Hmmm. Gee, anybody know what the pressure would be in Rome's municipal water system? I happen to know that 6" to 12" PVC water mains in the United States are generally rated for use at up to 150 or 200 psi, and that municipalities here try to maintain pressures of at least 50 psi in their systems. Are there pumps, which would be available to these charaters, that will move large volumes against pressure of, say, 70 or 80 psi?
Speaking as a degreed and registered mechanical engineer who has designed large domestic water systems for high-rise hotels and offices - yep, a pump able to do that is really not that big a deal. I'm not counting on all terrorists being stupid, though many sure seem to be.
FYI, from a civil defense readiness standpoint, I have a blue polypropylene 55 gallon drum of potable water (and a few ounces of bleach, of course) in my crawl space.
The problem is so easy to solve it borders on the trivial. It would be extremely easy to inject a concentrated cyanide solution into a pressurized force main. Many companies make the requisite equipment, and that equipment can be quite small.
Once contaminants are in the system past your local tap on the main, they circulate freely. Shut off any water tap branch, add contaminate upstream of the back-flow and its going to work its way back to the whole system diluted only by the volumn of water in the system in its local area.
Lets not get anymore specific about this. We don't need to make the job easier for anyone who hasn't figured this out yet.