I have a goofy suggestion, so feel free to point and laugh (set the milk down, and swallow before reading!).
A while back somebody posted about how he cheaply made 20 foot deep holes by hooking up a water hose to a section of pipe. He put a "T" on it just before the hose on top, so he could spin it. As he twisted and pushed down, the down-flowing water did the boring and mud ran up the outsides of the pipe. If you hit a rock, you'd have to start a new hole, but...
I'm guessing if you go down 8 feet the temps will be at least 50F.
Put a dark trough where it will get as much sun as possible, and a small Savonius VAWT could circulate the trough water through a loop of PVC pipe in the 8' well next to it.
Even if you still want to use a bubbler, by introducing bubbles near the bottom of the well-loop on one side, it would cause some circulation.