Groundwater circulation in deep aquifers in Northern Switzerland: Lessons learned from Nagra’s deep drilling campaign

For the assessment of the long-term safety and geological stability of sites foreseen for deep geological repositories for radioactive waste, it is essential to understand the hydrogeochemical evolution of regional deep aquifers. They represent the boundary conditions for the geological barrier and represent potential exfiltration pathways for radionuclides that may be released from the repository in the far future.

Here we present highlights of hydrogeochemical data and their interpretation collected during Nagra’s recent deep drilling campaign performed in the context of the site selection process for the Swiss deep geological repository. The extended hydrogeochemical investigation included groundwater and porewater hydrochemistry, rock properties, hydrotests, porewater tracer simulations, and groundwater modelling. For groundwaters, essentially all currently available analytical techniques for major ions, trace elements, stable and radiogenic isotopes, as well as common and noble gases were applied. Combining information from all these analyses allows to demonstrate that the aquifers above and below the Opalinus Clay host rock experienced a highly distinct, aquifer-specific evolution and do not hydraulically communicate across the geological barrier. For instance, groundwater in the Malm limestone aquifer located above the host rock contains a 16-20 Ma old marine Na-Cl signature and displays high salinities and ⁸¹Kr model ages of up to 15 g/L and 600 ka, respectively. Hence, it behaves almost like a stagnant water body with very low flow rates, which is due to the low permeability and the lack of major open karst features and highly transmissive faults.

In contrast, groundwater in the Muschelkalk dolostone aquifer located beneath the host rock represents a more dynamic flow systems with generally lower salinities and residence times. Nevertheless, two distinct components of different age and hydrochemical signatures can be distinguished. The first one represents a hydrochemically evolved groundwater with elevated concentrations of Na, Cl, and Li, indicative for rock salt dissolution and/or minor interaction with the underlying crystalline basement. The second component is of the Ca-SO₄ type and shows a clear glacial water stable isotope signature. Based on the spatial distribution of hydrochemical parameters, we infer that recharge of this glacial component occurred during a short period of time after the last glacial maximum. We link this glacial component to the deflection of the Wutach river towards the recharge area of the aquifer about 18 ka ago. The deflection forced the Wutach to flow directly across the outcrops of the Muschelkalk aquifer for a few thousand years, thus strongly increasing the recharge rate of glacial meltwater into the aquifer.

The two examples emphasize that extended hydrochemical investigations during site characterization may allow to unravel regional scale hydrochemical evolutions of deep aquifers on the time-scale of up to several million years. For the example of the Swiss program, the extended hydrochemical investigation significantly contributed to demonstrate the long-term geological stability of the site.