The temperature of groundwater is generally equal to the mean air temperature above the land surface. It usually stays within a narrow range year-round.
Stanley E. Norris and A.M. Spieker (1966, Groundwater resources of the Dayton area, Ohio, USGS Water-Supply Paper 1808) indicate that temperature surveys are valuable tools for identifying interbedded clay layers. The clay layers in
the Dayton, Ohio, area are the result of glacial deposition and are termed till. They consider three proposed flow systems in a pumped sand and gravel aquifer traversed by a stream that discharges water to the aquifer when the latter is pumped.
In the first system, the aquifer contains no till and a plot of water temperature related to depth yields a straight line.
In the second system, the aquifer contains a small till lens. Some water enters the pumped wells after leading downward through the till; another part of the discharge from the pumped well reached the unit by directly entering the screen through the sand
and gravel above the till. A third part of the discharge bypassed the till and entered the discharging well beneath the till. "Under these conditions the temperature gradient in the observation well will be approximately linear both above and below
the till. However … that part of the line representing temperatures below the till will be displaced, relative to the line above the till, in the direction of lower temperature. The position of the till layer in this idealized flow system is
indicated by a pronounced 'blip' on the temperature-gradient line, caused by the colder water in the till."
The third flow system is influenced by a laterally extensive till layer through which all recharge to the deeper aquifer tapped by the well must pass. The temperature-gradient curve indicates that the temperature in the observation well will always decline
with depth, but the rate of decline will increase significantly in the till because it contains significantly colder water than the overlying sand and gravel.
Another variation of the use of temperature surveys is the detection of lateral changes in permeability by monitoring grids of thermistors (temperature probes). K. Cartwright (1968, Temperature prospecting for shallow glacial and alluvial
aquifers in Illinois, Illinois Geological Survey Circular 433; 1974, Tracing shallow groundwater, systems by soil temperatures, Water Resources Research 10, no. 4) showed that changes in soil temperature can be used to detect shallow groundwater
flow systems and flow velocities. Another researcher, P.J. O'Brien (1970, Aquifer transmissivity distribution as reflected by overlying soil tempearture pattern, Ph.D. dissertation, Department of Geosciences, Pennsylvania State University),
used shallow subsurface temperatures to detect areas of increased flow to wells from riverbed infiltration, and to pinpoint zones of relatively better permeability in glacial outwash.
During 1974 to 1975, E.S. Bair and R.R. Parizek (1979, Detection of permeability variations by a shallow geothermal technique, Ground Water 16, no. 4) monitored a network of shallow temperature probes in a well field in Pennsylvania
to detect temperature variations. These variations in the smoother regional isotherm maps were found to represent locations where differing land-use practices or groundwater movement disturbed the normal thermal system. The process was experimental,
and the interested reader should consult the reference [Web editor's note: Ground Water article archives are available online to members]. Similar shallow thermal survey systems have been copyrighted under Thermonics and Geothermometry trademarks.
Temperature surveys of groundwater have taken on new significance as the several forms of geothermal energy become more important. Principally, shallow groundwater has great potential as a heat source or sink for domestic heating and cooling. Groundwater, heat pumps have already begun to assume a significant share of the domestic environment control load in many
areas and will therefore have an increased impact on groundwater systems.
Photo courtesy DOE/NREL, Craig Miller Productions.
Groundwater temperature data is necessary for the proper deployment of heat pumps. One, it is helpful to know the temperature of the groundwater to be used in order to select the proper equipment and configuration for the installation. Two, the potential
impact on the aquifer needs to be assessed to avoid overexploitation and excessive change in aquifer temperature. This is especially important in regions where either heating or cooling will predominate. Some localities in Florida have already driven
up groundwater temperatures significantly, reducing the efficiency of the heat pumps. Aquifer dewatering has occurred because discharge has been dumped in sewers and not recharged.
Further study is needed on the effect of solar energy on groundwater temperatures. Regions with balanced heating and cooling loads should not experience long-term alterations of groundwater temperatures, but colder climates expecting to use heat pumps
primarily to heat may cause long-term cooling if use of the technology is intense, just as primary cooling may cause long-term heating. Insolation, solar heat accumulated by the groundwater, will eventually return the groundwater temperature to normal,
but some maximum density of use may have to be determined to prevent overexploitation in many instances.
A computer simulation study conducted by Charles B. Andrews (1978, The impact of the use of heat pumps on groundwater temperatures, Ground Water 16, no. 6) illustrates both the potential problem and the type of study that has to be conducted
to make the best use of this energy-efficient technology.
The impact of the use of a groundwater heat pump for residential heating and cooling on groundwater temperatures was simulated by means of a mathematical model that couples the equations for groundwater flow with those for heat transport. It was found
in using data typical for southern Wisconsin that the injection of cooled waters back into the aquifer for 10 years only altered groundwater temperatures by more than 1 °C in an area with a radius of less than 40 meters from the wells.
A small regional groundwater flow rate significantly reduced the simulated impact of the use of a groundwater heat pump in southern Wisconsin on aquifer temperatures by dissipating the cooled waters of a larger area than was the case with no regional
flow. The thermally impacted area in both cases though was of large enough extent after 10 years to tentatively conclude that it is not feasible to use heat pump systems of this type in densely populated areas.
The long-term impact of groundwater heat pump use in areas where annual heating loads exceed cooling loads may be to lower groundwater temperatures. The environmental impact of lowering groundwater temperatures a few degrees Centigrade is not likely to
be large, especially if usage density can be planned to take solar heat recharge (insolation) and aquifer performance into consideration.
Recharging into ponds, above the saturation zone, or into other aquifers, rather than into the same aquifer, will change any calculations. In some instances, such practices may improve an area’s ability to use groundwater heat pumps intensively. With intense, unplanned use, an economical-impact point can be reached rapidly, and calculations show that even small cities in some parts of the country cannot rely on groundwater as a source of energy. A groundwater heat pump system as it is generally
visualized now is best suited for use in rural areas with low-density housing. Sophisticated district-heating and coordinated-use plans developed in Scandinavia and North America will increase their utility in urban areas.
The above information is excerpted in large part from Chapter 26 of the 1999 NGWA Press publication, Ground Water Hydrology for Water Well Contractors.