When the pond was full could you see the water exiting on the back side? That would give you an idea of how deep the problem is. If it's less than say 6ft I would drop the water level & dig down with equipment & fix it.
You're idea of pouring bentonite in may have merit if you have the patience. I would imagine it will be a very slow process.
I agree just plugging the hole is probably futile as the water will most likely find another route to the real problem.

Pond Boss Subscriber & Books Owner


If you can read this ... thank a teacher. Since it's in english ... thank our military!
Ric