big project to do it right, but should last for years if followed, not sure on rate of bentonite, depends on your soil. 1/2 to 2 lbs per square foot covers the whole range. I wouldn't skimp so you get it done in one shot. strip back a layer on the dam, till in the bentonite, to a 6 inch depth. compact with tractor tires is possible, then re lay the layer stripped off prior. you want it there to protect the sealed layer. figure that clay rate for a 10 foot deep pond, if it is 20, double the rate. is your pond leaking through the dam, or bottom? what kind of soil would you say you have.