Colloids can potentially enhance the transport of radionuclides in groundwater, meaning radionuclides could travel further than would normally be predicted by solute-only transport modelling. To develop understanding, potential radionuclide transport processes are investigated. Colloid-Facilitated Radionuclide Transport is investigated as part of the Colloid Formation and Migration (CFM) experiment at the Grimsel Test Site in Switzerland, where in-situ migration experiments have investigated the transport of tracers, bentonite colloids and radionuclides at a variety of flow velocities in a shear zone within fractured granodiorite.

This paper presents a transport model that aims to replicate the transport of tracers, colloids and americium in two different experiments using consistent parameters. Inverse modelling has been used to describe the hydraulic properties of the shear zone. Flexible transport equations are then used to simulate contaminant transport. The model is able to replicate the breakthrough curves for colloids and americium across two experiments with different dipole flow fields using consistent parameters. The parameter values used to describe colloid attachment and americium desorption are within the ranges used by other models in the CFM programme, but are different to ones generated by laboratory desorption experiments. It is planned to extend the model to other radionuclides in the future.