Introduction

Utilization of environmentally friendly and renewable energy sources has been on the rise around the world in recent years. One such source is the use of heat generated from a groundwater heat pump system as an alternative to burning of fossil fuel for residential and commercial space-conditioning applications. These include heating, cooling and supplying hot water. The US Environmental Protection Agency and the US Department of Energy concluded that geothermal heat pumps are the most environmentally friendly and cost-efficient way to heat and cool your home (Hughes 2008). Geothermal systems emit no carbon dioxide, carbon monoxide, or other greenhouse gasses that can be harmful to the environment and human beings. Groundwater can serve as both a heat source (for heating) and a heat sink (for cooling). The local temperature of groundwater varies little, if at all, on a seasonal basis, regardless of the temperature extremes on the surface. Thus, it is warmer than the outside air in winter and cooler in summer. Since heat pump capacity and efficiency vary significantly with the heat source/sink temperature (or temperature difference between the source/sink and the conditioned space), a groundwater source heat pump system, in principle, offers considerably more advantages over the traditional air-source heat pump system.

When boreholes are drilled to extract/inject groundwater for heat exchanges, they are called energy wells. The exact number of energy wells drilled in the world is unknown; however, it is estimated to be in the millions. In North America, half a million energy wells are drilled every year; around 20,000 such wells are drilled in Sweden yearly (Gehlin 2002). In China where groundwater heat pump systems have been introduced recently, tens of thousands of boreholes are being drilled to tap the energy source (Xu and Rybach 2005). Sales of the pumps exceed 8 billion yuan in China and are growing at 20 % annually. Many high-profile buildings such as the Olympic Village National Gymnasium, National Great Theater Waterscape, and the Water Cube facilities have utilized groundwater heat pump systems. The sizes of the systems vary from single boreholes for single family houses up to 600 boreholes for large facilities. Prospects for groundwater heat pump systems become more promising as China ramps up its efforts to reduce emissions and energy use as part of its 5 years plan for 2011–2015. The market for development and utilization of geothermal energy is estimated to grow to approximately US $15 billion over the next 5 years. Initial costs for installment of the pumps have decreased significantly, further paving the way for utilization of shallow geothermal energy across the country.

Use of groundwater is not without potential technical problems or economic and institutional constraints. The primary concerns are the cost of the wells and the availability of an adequate supply of suitable groundwater. Secondly, removal of significant quantities of well water without sustainable recharge may deplete the aquifer. Plans to re-inject or return the water underground may also encounter technical difficulties and/or be precluded by legal restrictions. If the system is permitted it could entail additional costs for a disposal well. Special provisions to prevent thermal alteration of the underground source may be required. This paper summarizes these and other issues related to groundwater quality and availability, potential environmental effects, and legal restrictions associated with application of groundwater heat pumps.

Shallow geothermal energy

Heat pump systems abstract water from shallow underground sources, reject or extract heat from the water, and discharge the water either to the surface, or more often back into the ground. In the groundwater heat pump industry, shallow refers to depths less than 400 m, depending on the geographic location of the system being constructed (Dochartaigh 2009). Temperature of the shallow subsurface is much influenced by annual mean air temperature; therefore the heat which is taken by geothermal heat pumps comes predominantly from the absorbed solar energy in the Earth’s surface. The external source of the Earth’s heat is the solar radiation. In summer, the Earth’s surface heats up due to intense solar radiation and increased temperatures. The most typical periodical variations are daily and annual temperatures on the Earth’s surface which have an influence on the shape of geothermal profiles (variation of temperature with depth). At soil depths greater than 30 ft below the surface, the soil temperature is relatively constant, and corresponds roughly to the groundwater temperature measured in monitoring wells. This is referred to as the “mean earth temperature.” Figure 1 shows the mean earth temperature contours across the United States. In each area of interest, a similar map may often be available or can be generated utilizing data available.

