I have a very similar situation. I sent you a PM message. Feel free to contact me, I may be able to offer some help. I also live in SW ohio. Our soils are generally heavy with clay, except in flood plains and such. Why is the site damp when it is sandy? Is there a high water table? What is the "muck" made up of and how thick? Seems odd that they would recommend the site with a guarantee when these soils are problematic to begin with.Have you had your soil and water district evaluate the site and observe the test? They will engineer your pond for 150-200 dollars which is refundable if you follow their plan
