In my extensive experience in Ancient (and not so ancient) history, every battle and siege seems to have a reference to “and they poisoned the wells so the enemy couldn’t drink.”
But how do you UNPOISON this font once the enemy has been gutted and sent home early? Especially the ancient times where I suppose it was harder to treat water. They had to have done something to fix the wells.
The history books never tell us the details . . .