I thought you could use a float (toilet) valve in an overturned container with insulation of some kind over the top. A pipe connects the container to the pond water water underground. A small vent hole in the overturned container acts as a dampener. Run the well water through the container and pipe, which if run often enough will keep the water thawed. The pipe must be large enough for the water flow from the well to prevent backing up and tripping the float prematurely.
Just some thoughts for electric free.
