The use of well water requires a disposal system for the large quantity of water to be returned to the aquifer. The least expensive disposal system, when local conditions make it practical, is surface discharge to a local river, lake, or stream.
This system eliminates the cost of a reinjection (or recharge) well. The supply well should be at least 100 feet or more from the discharge point. This permits thermal reconditioning.
The discharge point should also be above the surface of the water. If surface conditions are highly permeable, a tile field may be effective. A dry well is another alternative. Do not just dump discharge water onto a level field. It could create a lake. The situation may get worse in winter as the water freezes.
Where surface disposal alternatives are not suitable, a return well system must be used. This well should be drilled at the same time as the supply well, and be deep enough to receive the maximum amount of discharge water from the heat pump. It should be located at least 100 feet from the source well and should recharge directly into the same aquifer from which it came.
A major consideration is that a typical return well will only accept 50 to 75 percent of the water it will yield. When water is withdrawn from a well, a cone of depression is formed with the low point being at the well screen. This is the shape of the water flow into the well.
The opposite occurs in a return well. However, in this cone of impression, the high point is at the well casing. If there's a high static water level in the well, say 20 to 30 feet, the return water can rise enough to bubble out of the well cap or overflow pipe. A prudent driller should take this into account when drilling the return well.
Therefore, the return well should be larger or at least the same diameter as the supply well. It should also have twice the screen area. Finally, the return well drop pipe should terminate below the static water level to minimize encrustation and other mineral precipitation.
Most of the problems encountered with early installations have been overflow problems. In some cases, users have put all or part of the water back into the same well. If this is done, a control valve should be used to divert the water to the return well if the water temperature gets too cool (say below 38 F) in winter, or too warm in summer.
Another alternative where mineral precipitation is a problem is a two well reversible system. In such a system, both wells include a submersible pump to alternately pump and discharge into opposite wells. While this is more expensive, there are three major benefits:
- The return well can be heated to a warmer temperature in summer and reversed in winter;
- Aquifer replenishment may be more effective since both wells are redeveloped in the process; and
- Minerals and encrustation are periodically pumped out.
A word of caution: When switching, the first ten minutes worth of water should be pumped to waste. This keeps the accumulated encrustation material from being pumped through the heat pump's heat exchanger and returned to a well that was cleaned.