ymurf - I've got a very similar problem that I've been trying to figure out how to fix without breaking the bank. I've got an idea that I am going to try but have not been able to find any literature about or anybody else that has tried this yet. Since, like you, I can see the part of the dam that is leaking, I plan on focusing my efforts in the vicinity of the pond near the portion of the dam that is leaking. Anyhow, I am planning on buying several hundred pounds of the regular powder form of bentonite, mixing it with an electric trolling motor in a large trash can filled half full with water, adding the bentonite powder slowly until the fluid reaches a consistency like very very thin pancake batter. I will then try pouring this thin mud in the pond in the area of the leak. My thought being, that as a very thin mud, some of this fluid will reach the bottom and get drawn into the leaking area before the bentonite particles become fully hydrated. As this very thin bentonite mud travels through the seepage paths, I am thinking it will adhere, continue hydrating and swelling, and eventually seal the seepage path. I am thinking that I will begin mixing a very thin mixture and gradually thicken it up (to the point that the trolling motor can handle) after I have added a few trash cans full of very thin mud so that a thicker and thicker consistency of mud will get drawn into the seepage zone, eventually sealing it off. I want the thinner mud to start with so that is will hopefully get drawn into the seepage paths deep within the dam. Since the seepage zone in both of our situations is apparently near the bottom, as long as the pond has a decent level of water in it, the bentonite should stay fully hydrated, and shrinkage cracking should not be an issue. Also, once the bentonite "cake" stops, or substantially slows the leak, erosion of the seepage paths should slow to the point that natural sediment will eventualy more permantently seal the seepage paths.
Maybe somebody else here with more experience can provide some thought as to whether this idea has any shot of success?
Its going to be a while before I can try this on my pond, as I have a lot of work to do to clean up the bottom and make a few upgrades which will have to wait to the water level subsides after all of the recent rain we have had. I'll let you know if I try this and how it works.
