Marty, since you are in Oklahoma where the Oil and Gas industry has been active for many many years. Look for and contact any Drilling fluid companies around you. Contact them looking for what is called liquid mud. Some of these companies sell this liquid mud. This is or can be a bentonite mix or ask them if there are any drilling operations going on around you. When an Energy company is done with the drilling they have to clean things up and may have to haul off the drilling fluid for disposal. It might be cheaper for them to haul it off to your pond. This would be done with what is called vacuum trucks. They would pump the liquid to the pond where it might help to seal the ponds bottom. This could be at no cost for you. Over the past 30 yrs we have done this many times. The pond needs to be drained before applying it.