[jeff5may]

The easiest cheapest most reliable way to do a pump and dump system is drain to the surface. Any time you put water back into the ground, it could travel back to your source, robbing you of your heat advantage and poisoning your efficiency. People who do inject water back underground do it downstream in the aquifer or to another layer that is separate from the source layer. Either way requires a whole lot of testing and faith or courage.

Think of it this way: the ground is huge, but your borehole is not. Injecting nearly freezing water down 40 meters is only going to have 40 meters times the circumference of the borehole worth of surface area. For a 25 cm hole, that equals 31.4 square meters. Standard rules of thumb say 100 to 200 square meters per ton, so your hole is at least 5 times too small. In comparison, drawing from an effectively infinite source layer of heat, you'll have no heat transfer problems at all.