Nagra investigated the geological subsurface of Northern Switzerland using numerous methods, of which deep boreholes were the most important. Over three years, nine deep boreholes were drilled in the Jura Ost, Nördlich Lägern and Zürich Nordost siting regions. These three siting regions were examined in great detail within the framework of Stage 3 of the Sectoral Plan for Deep Geological Repositories.
The Sectoral Plan for Deep Geological Repositories regulates the site selection process for a repository in Switzerland.
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In-depth analysis
A drilling rig was used to drill several hundred metres into the ground. The complex deep borehole campaign allowed Nagra to retrieve many rock samples in the form of drill cores and examine them at the surface. Parts of the drill cores were sent to laboratories all over the world for further analyses.
Measurements were also conducted directly in the borehole, allowing conclusions to be drawn on conditions deep below ground. A long-term monitoring system was also installed in one borehole per siting region.
Completing the picture of the underground
Drilling was carried out to retrieve these rocks
The Opalinus Clay that will host the repository, as well as the rock formations lying directly above and below it, were of special interest. Investigations included studies of the thickness, tightness and exact composition of this geological safety barrier that will eventually reliably enclose the waste.
What is the purpose of the borehole investigations?
The insight gained from the deep borehole investigations was used to complement Nagra’s knowledge of the underground geological environment and to complete the overall geological image. This is crucial for the long-term, post-closure safety of a repository.
In-depth knowledge of the subsurface is also decisive for the comparison of the siting regions as it allowed Nagra to propose the most suitable site for a deep geological repository.
The subsurface in Northern Switzerland has been thoroughly investigated
Within the framework of the site selection process for a deep geological repository, Nagra drilled eight deep boreholes between 1982 and 1999. These reached a depth of around 1,000 to 2,500 metres.
Nine additional deep boreholes were drilled between 2019 and 2022, namely in the communities of Bözberg, Stadel (2 deep boreholes each), Bachs, Bülach, Marthalen, Trüllikon and Rheinau. These allowed Nagra to gain further knowledge of the rock formations in Northern Switzerland.
The 9 deep boreholes drilled from 2019 to 2022
Bülach borehole
Bülach was the first deep borehole to be drilled in the Nördlich Lägern siting region as part of Stage 3 of the site selection process for a deep geological repository.
Stadel-3 borehole
Stadel-3 was the second deep borehole to be drilled in the Nördlich Lägern siting region as part of Stage 3 of the site selection process.
Stadel-2 borehole
The Stadel-2 borehole was the third deep borehole to be drilled in the Nördlich Lägern siting region as part of Stage 3 of the site selection process.
Bachs-1 borehole
The Bachs borehole was the fourth deep borehole to be drilled in the Nördlich Lägern siting region as part of Stage 3 of the site selection process.
Trüllikon-1 borehole
The Trüllikon borehole was the first deep borehole to be drilled in the Zürich Nordost siting region as part of Stage 3 of the site selection process.
Marthalen borehole
The Marthalen borehole was the second deep borehole to be drilled in the Zürich Nordost siting region as part of Stage 3 of the site selection process.
Rheinau-1 borehole
The Rheinau borehole was the third deep borehole to be drilled in the Zürich Nordost siting region as part of Stage 3 of the site selection process.
Bözberg-1 borehole
Bözberg-1 was the first deep borehole to be drilled in the Jura Ost siting region as part of Stage 3 of the site selection process.
Bözberg-2 borehole
Bözberg-2 was the second deep borehole to be drilled in the Jura Ost siting region as part of Stage 3 of the site selection process.
Long-term monitoring in boreholes
A hydrotesting programme was carried out during the deep borehole campaign, investigating, among other things, the hydraulic properties of the aquifers, the confining geological units and the Opalinus Clay.
Additional measurements will have to be conducted over longer periods of time to determine the undisturbed conditions. In the case of the Stadel-3 deep borehole, it is clear that these measurements will continue for several decades. A similar system has been in place in the Benken borehole since 1999.
With the new systems, it is also possible to measure the temperature profiles using fibre-optic technology. The long-term monitoring systems are thus already contributing to the monitoring of the future deep geological repository.
What is being measured?
The main purpose of the measurements is to monitor the groundwater in the water-bearing strata in an undisturbed setting, i.e. some time after the borehole was drilled. The aim is to supplement the data obtained from the boreholes.
The data from long-term monitoring are not used in the site selection process but will merely document the initial state. This is why Nagra is installing monitoring systems such as these in all three potential siting regions. Later, the data can be used for groundwater monitoring during the construction and operation of the deep geological repository.
Sensors are being installed at different depths of the already existing borehole to measure, for example, the temperature and hydraulic pressure in the individual rock formations. To this end, individual measurement sections are isolated using packers (see photo on right).
To operate and maintain the measurement systems, the so-called rig cellar – a basement measuring around 3 by 5 metres – remains in place.
Where are the drill cores stored today?
They are being held at the drill core archive in Mellingen (Canton Aargau). Although primarily a science centre, the archive is not only reserved for scientists. Anyone interested in taking a look at the subsurface of Northern Switzerland is welcome to do so: Nagra offers free guided tours for groups of at least ten people: A small exhibition provides an overview of the geology of Northern Switzerland and the most interesting geological treasures in the archive.