Here's an engineering question:

How thick a clay blanket is required to support the hydrostatic pressure exerted by a 10' column of water??

Subsoil is quite sandy.

Troutgal,
I will respond to your question in regards to "bentonite thickness". Actually, the question you should be asking is "how many pounds of bentonite are required". You will need 3.5 to 4 pounds per square foot of surface area to be treated. Water depth is not an issue, so don't try to make this rocket science. A good quality bentonite meant for pond or lagoon sealing will do the job, and expect to pay from $7-$9 per 50lb bag. You do not need to pay $20-$30 per bag as I read on a previous post. I would not recommend buying bentonite from the local farm mill as this bentonite is used for something other than pond sealing. No sense paying $2 per bag less only to find out the pond did not seal. Hope this information helps.

While building my pond, I ran into an area that needed to be sealed as well...loose gravel type rock. I have clay available, the Dozer Expert, lol 40 years experience, is spreading 4-6 inches over it as a precautionary effort. After the tractor runs over it, the seal should be fine.

LOL..

Thanks for the responses. So far I've gotten anywhere from 2-4 pounds/sq ft suggestions. Better too much than not enough!

My only problem is how to get the bentonite to stay on a 2:1 slope. Not too steep to drive straight up or down so maybe it can be incorporated easier than I think.

Thanks for the info