Bob's response is the most feasible solution.

With enough well water at X temperature, you could reach any temperature down to X for a small enough pond and a high enough flow-thru rate of well water.

Lifting water does get expensive on the electricity bill.

If it is a rich guy, then I suppose it would be possible to use a heat pump with a ground loop and a loop in the pond. Of course, it would be least efficient in the summer as you are dumping heat into the ground around your loop at the time when the ground is the warmest.

We are quickly getting beyond any of my areas of even tiny expertise. (Although that hasn't always stopped me from running my mouth in the past!)