I don't think there is a fix to a structure problem like that unless you drain it and maybe concrete the bottom area. I once tried stopping up a creek with a gravel bottom. Never worked.

A friend of mine near you at Chalk Mountain had your situation. He broke thru the bottom to try to fix it. However, he found porous limestone below the surface and now has only a memory of a water hole.

Heck, mine goes down 5" a day in the summer when the temp gets up into the triple digits. Just hope it keeps raining enough to replace the leak and evaporation.