What if the waste is taken and dumped in a city water supply?
Well, it depends on what you mean by waste there are several different kinds of waste produced by nuclear power.
I will assume that you mean spent reactor fuel because that is what most people think of when they talk about nuclear waste.
First; of all it would be nearly impossible for terrorist to obtain spent nuclear fuel because it is not shipped off site for burial.
Second; if they did obtain spent fuel the containers that the spent fuel is stored in would be very difficult to open. If they did open it they would need very specialized remote manipulating devices to handle the spent fuel without killing themselves.
Third; if they managed to high jack a spent fuel shipment it would be very difficult to hide the trailer/cask that the fuel is shipped in. Nothing else looks like a spent fuel cask.
As I said before terrorist have easier targets available. The man power and resources necessary to pull off such an event would be too easily detected and too easy to stop.