When I dug my 1/10 acre pond it was in blue clay. The excavator smoothed out the whole surface to seal leaks but I didn't have to worry about water leaking out cause the water poked it's way through the clay and the pond filled on it's own. My water table is about 24" from the surface. But the pond has filled to the top. We had a ton of snow this year and all the runoff seems to have gone into the pond cause even with all the extra snow the surrounding area was drier than it was last year.(I just dug the pond last fall) My theory is that since the pond level is now higher than the water table, the clay must be containing the water near the top, but the pond may drain down to water table level. Time will tell, and good luck with your pond.

2 outdoor ponds- zone 6 1500gal backyard pond. zone 5 30,000gal mud pond 1 150gal indoor pond and 125gal aquarium.