Bobert... before you buy any bentonite, read up on how to use it. It can be a bit tricky.

As an engineer designing containment dykes, I never once spec'd that method associated with "tilling". I always spec'd the mats, but they are expensive and expensive to install. Of course, that was on large projects. You may find sommat available for a small scale project. And, as you won't have mechanical stress upon your pond liner, it may be the way to go.

Of course, I assume you are investigating all the options.

One of the problems with the "tilling" option is this. If you don't get the job don't right in just one small area, you will have to do the whole thing over because you will be looking for a needle in a hay stack. Actually, the needle would be easier to find.