Just a thought. I may try this. My pond has dropped a lot also. It has also slowed way down dropping and has seemed to nearly stabilize. That tells me any seepage must be in the upper few feet of bank and or dam. My pond only just filled to spillway this spring. I may take this time to get some sodium bentonite to go around this area...maybe sealing it off. I'm sure there are a lot of other things you can do that the experts will chime in on.