Bobbss....don't waste your time or money trying bentonite....it will not seal unless it's worked into the soil and compacted properly. IIRC, you were in a drought when building the pond, and if the clay was dry then, it could not be compacted well. Best and lowest priced "fix" would be to get a self driven sheepsfoot in there and compact it well while the clay has moisture....it will need to be drained and allowed to dry some to get excess moisture out...