Robinson,
I suspect that when the water drops enough your going to see a large hole somewhere in the side of the pond basin. A 400 gpm leak ought to leave a pretty good mark. Drain the pond down and cover the hole with clay and pack with a sheepsfoot. Scrape the top dirt off the entire basin, if you haven't already, and run the sheepsfoot over the entire basin 6-8 passes until it starts to walk up out of the ground. I think in a karst area the entire basin has to be attended to, not just the low spots. Don't give up on it. I live in a karst area and I know of several large ponds (lakes) built on the same topography near to me.
