rulgert, welcome to the forum. I was just in De Soto a few weeks ago, small world. Two things that come to mind are coring the dam and compaction. I am assuming you didn't use a sheepsfoot to compact the clay in 6" lifts.
You may look at trying to dig up a little more and compacting a clay liner around the entire bowl. Works better if the dam has a core trench but your dam has a lot of dirt in it already. Not going to say it is the best solution but definitely worth a look. I would think if you can get a total of 2 feet of known good clay compacted around the entire pond, you will get pretty close to sealing off most leaks.
I would also recommend a bucket test with clay from a few different locations around the pond:
Bucket Test Link 1Bucket Test Link 2Bucket Test Link 3TJ has also had some luck with
SoilFloc, post will explain.
