If you research my posts I have had the same problem you are having, trust me its not fun. Easy as you might think it is to plug the leaking overflow from the outside of the pond its not. In fact its impossible. That water has more pressure then all the dirt, clay and cement you could imagine. You got wait till the ponds water level goes below the leak then seal the pond from the inside with bentonite. Its not fun easy or fast but you just cant fix a leak from the outside.