Bentonite has to actually be tilled in to be used properly and it takes more than what you think as well. Also is this a spring feed pond or just collects water from every from runoff. etc ? I had a customer last year with a earthen pond that leak. It was roughly a 1/4 acre if i remember correctly. They needed the soil to create a new driveway so they dug a pond. I asked them why the just didnt put a liner in to start and they said contractor said it would hold water funny thing is i looked at the soil charts and concluded it wouldnt hold at anytime and called the extension office to confirm the site was sitting rock and shale deposites and just was a bad site. The liner was the only way to go the homeowner was not happy cause now its gona cost them double the amount cause they didnt line it in the begining. There is a liner designed especially for earthn ponds that i use and is half the weight of the traditional liners and is custom made to fit you pond size. Ther was no way i was gona use 45mil lol.. The weight of the liner was was roughly 5-6000 ponds.
The other liner was 2000 pounds. You will need to install a drain tile under the liner to relieve the pressure build up. Also you will have to place 10-12" of soil over the liner like tad said. This not your basic pond so you have to think about in different way.