Im a long ways from being a professional but have sealed a few lagoons and ponds up with bentonite and have had way better luck with incorporating it into the soil at least a ft or more deep into the clay, I actually sealed off almost clean sand up that way by mixing a generous amount of it with the sand for about 18" deep, but then that was in a sewage lagoon which only had about a 3 1/2' working depth, not near as much pressure.
