Consistency of the strontium transport parameters in Boom Clay obtained from different types of migration experiments

The consistency of migration parameters obtained by different methods is examined for strontium in Boom Clay. In addition to a previous in-situ percolation experiment and electromigration experiments (using an electrical field as driving force), a lab percolation test and two through-diffusion tests (on clay cores of different lengths) have been performed. All experiments lead to a robust value for the apparent diffusion coefficient of strontium between 5E 10^-12 m²/s and 1E 10^-11 m²/s. No reliable value is obtained for the product ηR of the diffusion accessible porosity η and the retardation factor R. For one of the through-diffusion experiments, the concentration decrease in the inlet as a function of time was fitted simultaneously with the strontium profile in the clay, leading to a ηR value around 400. Estimating ηR at the inlet/clay interface as the ratio of the bulk concentration and the pore water concentration leads to a similar value. The values obtained from the strontium profiles in the percolation and electromigration experiments are in line as well. However, fitting for both through-diffusion tests the outlet concentrations versus time leads io very low and unrealistic values of ηR (0.15 and 0.5). Modeling shows that the transport of strontium in Boom Clay cannot be satisfactorily described with the current assumptions (like e.g. instantaneous linear sorption equilibrium and boundary conditions), meaning that there is a problem to obtain a reliable value for the product ηR for strontium in Boom Clay.