It apears you have a clay layer with sand on top of it, the sand is holding the water. Most of Ohio was scoured by glaciers and they leave behind many layers of different material, you never know when one layer will start and another ends. I would think you can get a pond dug with out problems and as long as you do not dig through the clay layer, and you are correct the pond should be filled slowly or quickly by ground water. Whenever you put a bucket in the ground you never know for certain what you will find but if your contractor knows what he is doing he can tell by the material coming up what is going on with the soil.
