You are right.  Soil temps only rise on verticle systems which are cooling dominated.  Horizontal systems are more affected by seasonal swings than geosystems.

Yep. Water is most dense at 39F. So as the water at the surface cools, from say 50F, when it gets to 39° it sinks to the bottom of the pond. When all the water in the pond is 39 and the water at the surfaces continues getting colder, it stays on top until it finally reaches 32 and freezes. Even in the hot summer time a really deep lake will have 39° water at the bottom. Just nature's way of making sure lake trout always have correct temperature water to swim in, summer or winter.

Having access to 39 degree water in the summer as a heat sink should be ideal (very efficient) for cooling in the hot, humid Southeast.   Do you have any idea how deep the water should be to be an ideal heat sink for the geothermal system?

No, I don't. The lakes I have in mind with 39 degree water at the bottom year round are like 100' to 400' deep.

I don't think you will find any 39° water anywhere in the hot, humid Southeast.  You need cold winters to cool a body of water that cold.  Heat pumps are desinged to work with incoming water temps of 90° when in cooling mode.  Granted 39° would be better, but you have to work with what you have where you are.  Putting an aerator in a pond in the summer will help to keep it cool.  Even with incoming temps of 90° a heat pump is more efficient than most other options for cooling.

The data I have seen recommends a minimum of 5' water over your heat exchanger.  Here in Utah where we have cold winters, we go with 8' of cover.

Hope this helps.

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