An open loop system is employed where ground water is easily attainable, or alternatively the site is close to a river of stream.
As ground water remains at a relatively high constant temperature all year around, it is one of the best sources of heat to use.
A typical open loop operates with two wells. One extraction well and one injection well, with the heat pump in the middle drawing temperature from the water as it goes through.
If you have a stream or dyke close by it is equally acceptable to pump the water back into those rather than sink a second well.