Reactive transport of strontium in two laboratory-scale columns: Experiments and modelling

To support the environmental monitoring of nuclear sites, reactive transport models used to predict the migration of contaminants such as strontium-90 (⁹⁰Sr) in soils, sediments and aquifers are developed, continuously tested and improved. This study aims at assessing the adequacy of the multi-site ion exchanger model (MSIE) based on a component 'additivity approach' and coupled to the advection-dispersion equation (ADE) to simulate Sr transport in a clayey sandstone and a Bt soil horizon. We have also compared the performance of the modelling approach with simulation results obtained by considering a K_d approach (constant K_d). Transport experiments were performed in centimetre- and decimetre-scale columns in order to test the model sensitivities to the mineral abundance and the specificities of their reactive parameters. Non-reactive transport experiments with conservative tracers allowed us to determine the transport parameters, such as porosity and dispersivity. In this paper, we have compared the Sr transport simulation results with Sr experimental breakthrough curves acquired at various flow velocities. The simulations results show that the K_d approach can reproduce experimental data in the case of the clayey sandstone when a certain amount of uncertainty is accepted, whereas the additivity approach better fits the Sr reactive transport in both columns (especially the maximum value) without it being necessary to adjust the parameters. These results advocate for more complex retention models than K_d to better understand and improve the robustness of Sr transport predictions. The clay content, the relative abundance of illite and smectite, and the clay mineral specificity, are all sensitive parameters when it comes to defining the reactive system involved in Sr transport simulation. The results highlight the influence of illite in the spreading of the Sr breakthrough curve, especially through its low-capacity and high-selectivity site. This implies having access to a robust and extensive set of retention parameters acquired on reference minerals. In this study, the results obtained for the clayey sandstone confirm the robustness of our selected parameters when clay minerals have similar reactivity levels as the reference minerals. This set of parameters appears more limited in the case of the Bt soil containing weathered or evolved minerals. The choice of modelling approach is therefore crucial for accurately modelling and predicting Sr transport behaviour in porous media, as is the representativeness of the minerals in the database. (authors)