dhill1, I would also consider doing it a different way. I don't think anyone on this forum has sealed a leaky pond using bentonite/clay alone. Especially if you have rocks to contend with.

I think the only sure way to seal it would be if you could completely isolate the current bottom and the rocks in it with several, several thick layers of compacted clay. That means really good quality sticky clay, 6-8" layers, each layer compacted with the proper compaction tool (not just riding over it with equipment), and then do that several times. That means trucking in tons of clay and if you can do that and do it right, you probably will be happy.

If you cannot do that, then trying to do lesser clay quality, less thick layers or fewer number of layers and the hoping the bentonite will make up for that by its 'sealing' properties is probably not going to work.

Then your plan B is to read what you can about soilfloc on this forum. This is a plastic polymer that can make the best of a poor pond bottom when it is decided that proper construction was not used the first time, or that you can't drain, start over with proper clay, proper layers, proper compaction etc. The goal then would be to hopefully prepare the bottom by removing leaves, organic material, vegetation to allow the product to get to the bottom. Then when applied properly and in proper amounts, it does a pretty good job of going into the cracks and 'sealing' them. There have been many more success stories on this forum using soilfloc than just adding some bentonite and hoping it will seal the bottom for you.

Some have tried a hybrid solution with bentonite AND soilfloc but the jury is out on that from what I have read here on the forum.

If you are interested in trying that route, we have a PB expert here who can help and there are some good threads about that.