Jim, welcome to the forum. Get rid of the sand, excavate the clay and stockpile it.
Make the pond into the shape that you want, but make it 18"-24" bigger and deeper. Bring that clay back in, with proper moisture so you can roll it in your hands and make "pencils" or a hair wetter if it's really dry out.
Spread it out about 6"-8" deep. Run over it with a sheepsfoot roller until the roller starts to "walk out" on top of the clay. Repeat with another layer. Do that 3-4 times from bottom to past high water level in the pond and it should hold water like a bathtub with the drain closed (not open). Good clay, correct moisture for compaction and proper compaction are the keys to having a pond that holds water.
Layers thicker than that and the bottom of the layer won't compact, and the layers won't "knit" together. They will be like sheets of paper in a book. Tracked construction equipment is designed to lessen the psi on the ground, so the equipment doesn't sink. So, they are poor soil compactors. You could use a multi-tired pan scraper that is filled with dirt to get weight. Make many, many passes to compact ALL the clay. But, a sheepsfoot is what I'd use.