Fig. 1
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Groundwater temperature contours (in Fahrenheit) in the USA (Heath 1983)

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A good estimate of groundwater temperature is necessary for accurate design of the groundwater heat exchanger. Ground temperature increases with depth due to geothermal gradient, while the gradient varies over the world and is normally in the range of 1.8–3.6 °C per 100 m. Figure 2 shows a generalized temperature profile. When measuring the undisturbed ground temperature, the borehole must be at thermal equilibrium with the surrounding ground. Temperature logging by recording the temperature in a borehole provides the site-specific temperature profile.

Fig. 2
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Generalized temperature profile in the ground (Gehlin 2002)

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This shallow geothermal resource distinguishes itself from the widely recognized deep geothermal resource that produces higher temperature energy, above approximately 40 °C and is usually obtainable in areas of high magmatic activity or at depths of approximately 1 km and below. Deep geothermal energy is a non-renewable natural resource and has been used to generate electricity and space heating. On the other hand, the shallow subsurface serves as a huge heat reservoir and heat stored naturally in soils, rocks and groundwater occurs ubiquitously in the subsurface. Low thermal conductivity of the natural materials prevents stored heat from being immediately dispersed and allows use of heat from underground to be coupled with heat pumps to produce energy.

Coupling groundwater with heat pump

Groundwater abstracted for use can be either naturally flowing in aquifers, or it can be groundwater stored in mine workings. After circulating through the heat pump system, the thermally altered water can be disposed on the surface or injected back into the subsurface. Water use and discharge methods are important in the consideration of groundwater heat pump installations. Improper design may lead to a decrease in system performance or environmental problems or both. Groundwater depletion and local lowering of water tables should be avoided; thus, in most areas, recharge to the subsurface is recommended. In general, recharge using an injection well is the preferred method of water discharge. Due to regulations at the state and/or local level(s) of government and/or to geologic influences, alternative methods (e.g., leach fields, dry wells, or discharge to surface water bodies) could also be investigated.

Environmental impacts associated with usage of groundwater heat pumps are minimal or non-existent when the properties of the hydrologic system are evaluated and taken into consideration. In areas where well yields are low, however, consumptive use of groundwater due to over-pumping can lead to water depletion in the aquifer. In environmentally sensitive groundwater regions, careful planning is required to minimize environmental impacts.

Whenever possible, consumptive use of groundwater for heat pump applications should be carefully evaluated before implementation. Problems associated with widespread, high-density use of this method are usually too numerous to warrant its use. A possible alternative to consumptive use is the implementation of earth-coupled well systems. These closed-loop units do not withdraw water from the well avoiding problems of aquifer depletion.

Some problems are associated with the non-consumptive use of groundwater supplies. The most serious of these is the thermal alteration of the aquifer system; however, environmental effects of thermal alteration are minimal. The greatest effect will be in the performance of the heat pump system. 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 groundwater heat pumps could lead to thermal interference through improper well spacing. Efficiency of the 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 loads for the proposed number of residential units to be built at a given location, as well as the hydrologic properties of the aquifer to insure the efficient utilization of groundwater for the operation of groundwater heat pumps (Busby et al. 2009). In a single aquifer system, both the production and injection wells utilize the same aquifer. This may limit the number of heat pumps in a given area. A greater volume of the aquifer is needed for thermal reconditioning of the injected water as it is transmitted from the injection well to the production well.

A dual aquifer system enhances the feasibility of operating a greater number of heat pumps in a smaller area. This type of system can operate effectively only where two or more aquifers of sufficient capacity and of chemical compatibility exist. In addition, care must be taken that the quality of the water in the supply aquifer is at least as good as that in the aquifer to be recharged.

Groundwater heat pump systems consist primarily of four variations (Fig. 3), depending on how the groundwater is coupled with the heat pumps:

Fig. 3
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a Separate extraction and injection wells system. b Extraction well system with surface discharge. c Standing column well systems. d Single well circulation system

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  • Open loop groundwater systems with separate extraction and injection wells.

  • Open loop groundwater systems with extraction wells only while discharge occurs on surface.

