Guillory et al. (2025) Soil-to-river Cesium-137 transfer in a catchment coupling the SWAT model and a mass balance equation

Identification

• Journal: Journal of Environmental Radioactivity
• Year: 2025
• Date: 2025-11-04
• Authors: François Guillory, Hugo Lepage, J.-L. Sanchez, Clément Fabre, Patrick Boyer, David Mourrier
• DOI: 10.1016/j.jenvrad.2025.107849

Research Groups

• Nuclear Safety and Radiation Protection Authority, PSE-ENV, STAAR/LRTA, Saint Paul Lez Durance, France
• Centre de Recherche sur la Biodiversit´e et l’Environnement (CRBE), Universit´e de Toulouse, CNRS, IRD, Toulouse INP, Universit´e Toulouse 3 – Paul Sabatier (UT3), Toulouse, France
• Tour du Valat, Arles, France

Short Summary

This study developed a novel coupled SWAT and mass balance model to simulate Cesium-137 (137Cs) transfer from soils to the river outlet in the Ardèche watershed (2138 km2, France), revealing that 83 % of annual 137Cs transport occurs in the particulate phase, predominantly during high-flow events.

Objective

• To develop and apply a coupled modeling framework combining the Soil and Water Assessment Tool (SWAT) with a trace metal transfer equation to simulate Cesium-137 (137Cs) transfer from soils to the river outlet in a watershed affected by radioactive atmospheric fallout.
• To evaluate if this coupled model can accurately reproduce the observed dynamics of 137Cs within the river network.
• To quantify the total amount of 137Cs exported from the catchment, distinguishing between dissolved and particulate forms and analyzing its variation under different hydrological conditions.
• To explore the spatial and temporal evolution of 137Cs distribution across the catchment, providing insights into the processes governing radionuclide transfer under varying land use and flow regimes.

Study Configuration

• Spatial Scale: Ardèche watershed (2138 km2, France), discretized into 5679 Hydrologic Response Units (HRUs) and 283 sub-catchments.
• Temporal Scale: 7-year simulation period (2016–2022) for 137Cs transfer; hydrology calibrated from 1980 to 2000 and validated from 2000 to 2022; suspended sediment concentration (SSC) and particulate 137Cs concentrations calibrated from 2016 to 2019 and validated from 2019 to 2022.

Methodology and Data

• Models used:
  • Soil and Water Assessment Tool (SWAT) for hydrology and sediment transport.
  • Trace metal transfer equation (137Cs-Kd model) for solid/liquid fractionation of 137Cs, adapted from Tomczak et al. (2021).
  • Modified Universal Soil Loss Equation (MUSLE) for hillslope erosion.
  • Kodoatie (2000) equation for maximum sediment concentration in streams.
  • Stokes (1851) and Einstein (1905) equations for sediment deposition.
  • Bagnold and Beech (1977) sediment transport equations for floodplain deposition.
• Data sources:
  • Topographic map: High-resolution Digital Elevation Model (25 m) from Institut Géographique National (IGN) (2001).
  • Land use map: Corine Land Cover 2012 dataset (European Environment Agency).
  • Soils map: European Soil Database (ESDB) (2001) at 1:1,000,000 scale (European Commission).
  • Meteorology Data: SAFRAN model (Meteo-France) gridded (8 km x 8 km) daily values for precipitation, temperature, wind speed, solar radiation, and relative humidity (1980–2022).
  • River flow: Banque Hydro database (French Ministry for Ecological Transition and Solidarity) daily streamflow data from 10 gauging stations (2013–2022).
  • Suspended Sediment Concentration (SSC): Rhone Sediments Observatory turbidimeter data (10 min measurements, converted to SSC) at one station (2016–2022).
  • Median grain size (D50): BDOH database.
  • Particulate 137Cs: Monthly suspended sediment samples collected via metal traps at Saint-Martin d’Ardèche (2016–2022), analyzed by gamma spectrometry.
  • Soil 137Cs activity: Soil core samples (0–5 cm, 5–15 cm, 15–30 cm layers) from 31 sampling points in March 2022, analyzed by gamma spectrometry.

Main Results

• The total 137Cs stock in the Ardèche catchment soils was estimated at 7.7 TBq as of 1 January 2016.
• Modelled daily 137Cs concentrations in suspended sediments ranged from 0 to 43.0 Bq kg−1, while observed monthly concentrations ranged from 3.5 to 20.1 Bq kg−1.
• 83 % of the annual 137Cs transport at the river outlet occurs in the particulate phase.
• 75 % of the total annual 137Cs flux at the outlet is exported during the top 10 % highest flow days (flow rates exceeding 118.3 m3 s−1).
• On average, 263.0 GBq y−1 of 137Cs are eroded from the watershed soils.
• The river exports 1.62 GBq y−1 of 137Cs at the outlet and stores 256.0 GBq y−1 within the river itself and floodplains.
• The annual export of 137Cs represents only 0.02 % of the total 137Cs soil stock.
• Over the 7-year simulation period (2016–2022), radioactive decay accounted for a 17 % reduction (1.6 TBq) in the initial 137Cs watershed stock, while erosion-driven export accounted for a 0.14 % reduction.
• Modelled daily flow at the outlet showed good agreement with observations (R2 = 0.87, NSE = 0.87, Pbias = −1.36).
• Modelled suspended sediment fluxes were dominated by clay (93 %), followed by silt (4 %) and sand (1 %).
• The Sediment Delivery Ratio (SDR) for the Ardèche catchment was estimated at 0.0045 (0.45 %), indicating a storage-dominated system.
• Simulated dissolved 137Cs concentration at the outlet had a mean of 1.4 × 10−4 Bq L−1 and a maximum of 6.3 × 10−4 Bq L−1, with an annual flux of 0.3 GBq y−1.

Contributions

• Presents a novel coupled modeling framework combining the SWAT model with a trace metal transfer equation for simulating 137Cs dynamics.
• Represents one of the early applications of SWAT to radionuclide transport, particularly in a catchment without a nuclear power plant, focusing on historical atmospheric fallout.
• Provides a comprehensive quantification of 137Cs transfer, distinguishing between dissolved and particulate forms and analyzing its variability under different hydrological conditions.
• Generates spatially explicit results, including maps of 137Cs soil loss and storage within the river system, offering insights into radionuclide redistribution processes.
• Confirms the dominant role of particulate transport and high-flow events in 137Cs export in a late-post-Chernobyl context.

Funding

• Rhône Sediment Observatory (OSR), partly funded by the ‘Plan Rhône’ and the European Regional Development Fund (ERDF).
• "Nuclear, Risk, Society" Federative Project of the NEEDS (Nuclear: Energy, Environment, Waste, Society) program.

Citation

@article{Guillory2025Soiltoriver,
  author = {Guillory, François and Lepage, Hugo and Sauvage, Sabine and Sanchez, J.-L. and Fabre, Clément and Boyer, Patrick and Mourrier, David},
  title = {Soil-to-river Cesium-137 transfer in a catchment coupling the SWAT model and a mass balance equation},
  journal = {Journal of Environmental Radioactivity},
  year = {2025},
  doi = {10.1016/j.jenvrad.2025.107849},
  url = {https://doi.org/10.1016/j.jenvrad.2025.107849}
}