Numerical simulation of anomalous observations from an in-situ long-term sorption diffusion experiment in a rock matrix

At the in-situ Long Term Sorption Diffusion Experiment (LTDE-SD), performed in the Aspo Hard Rock Laboratory (Sweden), anomalous penetration profiles of tracers were observed. The concentration of sorbing tracers, particularly Cs-137, sharply decreased around the rock surface but was still quite high in the distance, which was known that it is difficult to model with the classic Fick's law. To understand the mechanism of matrix diffusion and sorption of solute by reproducing the observed penetration profiles in the LTDE-SD test, three cases for the matrix were proposed and evaluated: (1) homogeneous medium with a disturbed zone; (2) heterogeneous medium considering mineral grains with intragranular porosities and their intergranular pores; and (3) heterogeneous medium considering vein and microfractures as well as porous mineral grains and intergranular pores. The modelling domains for Cases 2 and 3 were established using 2-D microstructure characterization results from analyses including chemical staining of minerals and C-14-PMMA autoradiography. The numerical models for Cases 1-3 were calibrated by minimizing the discrepancies between simulations and observations of the penetration profiles and the changes in concentration in the reservoir for the tracers, Cs-137 and Na-22. The results show that only the calibrated transport parameter values for Case 3 were within the reported ranges, and we concluded that the sharp decrease in concentrations in the near-surface zone in the observed penetration profiles was likely to result from disturbed biotite grains and the long tails in the observed penetration profiles might originate from diffusion along the vein and microfractures transacting minerals.