Sodium Bentonite. I've used it and it works well. Depends on a lot of factors, like soil type and where the leakage is occurring.
I'd be happy to help if I can. Pics would be helpful.
Bubba

Bentonite Clay?

Bentonite based products have been used for many years to line lake bottoms and to seal foundations walls.

It may be used on ponds and lakes more often where you have it naturally occurring, so to defray some of the costs... I think parts of Colorado have it naturally occurring... lately, panel products have become available.

Here is a link that might be useful: Bentonite Building Products Info

I think the clay they use in kitty litter will work. You would have to get the cheap stuff that has no scent or other additives...only the clay.

I understand that kitty litter is fired or baked, so it doesn't work the same as raw bentonite. Of course,I could be wrong... I use kitty litter in the pots with pond plants; it stays in the pot better than soil.
Lee

Looking for a source of sodium bentonite for a40' x 40' pond. How much should I buy and what will it cost in bulk or by the bag?

Any good sources of "how to" information on the web?

Thanks in advance for your help.