Wow... Thanks for the replies. Those are pretty damning (excuse the pun) comments on the use of sodium bentonite. I'll keep thinking then. A very, very, expensive solution would be to construct a slurry wall behind the dam below the transmitting material. Another way would be to drill a well nearby, install a solar or windmill pump, and hope the daily pumping into the lake matches the leak flow rate thereby keeping a constant water balance. Mmmm... I wish there was a simple pill I could toss off the dock to solve this... Thanks again.
