Okay, there are two different types of bentonite, and I'm sorry, I'm not sure which is which, but one works better than the other. You will have to hope someone smarter than me responds.
Your pond is large enough, you should have a soil specialist come look at your site and make sure it is stable. I dug a much, much, much smaller hole to make a earth pond, six feet by three feet, by a foot and a half deep. That stupid pond threatened to take down my hillside.