  • Semi-open loop standing column well systems, which utilize a single well for both extraction and injection of groundwater without a hydraulic barrier between the pumping and injection zones.

  • Semi-open loop single circulation well systems, which utilize a single well for both extraction and injection of groundwater with a hydraulic barrier between the pumping and injection zones.

Selection of the coupling method is based on site-specific conditions. Table 1 summarizes advantages and disadvantages of each method (Orio et al. 2005; Ni et al. 2006; Sanner et al. 2003; Rybach and Sanner 2000). The first two systems are consumptive use of groundwater in nature, whereas the last two systems are non-consumptive use of groundwater. The most recent coupling technique is the single circulation well system, which was developed and patented by Ever Source Science & Technology Development Co., Ltd., Beijing, China (China Patent ZL200610002239.8). This system is different from the standing column well system that is widely used in the United States (Ni et al. 2006). The energy well is constructed of two sections: the low pressure water production space and pressurized water injection space. The water and heat exchange between the well and surrounding ground is through special metal meshes for heat balance sustainability.

Table 1 Summary of installation factors for groundwater heat pumps
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Hydrogeological and thermo geological parameters

The amount of heat available for groundwater heat pump systems is estimated by the following equation:

$${\text{Groundwater heat available }} = \, S_{\text{C}} \left( {T_{\text{g}} {-} \, T_{\text{h}} } \right) \, Q$$

where Q = pump rate in l s−1, S C = specific heat capacity of groundwater, 4,200 J l−1 °C, T g = groundwater temperature in °C, T h = water temperature from heat pump in °C.

For a unit pump rate of 1 l s−1 and a temperature difference of 1 °C between groundwater input and output, the groundwater heat available is approximately 4,200 J l−1  °C × 1 °C × 1 l s−1 = 4.2 kW. Similar to water, any earth materials store heat and their capability of storing heat is determined by their specific heat capacities, rock volume, and temperature difference.

At a constant pump rate, the heat energy available depends on the temperature difference between the groundwater and heat pump output. The heat pump output temperature is typically determined by the cooling and heating requirements and well controlled. Therefore, once the heating/cooling system is in operation, keeping the groundwater temperature as close to its original one is the key to keep the efficiency of the system. Temperature changes in the vicinity of the recharge well usually result from injection of the heat pump discharge water. If the discharge water from the heat pump is recharged to the supply aquifer, proper well spacing must be maintained to avoid thermal interference of supply and recharge waters. This “short circuiting” is not a significant detrimental environmental impact, but could lower the operating efficiency of the groundwater heat pump. If the discharge water is recharged to an aquifer other than the supply aquifer and the two aquifers are separated by a thickness of low-permeability material, thermal interference should be minimal or non-existent. Supply and recharge 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 recharge well.

Groundwater heat pump design needs to be adapted to the site-specific hydrogeological conditions and thus requires knowledge of hydrogeological data. Hydrological conditions have to be such that

  • Water can be continually pumped from the well at a rate that meets the heat exchange need from well(s).

  • Water from the heat pump can be injected continually into the aquifer.

  • An efficient heat exchange performance can occur in the aquifer dissipating temperature difference between injection water and natural groundwater.

  • No adverse environmental impacts such as groundwater quality deterioration and surface subsidence can be caused by the operation of the groundwater heat pump system.

Hydraulic properties

Certain general design criteria related to the hydrologic system have a profound influence on the operation of groundwater heat pumps. Depending on the mode of operation and on the manufacturer, a groundwater heat pump has a flow rate requirement of 1.5–3.0 gpm ton−1 (0.03–0.06 l s−1 kw−1) (a ton is defined as 12,000 BTU h−1 or 3.5 kw). Heat can be transported through an aquifer by the following three processes:

  • Through soil and/or bedrock by conduction,

  • through groundwater by conduction,

  • through groundwater by advection.

It is necessary to distinguish between the conductivity and thermal capacity of the water and the rock. This is to account for the fact that heat is stored and conducted through both water and rock, but heat is only advected by the groundwater.

