12 Feb 2026
| 12 Feb 2026
Pore water or groundwater chemistry: what governs uranium migration in Opalinus Clay?
Tim Schöne
CORRESPONDING AUTHOR
GFZ Helmholtz Centre for Geosciences, Reactive Fluids and Geomaterials, Telegrafenberg, 14473 Potsdam, Germany
University of Potsdam, Institute of Geosciences, Karl-Liebknecht-Str. 24–25, 14476 Potsdam-Golm, Germany
Theresa Hennig
GFZ Helmholtz Centre for Geosciences, Reactive Fluids and Geomaterials, Telegrafenberg, 14473 Potsdam, Germany
Lawrence Berkeley National Laboratory, Energy Geosciences Division, Cyclotron Road 1, Berkeley, CA 94720, United States of America
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Waiwera in New Zealand is a small coastal village with a geothermal reservoir underneath. Hydrogeological models support water management by providing insights into sustainable extraction from the system. It is artesian and the geothermal water is of meteoric origin percolating down to sufficient depth getting heated. It rises through a fault zone into the shallow and leaky aquifer. Most recently the springs have restarted discharging geothermal water on the beach.
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The effect of spatial variability on uranium diffusion in Opalinus Clay over a million years was assessed by 2D reactive transport simulations. Different rock types and porosity impacted results, with variability's influence growing with longer correlation lengths of geostatistical simulations (up to 12.9 %). This highlights that 1D models may underestimate uncertainty in long-term radionuclide transport at repository sites, and spatial variability must be considered.
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Uranium migration for a close to real case situation is quantified with reactive transport simulations using input data from the deep geothermal borehole Schlattingen, which is near the targeted area in Switzerland, and including the effect of the multi-barrier system on the source term. The hydrogeological system must always be considered in safety assessments since adjacent aquifers have a major impact on the pore water geochemistry, and hence sorption processes.
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Migration of uranium in the potential host rock Opalinus Clay is used as an example to demonstrate the extent to which simulated migration lengths can vary for a million years, depending on the model concept and on the underlying data and parameters. To reduce the uncertainty in this context, the calcite carbonate ion and the hydrogeological system at a potential disposal site need to be known, whereas the quantity of clay minerals plays a subordinate role, as long as it is enough.
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Safety assessments must demonstrate that radionuclides in potential disposal sites are retained within the containment providing rock zone using reactive transport simulations. Here, this is quantified for the example of uranium in the hydrogeological system of the Opalinus Clay at Mont Terri. Our work clearly shows how sensitive migration lengths resulting from simulations are to the model conceptualisation and selection of underlying data.
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Uranium migration in the Swiss Opalinus Clay is used as an example to quantify the influence of varying values of a stability constant in the underlying thermodynamic database within the law of mass action on the migration lengths. The difference of the stability constant of 1.33 log units lead to changed migration lengths of 5 m to 7 m. With a maximum diffusion distance of 22 m the influence of an uncertain stability constant is negligible for the host rock scale.
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Short summary
We studied how the chemical composition of water in rocks affects the movement of uranium, which is important for the safety of nuclear waste disposal sites. Using computer models, we found that water chemistry inside the host rock for the waste is more decisive for the transport of uranium than groundwater chemistry in adjacent rocks. These findings help improve site selection for a disposal site by indicating which conditions are suitable and less suitable to safely isolate radioactive waste.
We studied how the chemical composition of water in rocks affects the movement of uranium, which...
