Here's the scenario: Someone falls and drowns in a well dug into an aquifer, would this one corpse poison the water supply dangerously? How long would the water be dangerous for? Would they have methods for undoing it? (Early, mid, and late medieval periods) Or would they actually send someone in there to recover the body and prevent it poisoning the water? (assuming they knew what happened)
An additional question. How long would a well poisoned on purpose last fall? I've heard that some cities used to poison their own water supplies in the countrysides before retreating behind city walls for sieges.