In an open loop geothermal system, the heat exchanger uses well water to supply tor remove the heat energy from the home.
In some cases, these systems are the best choice but generally not, in my opinion. When you draw water from the ground, there is most often a confining layer that prevents that water from recharging the aquifer.
Usually, it just goes away. It goes downstream, evaporates, maybe penetrates into the shallow water layer. But it usually does not go back to where you got it from.
if your heat pump uses 12 gallons of water per minute and runs 45 minutes per hour (not unreasonable in the cold weather), you will flow 388,800 gallons of water per month from your aquifer.
Also, even though some will say that it is the most efficient source for a heat pump, they rarely factor in the cost of pumping the water.
While, technically, the coefficient of performance can be a little bit higher but if you have (deep breath) a 1/2 HP submersible pump drawing just 6.2 amps at 250 volts running 45 minutes per hour at 13.5 cents per Kilowatt/hour, you will spend $101.25 for 30 days per month just to pump the water! That takes a real bite out of the "most efficient" argument!
Then factor in the cost of replacing your pump more frequently than what you would if you were simply using it for your domestic water supply...
Just make sure that you weigh all of the options