From what Ive experienced with bentonite, I have fixed leaks in a number of ponds and lagoons with it, always the powdered stuff, never tried the granulated stuff, but it will have to be incorporated into the soil a good ways to be effective, even if it did temporarily stop the leak for a while by applying it to the surface, a simple rooting from a catfish, or even a bluegill building a spawning bed, or a crawfish, would open it right back up. imo
Good Luck,, let us know how it goes.
