In a recently launched project, the aquatic research institute Eawag, together with Empa on the joint campus in Dübendorf, Switzerland, is investigating how the use of borehole heat exchangers affects the surrounding soil, the groundwater and the microorganisms living in it.
Traditional geothermal probe heat pumps extract heat from the ground in winter to heat buildings. However, the borehole heat accumulators installed on the new research campus of the Swiss Federal Laboratories for Materials Science and Technology (Empa) and the Swiss Federal Institute of Aquatic Science and Technology (Eawag) are geothermal probes that can not only draw heat to the surface in winter, but also return the heat from the summer months to the ground so that it is available in the colder months of the year. Temperatures of up to 65 °C are fed into the storage tank. As a result, temperatures of up to 50 °C can be reached locally in the ground.
So far, however, little is known about how the subsoil and ecosystems at these depths react to such warming. The regular heating and cooling of the probes at soil depths of up to 100 meters can affect the chemical components in the groundwater as well as the microbial communities in the soil and water. Exactly how and to what extent is now part of the new ARTS (Aquifer Reaction to Thermal Storage) research project.
The high-temperature ground probes under the campus reach down to a depth of 100 meters. Three groundwater pumps bring the groundwater to the surface at three different locations - Graphic: Eawag
A unique setting
144 geothermal probes were "sunk" on the new research campus in Dübendorf. They go down to a depth of up to 100 meters and converge in a basement next to the new multi-storey parking lot. The geothermal probe field is being used by Empa's ehub ("Energy Hub") team to investigate the experimental design of such storage systems and their interaction with other heat sources. Initial results show that it can make a valuable contribution to the decarbonization of a local energy system.
Three new holes were drilled into the ground in January: the ARTS groundwater observation points. Over the next three years, water samples will be brought to the surface from underground to provide information on how the microbiology of the environment reacts to the probes and to what extent the chemical composition of the groundwater is affected.
The researchers use five pumps to extract groundwater samples from the three boreholes before, during and after it comes into contact with the geothermal probes. In the first few years of the project, only two of the three monitoring stations will be relevant, as comparisons can be made just a few months after the probes go into operation. However, it may take several years before the groundwater from the immediate vicinity of the probes reaches the third station further away - that's how slowly the water flows through the subsurface.
The box on the surface contains the sensors together with the mass spectrometer - Image: Eawag, Joaquin Jimenez-Martinez
Mass spectrometer in miniature
The aim of the project is to gain insights into the reactions that this type of heat storage triggers in the groundwater. This includes not only hydrogeochemistry and microbiology, but also the analysis of gases such as oxygen, methane or carbon dioxide produced by the effect of heat in the soil. Such gases are mainly consumed and produced by bacteria underground - depending on the effects of heat and cold. For this purpose, the water flows into a GE-MIMS mass spectrometer (also known as "Mini-RUEDI") developed at Eawag. "For the next three years, devices will measure the dissolved gases in the groundwater every hour, while 2.4 liters of water pass through the mass spectrometer every minute," explains Joaquin Jimenez-Martinez, head of the project and researcher in Eawag's "Water and Drinking Water" department.
The water samples are also regularly analyzed in the laboratory by researchers from Eawag's "Environmental Microbiology" and "Aquatic Ecology" departments. They focus on the question of how microbial diversity changes under the influence of temperatures of this magnitude. DNA traces (eDNA) can also be used to determine which organisms populate the groundwater and whether their numbers and distribution change as a result of the geothermal probes.
Our soil consists of several layers. It is porous and loose near the surface, but compact like concrete at depth. Image: Soil from the boreholes for the necessary groundwater pumps - Image: Eawag, Joaquin Jimenez-Martinez
Great interest from the federal government and cantons
Switzerland already has the highest density of geothermal probes in Europe, which is why the project is attracting a great deal of interest from the federal government and cantons. The demand for new options for energy generation and seasonal storage has also increased as part of the energy transition. The effects of temperature input on the groundwater as an overall system are also of interest. ARTS is therefore supported by the Swiss Federal Office of Energy (SFOE) and the cantons of Zurich, Aargau, Thurgau, Zug and Geneva and is run in cooperation with Empa and Eawag. Employees from the environmental offices of Zurich and Thurgau also contribute to the hydrogeological understanding. Collaboration on this scale is not commonplace and the speed with which the project was developed is unprecedented. "It took just ten months from the initial idea to drilling the holes on the campus for the sensors," says Jimenez-Martinez. This shows how urgent the issue is.
