Prof. Child doesn't say much about this. It was the title of the song in the book he got it from. (He "collected" from books, not people).

I would not be surprised, but I certainly do not know for a fact, if the well was one of those enchanted or "holy" wells. Since she was clearly a wisewoman or witch (sent the children to learn gramery (sorcery)and conjured them back from death) being the guardian of an enchanted well would be in character.