Some time ago I talked to an Eastern European who used to be on an "unconventional warfare" team in the 80's. They were trained for covert operations in foreign countries, including sabotage and terrorist attacks in the event of an all-out total war. I have seen some of their training manuals and I recall reading a chapter specifically dedicated to poisoning water and food sources. IIRC botulin was the toxin of choice.

Another thing which caught my attention was that the primary targets for such attacks were smaller cities with a population of about 100.000 and not much more. The idea being that the water supply in smaller cities is likely not as well guarded and any attack will possibly have a greater effect because a smaller quantity of toxin is necessary.

There was also some information on where the toxin is best placed. Very large reservoirs were not to be targeted because of the quantity of toxins needed. Instead, the emphasis was on finding weak spots in the system of pumps and piping that supplies water to smaller, relatively contained areas, eg. residential neighborhoods, hospitals, schools, military installations etc.

I don't remember half of what I read in there but it looked like it could actually work. Somebody really did their homework. Scary stuff.