As part of the INTERREG VB project GRETA (near-surface geothermal resources in the territory of the Alpine Space), geothermal resources near the Alpine landscape and their sustainable use as a contribution to the reduction of CO2 emissions are to be investigated. Together with the project partners, the Office of Nature and Environment canton Grisons (ANU), the Federal Office for Spatial Development (ARE), the municipality of Davos and the Geotest AG Davos, the Applied and Environmental Geology (AUG) is working on the pilot study Switzerland in Davos (canton of Grisons) and the surrounding area (Fig. 1).
There are some limits with respect of the number of shallow geothermal systems which could be installed in touristic centres in alpine areas, which take the heat from the shallow groundwater because of groundwater protection issues and critical temperature decline in the vicinity of geothermal heat pumps (GHPs). However, in the Alps the distribution of regional aquifer and aquitard systems is not restricted to the shallow subsurface. Complex 3D tectonic settings of the different nappe structures, the individual nappes often separated by Triassic evaporitic and dolomitic units, which represent regional aquifers, are often characterized by recharge areas which are hydraulically connected to deep aquifers below the valley bottoms (Fig 2).
The innovative aspect of the GRETA pilot study Davos is the development of methods for the estimation of the geothermal utilization potential of deeper alpine aquifers and the development of model-based scenarios of the exploitation of the deep aquifers and their computer-aided use optimization. A major challenge is the integrating of complex geological geometries from a 3D geological model (GOCAD) at different scales into a numerical groundwater flow model (COMSOL Multiphysics). Starting point is a 3D geological model representing a regional tectonic system of the eastern alpine and pennine system of the Davos area. The developed 3D geological model is transferred into regional and local scale groundwater flow models.
We started from a purely conceptual approach using the 3D hydrogeological model of the Davos area and defining hydraulic boundary conditions. The driving forces of regional groundwater flow systems are largely controlled by the configuration of the water table, which under most conditions is a subdued replica of the topography (Toth 2009) and which determines the hydraulic head distribution. As a consequence regional flow in mountain areas can in a first approximation be regarded as driven by variations in hydraulic head. This concept lead to the idea to explore aquifers at greater depth, in the artesian confined Arosa dolomite at about 100 m to 400 m depth.
The availability of data on the hydraulic properties and the hydraulic boundary conditions of the subsurface is generally very limited in alpine areas. Developing regional groundwater flow models, based on the 3D geological conditions which determine the large-scale groundwater circulation systems, are the greatest challenge for using the energy of deep aquifer systems in the Alps.
The 3D geological-hydrogeological model allows evaluating pumping tests in the 400 m deep exploration well and to understand the dynamic character of capture zones of pumping wells as well as to test how different boundary conditions and hydraulic property distribution influence calculated flow regimes. In addition, the model enables us to test the effects of hydraulic regimes changes at different scales. In the year 2016, the focus was laid on the development of the 3D geological model and in 2017 on the hydraulic model for understanding the dynamics of deeper confined and artesian aquifers including the interaction with near surface groundwater resources in the unconsolidated rocks of valley fills. Results of the regional flow can be seen in Fig. 4 and 5.
The energy city of Davos has already drilled an exploratory well (EKB) in 2012 down to 400 m, whereby artesian tense groundwater was found, which had a free discharge of about 1200 l/min. By installing a pump, the yield could be increased to 1760 l/min, with a lowering of the groundwater level by 31 m. Without groundwater extraction, the groundwater level recovers quickly, which suggests abundant inflows. A measurement program (GNAMA) around the exploration well at the Davos Kurpark served to study the sustainable operation of production and to monitor potential negative impacts on existing geothermal applications. By connecting the heat extraction system of the exploratory well with existing heat supply systems of the congress center and the ice hockey center (Fig. 3) it was possible to increase heat generation from 1200 MWh to 1600 MWh per year. This increased the heat supply to the congress center and the indoor pool from 12 hours to more than 20 hours per day. The extensive previous and current investigations in Davos due to the use of GHPs are listed in Table 1.
The success of geothermal energy in Davos largely depends on whether or not the water resources permit sustainable use. This question should be answered by the model scenarios based on the groundwater model. A comprehensive measuring program (GNAMA) during a large scale pumping test in the exploration borehole in the Kurpark of Davos (400 m deep) served to examine a sustainable operation of the production well and to monitor potential negative impacts on existing geothermal applications. The pumping tests allowed to derive the hydraulic properties of the subsurface and to calibrate the hydraulic models. In a second phase the productivity of the well, possible interference with other or planed use of the aquifer system and the 3D evolution of the capture zone of wells could be derives. Further hydrogeological data concentrate on the Landwasser valley of the Davos area which are the basis for the setup of the models and the hydraulic calibration.
According to the groundwater protection legislation, the sulphate rich water from the Arosa dolomite should be reinjected into the same aquifer. Since this would be very (energy-) consuming, it would almost represent a killer argument for using the Arosa dolomite as an energy resource. Due to the fact, that the water from the Triassic dolomite already infiltrates into the Quaternary aquifer and that the introduction of the mineralized water into the surface waters only lead to negligible change of the chemical composition of the surface water, a re-infiltration of the water from the Triassic aquifer into the surface waters can be envisaged under certain constraints with respect to the infiltration.
Prof. Dr. Peter Huggenberger
Dipl. Hydrol. Stefan Scheidler
Dr. Birte Anders
Applied and Environmental Geology, University of Basel
Fig. 1: Overview of regional and local study area.
Fig. 2: Geological model from the local study area with cross section through the geothermal drilling. Target horizon is the Arosa Dolomite with a change of hydraulic conductivity (the dashed line is based on information’s from the drilling).
Fig. 3: Conference and sports facilities are the main user of Energy in Davos.
Tab. 1: Previous work and current activities in the pilot project area of Davos.
Fig. 4: Result of regional scale groundwater modelling: the arrows represent the flow direction. The density of the arrows is based on a Gaussian distribution and is not to be equated with a volume or a velocity.
Fig. 5: Result of regional scale groundwater modelling: the arrows represent the flow direction. The density of the arrows is based on a Gaussian distribution and is not to be equated with a volume or a velocity. This figure shows the view from the north, where the influence of the topographical and tectonic structures on the groundwater-flowfield is clearly visible.