[piasa]

If they were going to just toss it into a reservoir, they wouldn't need a map of the water system.

[HiTech RedNeck]

Possibly the terrorists were planning to pump the poison into the distribution side of the water supply. The idea would not necessarily be to kill, but to create a panic (some people get ill and the water is tested and voila, there's cyanide and nobody has any idea how it got there).

[P-Marlowe]

Once the water is in the water system, the water pressure alone will keep out any attempt to insert contaminants. They could put it into water towers, but it is probably still going to be diluted to the point of ineffectiveness.

What they might accomplish is to put a little panic into the people. A lot of people might freak out if the water supply was contaminated, but how many Italians actually drink WATER? Most Europoeans don't bathe too often either. So what would be gained by poisoning the water supply in Europe?