Hydrogeological investigations in deep exploratory boreholes for site selection of a deep geological repository for radioactive waste in Switzerland

In Switzerland the site selection process for a deep geological repository for radioactive waste is defined by the Sectoral Plan for Deep Geological Repositories and is led by the Federal Government (Swiss Federal Office of Energy). The aim of the process is to identify sites for deep geological repositories as part of a broadly-based stepwise procedure. Stage 2 of the process was approved by the Federal Council on 21st November 2018, and we are currently in the third and final stage.

At present, Nagra is investigating three siting regions in detail – Jura Ost, Nördlich Lägern and Zürich Nordost. The investigation programme includes seismic campaigns as well as shallow boreholes (< 350 m) focussing on Quaternary sediments and deep exploratory boreholes (> 1000 m). Based on the results of the geoscientific investigations, Nagra will submit general licence applications for the selected site (if a combined repository is proposed) or sites for a low- and high-level waste repository.

The deep exploratory borehole campaign is comprising nine boreholes and is establishing a comprehensive dataset covering various geoscientific disciplines. In this presentation we are focusing on the investigation strategy, performance and results of the in-situ hydraulic borehole investigations carried out from 2019 to 2021. These investigations consisted of active hydraulic packer testing, fluid-logging, and passive long-term monitoring of the hydraulic heads by means of multi-packer systems. They have the following objectives:

• Determination of hydraulic conductivity, transmissivity, hydraulic head as well as flow model identification.
• Localisation of groundwater flow zones.
• Groundwater sampling from the aquifers. 

The hydraulic in-situ data are needed for:

• Characterisation of hydraulic barrier properties of the containment-providing rock zone, which consists of the host rock Opalinus Clay and the confining geological units above and below.
• Characterisation of the aquifers to identify potential release paths and facilitate the planning of the future access structures. 

Nagra’s hydrogeological investigation program is characterised by state-of-the-art testing and analysis techniques performed by different contractors and a thorough quality control process from data acquisition to the final reports. Our presentation will give an overview of the different in-situ investigations, how they complement each other, as well as challenges encountered. In addition, we will present a comprehensive dataset of the hydrogeological properties of the aquifers, the Opalinus Clay and its bounding formations in Northern Switzerland from the completed campaign.