Before you go through that expense, can you give that Polymer Innovations material a chance? They have two products that apply to us: Seepage control, and Leak control.
http://www.polymerinnovations.com.au/You seem to have the perfect scenario to try the seepage control stuff out. You would likely need at least 2 pails, which is about $350 a piece. A heck of a lot less than bringing in clay. If it works, no more needs to be done. If it doesn't, the material will still be integrated with the soil assisting a clay liner. From what I understand of the materials is it will work best on a fresh pond without muck and vegetation.
I am dying to know if this works or not. I didn't get any this year since my wife is threatening to quit her job and we have had enough rain and runoff to keep things topped off. I may have to pull the trigger this fall as we have not had much rain recently and are back to loosing water.