The governing equation describing groundwater flow is Darcy’s law, in which the velocity can be calculated by:

$$v \, = - K I/n$$

where v = groundwater velocity (m s−1), K = hydraulic conductivity (m s−1), I = hydraulic gradient, n = porosity.

Naturally occurring ranges of values of hydraulic properties of soils and rocks are summarized in Table 2. Hydraulic conductivity is well applicable to porous media aquifers. In fractured bedrocks, the interconnected fractures are the primary passages for groundwater flow. Most research has shown that the macroscopic hydraulic flow in a large enough volume of fractured rock can be reasonably well represented by the flow through an equivalent porous medium. The US Environmental Protection Agency (1996) reports a typical range of hydraulic gradient values of 0.0001–0.05. Site-specific values of these parameters are to be determined with traditional hydrogeological investigations such as pumping tests, in situ permeability tests, and geophysical surveys.

Table 2 Typical values of hydraulic properties of soils and rocks (Chiasson et al. 2000)
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Groundwater flow transports heat and affects the heating and cooling performance for the groundwater heat pump systems. Both lateral and vertical hydraulic gradients are important in designing the systems. Downward groundwater flow is detrimental for heating applications as it generally transports cooler water to depth, while upward flow has the opposite effect.

Groundwater chemistry

The presence of certain chemical elements in the water may result in corrosion or incrustation of the water-side heat exchanger and prevent efficient return of water into the aquifer. For these reasons, groundwater sampling should be performed for chemical quality of the water before designing the heat pump systems.

In its natural state, groundwater is in chemical equilibrium with the subsurface environment. Chemical reactions that occur are responding to changes in temperature, pressure, and mineral composition of the aquifer. These changes are generally gradual in space and time, thus allowing groundwater to maintain equilibrium conditions with the surrounding rock body. In theory, the relatively rapid alteration of physical or chemical groundwater qualities by man-made devices has the potential to upset this natural equilibrium. This could change the capacity of the groundwater to dissolve or retain in solution certain chemical elements or compounds.

Water quality, as well as possibly being corrosive to any casing, is of importance in open loop systems, particularly if the water is pumped through the heat exchanger. If ions such as iron and manganese precipitate due to oxidation, they can cause clogging. It is, therefore, important to keep reducing groundwaters in a closed system, preventing them from coming into contact with oxygen in the atmosphere. Calcite may precipitate due to differences in carbon dioxide concentration and/or temperature and cause scaling to occur and hydrogen sulfide to accumulate. Groundwater containing dissolved gases needs to be kept at a high pressure to prevent degassing. If the water quality is particularly poor, a ‘prophylactic’ heat exchanger, often with a storage tank to buffer variations in flow, can be installed (Banks 2008).

Poor quality groundwater can also be an issue, as high total dissolved solids content and particularly high chloride and sulfate ion concentrations, can be corrosive to some casing materials. The oxidation of sulfide minerals by exposure to air, in underground mines or surface spoil heaps produces acidic, dissolved metals and sulfate. Acid mine drainage is a major source of pollution in many mining areas, however, as the sulfide oxidation reaction is exothermic, Banks (2008) have speculated that this geochemical energy could be used by heat pumps.

Corrosion of the water-side heat exchanger is inhibited by the formation of an oxide or hydroxide film. Any groundwater constituent that prevents the formation of this film or removes it will cause degradation of the metal. Hydrogen sulfide (H2S) is the most common corrosive agent with water-side heat exchangers. Although the cuprous-nickel alloy heat exchanger is more resistant than the copper variety to mechanical erosion and corrosion by brackish waters, neither metal shows acceptable resistance to dissolved hydrogen sulfide. Concentrations as little as 0.5 parts per million are known to cause corrosion of the metals. Chemical incrustation has not been a significant problem in existing installations. Analytical techniques are available to predict the tendency of a water sample to corrode or scale. Biological incrustation (fouling), if encountered, is likely to be a problem throughout the entire domestic water supply system. If bacterial infestation and subsequent blockage of the heat exchanger is a chronic problem, water treatment before passage through the heat exchanger would be required. Due to the large volumes of water used, this would substantially increase system operating costs.

