Highlights and lessons learned from groundwater investigations in the framework of Nagra’s exploratory boreholes for a deep geological repository

The Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) carried out an underground investigation program for a comparative evaluation of possible sites for a deep geological repository (DGR) for nuclear waste. Between 2019 and 2022, one inclined and eight vertical multi-purpose exploratory boreholes were drilled in three siting regions in northern Switzerland.

In the frame of these works, accurate groundwater sampling and analytical data interpretation has been fundamental to derive solid information on groundwater composition, evolution, and residence times in aquifers limiting the low permeability sequence of the host rock (Opalinus Clay) and the confining units.

Eighteen deep groundwater samples were successfully collected from the Malm, Keuper and Muschelkalk aquifers at depths between approx. 350 m and 1150 m below ground level. The following selected highlights and lessons learned regarding hydrochemical groundwater characterisation will be presented:

• Procedures to correct analytical results for the contamination of groundwater with different types of drilling fluids, using artificial and environmental tracer data and geochemical modelling approaches;
• Krypton-81 (T_1/2 229 ka) in the groundwaters was successfully analysed and, combined with hydrochemical and isotopic data, helped to constrain groundwater mixtures and model residence times far beyond the classical carbon-14 method;
• Tools to cope with technical challenges at a drill site resp. in deep exploratory boreholes (i.e. time constraints, low groundwater flow rates).

We demonstrate that a high level of quality for hydrochemical and isotope data of groundwaters can be reached even under challenging operational conditions.

The hydrochemical data of groundwater and the profiles of natural tracers in pore water are relevant datasets to constrain the site model used in the site selection process and in the long-term performance assessment for a DGR.