After my vampire mayor got hammered into a coma I was concerned that he would die of infection and that I would run out of vampires, so I decided to try infecting a peasant girl. First I tried to drop him down an existing well, thereby polluting the water. However, since every bone is his body was broken and overlapping another, the surgeons were unable to get him mobile before infection claimed him. I savescummed and tried a different method, which was convicting him of another crime he had committed in the past. I got quite fortunate at this point because the hammer smashed a rib through his heart, causing massive blood loss. As soon as he was dead I walled the area up and constructed a pump nearby to flood the chamber with water. After it was flooded I constructed a chamber above, with a little water hole, and locked the peasant girl inside.
What I have noticed and learned so far:
Pumping the water in either removed or moved the blood. This may be related to water's ability to push blood onto walls. I noticed a lot of blood on the walls, where there had been little beforehand. However I had expected there to be some blood still visible on the floor. Secondly, after the peasant girl drank from the water she did not complain of its nastiness (despite having a rotten corpse right where she drank from), leading me to believe that the water had indeed not become contaminated. I still need to make sure she hasn't become a vampire, but she ran off to eat as soon as I let her out which is not promising. It may be that dropping down the well or spike trapping may be the only way to pollute water.