I'm in southwest Louisiana and have completely different soils, but here is what I was told by a trackhoe owner/operator who digs pond all year long. "If you cut into a "sand" you better know if it will sustain the water level in your pond all year long!" You may have encounter a water table that is controlling the level of your pond. My suggestion is mark the water level,pump it down, and see if it rises back to the old level without any rain runoff. With the amount of area that you have running into your pond it should have overflowed many times. If it does refill without any rain then you should be able to find the spot where the water is seeping in and out and cap it with more clay. Good hole hunting!!