Where the collector loop is below the water table in an aquifer with significant groundwater flow, heat transport away from the site will occur. This can be helpful for single borehole cooling or heating systems in dispersing warmth or coolness, respectively, away from the borehole and bringing new cooler or warmer groundwater toward the borehole. Where several boreholes are drilled for a larger cooling or heating system, or separate schemes are in close proximity, thermal interference can potentially cause problems.

Thermal properties

The rate, at which heat can be transferred to the heat exchanger from the ground, or to the ground, is determined mainly by the thermal properties of the earth, i.e. thermal conductivity and thermal diffusivity. Thermal conductivity is the capacity of a material to conduct or transmit heat, while thermal diffusivity describes the rate at which heat is conducted through a medium. It is related to thermal conductivity, heat capacity and density via the equation:

$$\alpha = \lambda / \, (C_{p} \rho )$$

where α = thermal diffusivity (m2 s−1), λ = thermal conductivity (W m−1 K−1), C p = specific heat capacity (J kg−1 K−1), ρ = density (kg m−3).

Naturally occurring ranges of values of thermal properties of soils and rocks are summarized in Table 3. Site-specific thermal response tests are typically used to determine the effective thermal conductivity over both formation and groundwater. Since the introduction of the thermal response test method in 1995, it has become the most commonly used method to measure in situ thermal conductivity for ground source heat pump system design (Gehlin 2002). Figure 4 shows a general setup associated with the thermal response test. Different types of devices are available for the in situ tests in different countries (Lo Russo and Civita 2009; Sanner et al. 2005). In a typical thermal response test, a constant heat injection or extraction is imposed on a test borehole. The resulting temperature response is used to determine the ground thermal conductivity, and to test the performance of the borehole.

Table 3 Typical values of thermal properties of soils and rocks (Chiasson et al. 2000)
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Fig. 4
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General setup of thermal response test and the Norwegian response test rig (Gehlin 2002)

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As discussed by Chiasson et al. (2000), thermal conductivity varies by a factor of more than two for the range of common rocks encountered at the surface. Superficial deposits and soils are complex aggregates of mineral and organic particles and so exhibit a wide range of thermal characteristics. The level of water saturation has a significant impact on the thermal conductivity of superficial deposits. For sedimentary rocks, the primary controls on thermal conductivity are the lithology, porosity, and extent of saturation. Mudstones and clays have thermal conductivities in the range 1.2–2.3 W m−1 K−1. For low porosity (<30 %) shale, sandstone, and siltstone, the mean thermal conductivity is in the range 2.2–2.6 W m−1 K−1. Water has a thermal conductivity of 0.6 W m−1 K−1 and air a thermal conductivity of 0.0252 W m−1 K−1. A saturated quartz sandstone with 5 % porosity might have a thermal conductivity of about 6.5 W m−1 K−1 but this would decrease to about 2.5 W m−1 K−1 if the rock had a porosity of 30 %. Porosity is also the main influence on thermal conductivity of volcanic rocks. Low porosity tuffs, lavas and basalts may have values above 2 W m−1 K−1, but at 10 % porosity with water saturation this might reduce to about 1.5 W m−1 K−1. For intrusive igneous rocks, which generally have a much lower porosity, the thermal conductivity variation is less. Intrusive rocks with low feldspar content (<60 %), including granite, granodiorite, diorite, gabbro and many dykes, have a mean thermal conductivity of about 3.0 W m−1 K−1. For metamorphic rocks, porosity is often very low and thermal conductivity can be related to quartz content. The thermal conductivity of quartzite is high, typically above 5.5 W m−1 K−1. For schist, hornfels, quartz mica schist, serpentinite and marble the mean thermal conductivity is about 2.9 W m−1 K−1.

