The close-up photo is of a 3/4″ diameter hole in my well cap that is normally filled by a simple plug.  The well is 47 years old; these days, a vent would probably occupy that hole.  I was about to introduce a diluted bleach solution to address some coliform bacteria, but inside this hole, as you can see if you look carefully at the photo, are a tube of slightly smaller diameter and within that tube, another of slightly smaller diameter.  Not knowing what those are, I’m hesitant to pour the diluted bleach.  The other photo is of the whole well setup, with the square-headed plug occupying the hole in the close-up photo.  Do you have any idea what the tubes inside the hole are?