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A: Most problems regarding geothermal heat pump dewatering have stemmed from improper determination of well yield. Domestic wells are normally designed to produce enough water for household use only, which is usually 300 to 400 gallons per day. Ground water heating may require 10,000 gallons of water or more per day in extremely cold weather. An adequate water well design for household usage may not be sufficient to also sustain a geothermal heat pump.

Uncontrolled overpumping and overdevelopment of the ground water may cause problems such as aquifer drawdown and well interference. Aquifer drawdown indicates that more water is being withdrawn from the aquifer than is being replaced and usually manifests itself by smaller yields, lower water levels in wells, and higher pumping costs.

Pumping a well usually creates a cone-shaped depression of the water table (the two-dimensional surface representing the top of the ground water), with the lowest point on the cone being the well location where the water is being pumped. With overpumping, two or more closely spaced wells may have an overlapping of their individual cones of depression. This is called well interference and also is reflected by lower well yields, as some of the available ground water must now supply two or more wells instead of one.

The way to prevent water-level declines is not to pump water to waste when the well is used to supply water to the geothermal heat pump, but to use it and then return it to the aquifer through a return well.

The spent water, which may have been used in the summertime to remove heat from the interior of a building, will be warmer than the native ground water. In the winter, on the other hand, it is cooler than the sources of ground water. The spent water, if returned too close to the production well, will break through or shortly appear in the production stream. Thus, if the volumes pumped during each season are nearly equal, the temperature of the ground water will not change significantly even after a decade or more of operation.

Whether the water is returned into the same producing aquifer by way of the supply well or a second well, no aquifer drawdown or well interference will occur.

Management of the heat balance within an aquifer is essential in urban areas where heat transfers between several users may have to be coordinated. Spacing of private domestic wells is likely to depend on property boundaries rather than the hydrologic characteristics of the aquifer under development. Random installation of GeoExchange units could lead to thermal interference through improper well spacing. Efficiency of the geothermal heat pump system would be considerably reduced where a sufficient amount of interference exists.

Through careful planning and analysis of the aquifer prior to housing construction, it is possible to avoid the problem of well interference. Production and injection wells can be spaced for optimum dissipation of thermal energy within the aquifer. Well spacing should be based on the heating and cooling load for the proposed number of residential units to be built at a given location, as well as the hydrologic properties of the aquifer to ensure the efficient utilization of ground water for the operation of geothermal heat pumps. Thus, it is crucial that the well yield be correctly and accurately determined by a qualified contractor.