Conduction and advection

Groundwater flow can have a significant impact on groundwater heat pump system design. The complexity of both the heat pump system design and the heat flow equations means that analytical solutions are not suitable and numerical modeling may be required. An indication of whether groundwater flow might be significant, however, can be gained by comparing the relative impact on the heat transport caused by conduction and that caused by advection (also referred to as forced convection). This comparison can be used to decide whether further numerical modeling is required. The parameter that is used for this comparison is the Peclet number for energy transport (Busby et al. 2009), which is a dimensionless variable obtained from consideration of the advection-conduction equation:

$${\text{Pe }} = \, \rho \, C_{\text{p}} \;q\;L/ \, \lambda$$

where Pe = Peclet number, ρ = density of water (kg m−3), C p = specific heat of water (J kg−1 K−1), q = specific discharge (m s−1), q = hydraulic conductivity × hydraulic gradient, L = length scale (m), λ = thermal conductivity of aquifer (W m−1 K−1).

Large Peclet numbers mean that advective transport is dominant while small numbers imply that conductive transport is dominant. In principle, this advection becomes important when the Peclet number exceeds 1, but this is slightly dependent on the choice of the characteristic length. This length scale is an indication of both the borehole spacing and depth (thickness of saturated aquifer penetrated). In general, the Peclet number exceeds 1 when the groundwater flow velocity is greater than 10 m year−1. The advection transport of heat can be dominant in gravel, coarse sand, and karst limestone aquifers.

Environmental aspects

Armitage et al. (1980) summarize the potential environmental impacts of using groundwater heat pump systems. Use of groundwater for the operation of groundwater source heat pumps is expected to have a minimal impact on the environment. In fact, in some cases positive effects can be expected. The effect on the environment depends on the following factors:

  • whether water is recharged,

  • variations in heating and cooling load,

  • density of use.

The consumptive use of groundwater involves the withdrawal and subsequent remote disposal of the spent heat pump water. Remote disposal can be accomplished in several ways, including discharge into nearby streams, ditches, and reservoirs. Without proper planning, this type of groundwater use may have certain undesirable effects on the environment.

The non-consumptive use of groundwater involves the removal of water from the aquifer and its subsequent reinjection into an aquifer near the site of withdrawal. This method eliminates many of the problems associated with the consumptive use of groundwater but creates several of its own. Being re-injected, the water will have been thermally altered (that is, its temperature will have been raised or lowered several degrees). The effect of this thermal pollution on the aquifer should be considered. Long-term monitoring (Armitage et al. 1980; Xu and Rybach 2005) have indicated that the non-consumptive use of groundwater for heating and cooling demonstrates no serious environmental impacts.

The consumptive use of groundwater in heat pumps involves the withdrawal of water and its subsequent surface disposal by any of several methods. The term “consumptive” is misleading in that it implies permanent removal of the groundwater. Actually, the water is eventually returned to the hydrologic cycle. On a small scale or when density of utilization is low, the consumptive use of groundwater will have minimal effects. It is only when high-density usage occurs that the impacts may be serious.

Once water has passed through the heat pump system various alternatives for surface disposal include discharge into nearby ditches, streams, storm drains, sewers, septic tanks, or cesspools. Disposal into septic tanks and cesspools is considered surface discharge only in areas where the water table is not present at shallow depths. In areas where the water table is close to the surface, discharge into septic tanks and cesspools is a form of recharge.

Problems associated with consumptive use of groundwater

  • The most common problem to be anticipated is the drop in groundwater levels, creating a cone of depression at the pump well. Control of the cone of depression can be challenging for closely spaced high-density usage of heat pumps because of well interference. In an area where there are numerous wells, the water table may be depressed to the point where some of the shallower wells no longer produce sufficient quantities of water.

  • The consumptive use of groundwater along coastal areas may change hydraulic gradients that favor the intrusion of salt water. In other areas, the establishment of new gradients may result in the upward coning of poor quality water from deeper aquifers or zones.

