Check your stocks list for items which are still marked for dumping yet not forbidden. There is one, a rough pyrite, which appears to be underwater. It is probably invisible because it was washed to its current location by water movement. I haven't had time to examine the well situation yet and it is time for me to cook breakfast.
That was fast, for the problem I thought would take longer to fix. Thanks!
Second Edit: You have water falling into the well from above. The well bucket displays as full, which I have never seen before. It is my guess that people are being knocked into the well by falling water when they are trying to get a drink, leaving the bucket full because the drinking task is unfinished.
...Hm. Problem is, I spent a lot of time on this mist generator, and don't want to just scrap it. So I'll deconstruct the well, pu in a floor grate, channel a hole in the dining room floor, and build a new, off-center well.
Or maybe rebuild the well somewhere else. That could work, too.
EDIT: Wait, my tribe speaker just fell in, cancelling "store item in a barrel"! I hope this still wor...wait, I'm putting a floor grate in, of course it'll work.