You can also use the thick, silver goop in a yellow tube (from a big box store) used to seal raingutters to seal buckets and plastics. I've used it and the seal is still working after 2 years.

Thank you for that suggestion.

I had considered that idea but felt the ponds were starting to deteriate from the harsh summers here. I figured I'd likely be dealing with leaks from that point on. Since the ponds are not that expensive, I decided that was the way to go.