  • In areas where withdrawal exceeds available natural recharge and storage, land subsidence may occur. This subsidence is caused by an increase in pressure which compacts the aquifer matrix. For unconfined aquifers, this increase results from a loss of buoyancy of the aquifer matrix in the dewatered zone. For confined aquifers, the increase in pressure is a result of decrease in hydrostatic pressure against the bottom of the upper confining layer. Bouwer (1977) has calculated that the rate of subsidence will range from 2 to 20 in. (5–50 cm) for every 33 foot (10 m) drop in groundwater level for unconsolidated deposits. Surface cracks or faults may result from non-uniform subsidence induced by differential water declines or differences in compressivity of subsurface materials.

A more localized form of land subsidence occurs in areas underlain by carbonate formations. In carbonate formations that have undergone extensive karstification, subsidence occurs through subsurface erosion. Joints and fractures within these formations may be widened by solution-formed cavities. When water levels decline, there is a reduction in hydrostatic pressure. The roof of a cavity may slowly lose its underlying support to the point where it collapses under its own weight (Zhou and Beck 2011).

The consumptive use of groundwater in environmentally sensitive regions may aggravate and even directly contribute to the imbalance of the hydrologic system. Additional stress imposed by groundwater heat pumps within such regions may create a situation of groundwater mining, i.e., groundwater withdrawals exceed storage or recharge or both. Priorities and goals must be established so that this limited resource can be efficiently utilized. The removal of groundwater and the method of disposal must be evaluated on a site-specific basis as discussed in this paper. Monitoring and careful management of groundwater resources can avoid many of the problems associated with the intense development and consumptive use of groundwater.

Problems with non-consumptive use of groundwater

  • The non-consumptive use of groundwater for the operation of heat pumps entails the discharge of water into an injection well (or other subsurface disposal device) after it has passed through the system. The hydrologic system would maintain equilibrium between inflow and outflow where the heat pump water is re-injected. The discharge water, however, will be thermally altered, that is, the temperature of the water will be raised or lowered, depending on the mode of operation of the heat pump. The areal extent of thermal alteration within an aquifer depends on the rate of groundwater movement, the amount of thermal energy added to or subtracted from the system, as well as the thermal properties of the aquifer. Variations in heat transfer can result in changes in porosity, transmissivity, or lithology. Other perturbations of the thermal regime can result from the recharge of phreatic waters in an unconfined aquifer (Werner and Kley 1977). By proper spacing of wells, thermal interference can be avoided. The effect of this thermal alteration on the aquifer’s hydrologic properties is the primary environmental concern associated with non-consumptive use.

  • Several studies have examined cyclic heat extraction or heat transfer through aquifer systems. Andrews (1978) simulated the impact of heat pump use for residential heating and cooling of groundwater temperature by means of a mathematical model. A hypothetical case using data for southern Wisconsin was considered. Ten years of operation of a dual well system in a subsurface aquifer produced a change in water temperature of less than 1.8 °F (1 °C) at a distance greater than 132 ft (40 m) from the wells. This evidence indicates that reinjection of spent heat pump water would not adversely affect groundwater temperatures if use was restricted to areas of low population density.

  • High-density usage of groundwater heat pumps could have a more pronounced effect on the environment. In one study done in northeastern Illinois, the wells for cooling installations penetrated sandstone aquifers. The aquifers showed no evidence of undesirable effects related to the injection of higher temperature water. Four installations each recharged 25,000–345,000 gpd (1.1–15.1 l s−1) at an initial temperature of 85°–110 °F (29.4°–43.3 °C) (Sasman 1972). Another study concerned the use and subsequent recharge of groundwater for air conditioning of commercial buildings on Long Island (Armitage et al. 1980). In several cases, thermal well interference occurred between the production and injection wells. The increased groundwater temperature resulted in lowered efficiency of the cooling system, which therefore increased operating costs; however, no adverse effects on the aquifer were noted.

