Thank you for the reply. The pond is dry, I've just finished digging it. I have now been in contact with several distributors of bentonite and all of them say the same thing, sodium bentonite is not a good option for me as it tends to creep downwards on slopes steeper than a 1:3 ratio (V:H). Guess I'll need to look for another solution.
Ducks and pigs can be used to seal ponds.
