I think it would need to be much closer to the surface--no more than 6 inches. And you'll need a very thick chunk--probably 1/2 inch or more. Might be costly. What about putting something like cactus or roses around the edge? Means a PITA for you to maintain the pond, but a thin barrier should be a significant deterrent, I'd think.
Finding the leak won't be fun--wish I could think of a clever way to make that easier for you!
Finding the leak won't be fun--wish I could think of a clever way to make that easier for you!