  • Thermal alteration of groundwater may also have an effect in the chemical equilibrium of the aquifer system. The chemical composition of groundwater is subject to a variety of complex interactions that are dependent on its geologic, hydrologic, and biologic characteristics, as well as the presence of man-made contaminants. Of concern is the extent to which temperature changes affect water–rock or water–water chemical reactions. The effects of thermal alteration on the natural chemical equilibrium of groundwater can best be understood when the chemical factors involved are considered. In groundwater systems, the types of chemical reactions that are reversible and rapid enough to be of concern here include adsorption or desorption of cations or anions held at the surface of the aquifer matrix, solution or precipitation of mineral salts, and oxidation, reduction, or hydrolysis of certain metals. The most serious potential impact would be the precipitation of mineral salts that could clog the pore spaces of the aquifer, inhibiting water injection processes. The solubility of common salts, mainly magnesium and calcium, is highly dependent on pH-Eh conditions, and only to a lesser extent on temperature. Therefore, the impact of changing the solubility of salts in aquifers at the anticipated temperature ranges, resulting from the discharge of heat pump water, could only be measured in terms of geologic time. It is unlikely that the operation of the heat pump units would affect the pH-Eh of the aquifer system, or alter the existing chemical equilibrium of the system.

  • The non-consumptive use of groundwater for heat pump operation is expected to have some positive effects on the environment. Groundwater source heat pumps conserve energy and reduce pollution produced by more conventional systems. The favorable environmental impacts more than offset the slight and often insignificant thermal-fluid alterations that occur through the operation of groundwater source heat pumps.

Regulatory restrictions

Federal, state, and local regulations impose restrictions and mandate various requirements for well construction, groundwater use and quality, and effluent disposal, but they will not significantly obstruct the implementation of groundwater heat pump technology (USEPA 1999; Armitage et al. 1980). The Safe Drinking Water Act was enacted to insure that public water systems are adequately supervised by the states. This act requires the EPA to adopt regulations for state underground injection programs. Heat pumps which use a reinjection system or discharge water in a manner which could affect drinking water supplies may be subject to regulations as a Class V well discharge system. States are required to participate in inventory and impact assessment of all Class V wells. This may include heat pump reinjection wells, depending upon the state’s interpretation of the Act.

At the state level, well construction requirements may be numerous or non-existent. Concern here is more a matter of cost than of limitation; the expense involved in meeting such requirements can rule out the feasibility of heat pump utilization. Where water-use restraints exist, usually in the form of permit requirements, they are not serious deterrents to heat pump use. The disposal of effluent to a recharge well is uncontrolled in some states and prohibited in others. Where this disposal method is forbidden, heat pump viability may suffer. Alternative disposal methods are available, however, and are generally subject to less regulation than are recharge wells. Since other disposal methods, such as discharge to land, surface water, septic tank, or sewer, are usually possible, state limitations on recharge wells should not hinder heat pump utilization. Some local controls on well construction, groundwater use and quality, and waste disposal may adversely affect heat pump utilization. Most local regulations, however, will not seriously impede widespread development of this alternative energy source. Most regulations and restrictions enacted at the Federal, state, and local levels will not present serious obstacles. Knowledge of their existence and their legal implications is vital to the implementation of groundwater heat pump technology.

Conclusions

Hydrogeological and thermo geologic factors influence the performance and hence design of a groundwater heat pump system. For design of any individual system, site-specific information is required. For high capacity groundwater heat pump systems site investigation will be necessary. Pump tests can provide the aquifer characteristics, whereas thermal response tests provide the estimate of the thermal properties at a site. Such investigations are expensive and are unlikely to be undertaken for individual domestic properties. For these cases the data sets described in this paper can be of use in helping design the system. Potential environmental impacts and legal constrains shall be considered as well in designing the proper coupling systems. In principle, groundwater heat pumps are an economically feasible and environmentally sound alternative to conventional heating/cooling systems. Monitored installations where meaningful data can be accumulated are needed to substantiate this at each installation site. Establishment of monitoring networks and careful management of groundwater resources can help sustainable use of groundwater and prevent any adverse environmental impacts.