Modelling water circulation and solute transport at a former french uranium mining site

Within the frame of the RadoNorm project (https://www.radonorm.eu/), the goal is to better understand the importance of hydrogeochemical and chemical processes for radionuclides’ transfer in the environment with a focus on Naturally Occurring Radioactive Material (NORM). In that context, a wetland in a downstream area of a former uranium mine tailing, i. e. the Rophin site, has been selected and the following water flows and solute transport models, in 1, 2 or 3 D could be used: HYTEC [1], HYDRUS [2], CIEMAT model [3], and MELODIE [4]. This application should allow to understand the fate of uranium and radium observed in the wetland area, the link between radionuclides’ concentration in the wetland and in the crossing stream and also to highlight a possible transfer of radionuclides to the groundwater.
The Rophin site located in the department of Puy-de-Dôme of the region Auvergne-Rhone-Alpes (France) where measured dose rates are 20 to 30 times higher than the background along the Gourgeat stream and in a so-called “wetland area”. A source of contamination is present due to successive discharges from settling ponds during the operational phase of the mine and located in the whitish layer of the wetland, where three different soil horizons are identified (an organic-rich surface layer, a whitish layer, and a paleosol layer).
The watershed has been equipped since 2019 with piezometers and surface sensors to monitor the water (electrical conductivity, temperature and water flow). Metereological data is available to identify patterns for precipitation regimes for the wet and dry seasons. First results from measuring campaigns in 2021 and 2022 confirm the presence of a groundwater table in the wetland, which is rather close to the surface and varies depending on meterological conditions. To characterise the different horizons in the wetland, a field study was carried out in October 2022 which provides information about geophysical parameters. In parallel, parameters such as soil permeability, soil porosity, diffusion coefficients and dispersivity are going to be obtained by laboratory measurements in spring 2023. Surface mapping data (dose rates and gamma-ray spectroscopy analysis) and Light Detection and Ranging (LiDaR) data are available as well as information about water chemical composition and soil/sediment mineralogy both in the wetland and in the Gourgeat stream. To evaluate radionuclides sorption onto the various soil layers and partition of radionuclides between solid and liquid phases, distribution coefficients are determined either experimentally (Kd) or modelled with the mechanistic "Smart Kd" approach [5].
A description of water flows and soil layers to be modelled is presented. In a first approach assuming a saturated zone, modelling of the three layers system will be performed in 1D and 2D. Two types of simulation are considered, one with a low flux regime when the stream and the wetland seems to be disconnected and the other with a high flux regime when the stream and the wetland should be hydraulically connected. The aim is to evaluate the evolution of uranium and radium concentration within the layers of the wetland and also to quantify the concentrations of those radionuclides released into the environment, i.e. groundwater and Gourgeat stream.

References
[1] Van Der Lee, J., De Windt, L., Lagneau, V., & Goblet, P. (2003). Module-oriented modeling of reactive transport with HYTEC. Computers & Geosciences, 29(3), 265-275.
[2] Simunek, J., Jacques, D., van Genuchten, M. T., & Mallants, D. (2006). Multicomponent geochemical transport modeling using HYDRUS-1 D and HP 1. Journal of the American water resources Association, 42(6), 1537-1547.
[3] Pérez-Sánchez, D., & Thorne, M. C. (2014). Modelling the behaviour of uranium-series radionuclides in soils and plants taking into account seasonal variations in soil hydrology. Journal of environmental radioactivity, 131, 19-30.
[4] Mathieu, G., Dymitrowska, M., & Bourgeois, M. (2008). Modeling of radionuclide transport through repository components using finite volume finite element and multidomain methods. Physics and Chemistry of the Earth, Parts A/B/C, 33, S216-S224.
[5] Stockmann, M. & Schikora, J. & Becker, D.-A. & Flügge, J. & Noseck, U. & Brendler, V. (2017). Smart Kd-values, their uncertainties and sensitivities - Applying a new approach for realistic distribution coefficients in geochemical modeling of complex systems. Chemosphere. 187. 10.1016/j.chemosphere.2017.08.115.
Acknowledgments

The RadoNorm project has received funding from the Euratom research and training programme 2019-2020 under grant agreement No 900009.