EGU2020-8972
https://doi.org/10.5194/egusphere-egu2020-8972
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Long-term coupled permafrost-groundwater interactions at Olkiluoto, Finland

Denis Cohen1, Thomas Zwinger2, Lasse Koskinen3, and Tuomo Karvonen4
Denis Cohen et al.
  • 1Earth and Environmental Science, New Mexico Tech, Socorro, NM, United States of America (denis.cohen@gmail.com)
  • 2CSC - IT Center for Science Ltd., Espoo, Finland (thomas.zwinger@csc.fi)
  • 3Posiva Oy Olkiluoto, Eurajoki, Finland (Lasse.Koskinen@posiva.fi)
  • 4WaterHope, Helsinki, Finland (tk@waterhope.fi)

Understanding permafrost development and its effect on groundwater flow patterns and fluxes in the event of future ice-age conditions is important for the long-term safety of spent nuclear fuel repositories. To assess the evolution of permafrost thickness, talik development, and groundwater flow and salinity changes at Olkiluoto, Finland, during the next 100,000 years, we solve Darcy flow coupled to heat and solute transport in three dimensions in a rectangular block representing an area of 8.8 km by 6.8 km, and down to a depth of 10 km. The set of equations is based on continuum thermo-mechanic principles. Important and highly non-linear coupling processes such as the exponential decrease of permeability with ice content in soils and rocks, solute rejection during freezing, and variable-density Darcy flow are fully taken into account. Model equations are solved using the finite element method implemented in the open source software Elmer.  High-resolution data of rock and soil permeability, thermal and physical properties, are mapped onto a 30-meter resolution grid resulting in a system of about 5 million nodes and 5 million elements. Soil layers at the surface are vertically resolved down to 0.1 meter. High contrast in permeability over short distances (from soil to granitic bedrock) make the system of equations challenging to solve numerically. Simulations are driven by RCP 4.5 climate scenario that predicts cold periods between AD 47,000 and AD 110,000. Surface boundary condition for temperature is calculated based on freezing and thawing n-factors that depend on monthly temperatures and the topographic wetness index that defines different zones of vegetation and ground cover. The thickness evolution of the six upper soil layers, including peat, above the granitic bedrock is also taken into account. Preliminary simulations are able to represent permafrost development at a high spatial resolution with evidence of important feedbacks due to permeable soil layers and faults in the bedrock that focus groundwater flow and solute transport.

How to cite: Cohen, D., Zwinger, T., Koskinen, L., and Karvonen, T.: Long-term coupled permafrost-groundwater interactions at Olkiluoto, Finland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8972, https://doi.org/10.5194/egusphere-egu2020-8972, 2020