Will you be on location while they are taking the soil samples?

You do NOT need 100% clay to make the core trench in your dam. It can have some silt and/or sand particles in the clay.

If you can be there, take a bucket of water with you. After they take enough material for the lab sample, you should grab a handful of the material that appears to contain clay.

Wet the sample a little bit. Can you roll it into a ball? Can you smush the ball on the side of your truck and have it stick? Can you roll it into the shape of a pencil? If so, then you have found your good clay.

Keep taking samples in your projected pond basin that would be adjacent to your dam location. If you can confirm good clay in a few spots in that basin, you should be good to go. If you find clay in another location, you should still be good, BUT your pond construction costs will go up because transporting clay to the dam location adds to the machine hours and diesel bill.

If you don't find any suitable clay material, then you are going to have to switch to Plan B.