I do thermal gradients in my day job, but NOT for home geothermal systems.

Have you called a contractor yet?

The reason I ask, is that I believe that during peak summer air temperatures, your soil temperature will be lower than your pond temperature. In June, the water temps in a stratified pond will probably be lower than the soil temps. However, that may not be the case in July and August when you need maximum cooling for the house.

If you are bringing in heavy equipment to excavate the pond, it might be very cheap to install ground loops at that time.

Good luck on your new pond project!