Why not use a closed system, at least as far as the well is concerned?
Maybe a big coil of 3/8" SS tubing down in the well?
Maybe 3/4 lines in the casing, 1" in the long runs?
Basically an upside down and inside out solar drain-back system. Use the water, casing and earth as the heat sink.
Low head, low volume, low power pump.
It would not be as astoundingly cold, but it would be less wasteful, use less power, and not risk contamination.
Before the laughter begins,
Consider the BTUs exchanged in the test setup.
Consider the power the well pump uses to supply the well water (not an issue if the water is needed for outdoor use).
Compare the power the well pump uses to exchange those BTUs with a small and efficient window A/C unit.
My little (only, and very little) high SEER window A/C unit must use less than 8A at 110V.
Yes. The well temp will increase, and that will save money on the hot water heating.
G-