A: A geothermal heat pump does not consume any water in the heat exchange process. If eight gallons of water go in the geothermal heat pump, eight gallons of water will come out. Thus, you will need a method to discharge this water.
Typical methods of discharge include returning the water to the aquifer from which it was extracted; secondary use of the water, such as industrial process applications or agricultural uses; evaporation and/or seepage ponds; and discharging to ponds, streams, rivers, lakes or the land surface. Naturally, it would be desirable to have plumbing installed to allow for secondary use of the discharge water, if this is permitted, for lawn and garden watering when the ground conditions warrant it.
Most sewer systems cannot handle the large volumes of water which geothermal heat pumps can produce (up to 10,000 or more gallons per day). Sewer lines may back up and tile fields will become overloaded, so disposal in sanitary sewers or septic systems is not recommended.
While ponds could be attractive and have multiple uses, they consume large amounts of land area, which would restrict their potential use in more densely populated areas. Discharge to the land surface may create bogs, accelerate pollutants leaching into ground water, and form icy areas in the winter. In addition, surface discharge in some areas may accelerate sinkhole activity.
Disposing of the water to a stream, river, or lake may be effective, but your local government may have specific regulations dealing with this issue. If the ground water used in the geothermal heat pump was of poor quality, discharging it to higher quality water will contaminate the discharge aquifer or body of water. Also, heating ground water may raise the temperature enough to be harmful to surface water dwelling creatures. Check with your local authorities before making your decision.
In close quarters, care must be taken so that the discharge doesn’t infringe on a neighbor’s property. Often, local ordinances prohibit any kind of discharge, and although variances can sometimes be obtained, they can be costly and time-consuming.
The disposal methods preferred by the National Ground Water Association, from a water conservation standpoint, are secondary use of the water and returning the water to the production aquifer.
When returning the water to the original aquifer, the water should remain in a closed system. There are several methods for returning the water to its aquifer. They are: using a single well for supply and disposal; using two wells and alternating withdrawal and disposal between the wells in winter and summer; and using two wells, but not alternating their function.
Single well — For return of water in a supply well, approximately 100 feet of vertical separation is required for every 12,000 Btu of heating or cooling produced per hour. A typical domestic geothermal heat pump, with a 60,000 Btu capacity, would require 500 feet of separation in the well from the point of disposal to the point of supply. If 1000 feet of vertical separation were needed, it probably would be inadvisable to use this system. Higher drilling and pumping costs could make this system impractical in many areas. Also, local regulations may restrict the use of a single well for both potable water and return.
Two-well system (alternating) — This method could take advantage of any temperature variations that may exist in the ground water from this process. In winter, a return well injects cooler water back into the aquifer. summer, the return well (now acting as a supply well) could withdraw this cooler water from the aquifer for more efficient cooling. The second well could be used in a similar way for warmer water. Alternating supply and return may also be beneficial as a preventive maintenance measure. The alteration in each well helps prevent clogging of the well screen, a common problem. This method of design is rarely practiced, probably due to its larger initial investment. The system would require an additional pump (one per well) and more piping compared to other systems.
Two-well system (nonalternating) — A general rule to follow is to drill the return well to the same aquifer at the same depth as the producing well. Return wells in sand and gravel aquifers must have a screen to help prevent incrustation and facilitate water movement. Water is always returned below the water level to reduce precipitation of dissolved solids on the well casing.
If a return well is drilled, it must be placed far enough from the supply well to prevent an overlapping thermal effect between the two wells. Raphael G. Kazman in his report, Use of Twin Wells and Water-Source Geothermal Units for Energy Conservation in Louisiana, created a table for use by the project engineer or designer to approximate the needed spacing between the production and return wells. These wells alternate their roles: the well used for production in the summer is used for return in the winter and vice versa.
Kazman reported that if each house in an entire housing development were equipped with twin wells and geothermal heat pumps, the return water of one property owner might be pumped by his neighbor. The studies he conducted indicate that to maintain the same well discharges without danger of interference with neighbors, the spacing between the twin wells must be increased, possibly as much as 20 percent or more. To minimize interference and short-circuiting between neighboring sets of twin wells, the summer well should always be located on the street side of any lot, and the winter well should be drilled in the backyard, he says.
Two wells, even in the same aquifer, keep the temperature difference separate, if properly spaced, because underground water flows so slowly. The spacing of the supply and return wells is dependent upon several variables, among which are:
- selection of the optimum geothermal heat pump based on available well capacity and operating cost
- the length of the critical season of operation (overall duration of the heating season in the North, air-conditioning season in the south)
- the actual number of days that the GeoExchange unit will be called upon to operate and the percentage of time during those days that it will actually be in operation
- the aquifer characteristics: permeability, thickness, and specific capacity.
Spacing the supply and return wells at least 100 feet apart are best for most locations. For good aquifer conditions, this spacing might be less, while greater spacing may be necessary for poor aquifer conditions. The return well has to be constructed as well as, or better than, the supply well–it is not just a dumping hole.
Theoretically, an aquifer will accept the same amount of water that it will yield. In reality, however, it will only accept 75 to 80 percent of its yield in return. Therefore, an aquifer that will yield 18 gpm will only accept approximately 14 gpm and the remaining 4 gpm will run out on the ground. The contractor and homeowner should be concerned about the possible plugging of the return well by deposits of sand or high amounts of iron being present in the ground water. Air dissolved in the water can also induce corrosion.
It should be noted that in aquifers with low permeability, gravity feeding of return water might not provide sufficient pressure to allow infiltration into the aquifer. If the discharge water is returned to an aquifer other than the supply aquifer, and the two aquifers are separated by a thickness of low-permeability material, interference should be minimal or nonexistent.
Supply and return aquifers must be chemically compatible to assure that mixing of the two water types does not result in precipitation of salts or hydroxides from solution, which might lead to eventual plugging of the aquifer surrounding the return well.
Local policies vary widely concerning the use of return wells. Several states, including Wisconsin and Minnesota, do not allow any type of underground injection return at this time. Several others have adopted a general policy to discourage this type of disposal method. On the other hand, Oregon encourages return of this water. Many of the other states have some form of permit or notification procedure.
Enforcement is often lax, however, particularly for small domestic systems. Regulatory agencies commonly have statutory authority to regulate these systems but no formal program. More than one-third of the states have no permit or notification requirements.
Regulations for return of water to surface water generally correspond to the federal National Pollutant Discharge Elimination System standards. Disposal to a leach field and septic tanks is generally not a problem, but is usually not recommended. However, many states require that septic tanks be spaced a certain distance (generally around 50 feet) from a well.
Disposal to sewers is regulated on the state level in only four states. The state of Minnesota, for example, had forbidden for many years the use of any type of return well. The law was amended in late 1981 to allow the construction of a minimum number of return wells.
Provisions of the law said that the wells must withdraw from and return to the same aquifer, the wells must be constructed so as to allow for inspection of water quality and temperature, the system must be constructed as a completely closed system which is sealed against the introduction of foreign substances, and the owner must agree to allow inspections by the health department. The law does not allow for a domestic supply well to be used in conjunction with a geothermal heat pump. Again, remember to check with your local officials.