So, it's been a while. Curious how the fix turned out? I live in SE Michigan and have the same soil. I bought my house in 2012 with the.6 acre pond with the same problem. It's beautiful in the spring but as ground water drops so does my pond. Previous owners brought in a few truckloads of water to bring the level up just before selling, bless their hearts. I'm learning a lot though haha. I've looked into bentonite clay as you have done and liners. I was told ground water would push the liner around as well as the clay, which makes sense. I have a backhoe, just not a solution I'm sold on. Pond is 15-20' deep max. I've considered seawall sections but that's a lot of work and expense but I could do it myself. I'm leaning toward a trench around the perimeter down to the clay then putting a plastic, rubber, or clay liner or combination but wanted some experienced feedback on it before committing. Lots of large trees close by which is probably giving problems from root growth now and in the future so clearing some out is probably on the horizon.
