Pond Boss Forums POND CONSTRUCTION Selecting a site Pond Repair

I have several small ponds in addition to a larger pond that the original owner dug out made concrete walls and put some type of roofing underlayment to create the vapor barrier. Some of them clearly have been compromised and don't retain water. These ponds are small like 25'x25' max and he has put about 10" of gravel at the base of all these. I would like to repair them and am looking for the most cost effective and labor unintensive solution.

Here's what I've considered....
Moving all the gravel to one side and hot tar over the underlayment.... move gravel to other side hot tar there. I think this is my favorite solution so far but not sure how well hot tar will hold up.

Move gravel in similar fashion and put new waterproof barrier (synthetic underlayment) over the existing stuff. I am concerned about this method meeting the test of time, possible sagging etc. I also imagine it is more expensive than hot tar would be but I am not sure.

Appreciate any Ideas or critiques!

Contact some of the liner companies. They might make a repair patch kit, somewhat like a pool liner patch kit. Tar/liner might expand/contract at different rates and it might not be stuck together for very long. Hot tar is pretty brittle once it cools down.