be fully armed, but many are), these rag-head whack-jobs are wanking in their apartments, for the most part. When they stick their heads out, ther are being wacked -- despite the best efforts of the anti-American Democrat Party and LMS media.
It is a stupid way. I work for a state agency that teaches water & wastewater as well as a DHS program to prepare water systems for a poison attack. Everyone in the DHS program believes the odds of it actually happening are ZERO. The dilution factor alone means they'd have to bring so much material it would takes a large number of semi trucks. Kinda hard to do that without being noticed. Also, anything dumped in the water can be removed using the proper processes.
If terrorists want to screw with the water supply, they'll blow up the dam, treatment plant, or pumping stations.