Methodology for evaluating matrix diffusion and sorption parameters in crystalline rocks: Application to laboratory...

For long-term safety assessments of deep geological disposal, evaluating effective diffusion coefficients (D_e) and distribution coefficients (K_d) under in-situ conditions remains a critical yet challenging task due to practical limitations. This study aims to establish a methodology for evaluating these parameters under realistic geological conditions through an integrated analysis of a long-term in-situ diffusion (LTD-II) experiment and complementary laboratory through-diffusion experiments in Grimsel granodiorite. The proposed approach combines (i) post analysis of cored samples to characterize spatial variations in pore connectivity and anisotropic transport, (ii) numerical modeling that accounts for the borehole disturbed zone (BDZ), and (iii) laboratory diffusion experiments to verify the consistency and reliability of in-situ parameter estimation. The modeling reproduced both the depletion curves and the tracer concentration profiles observed in the LTD-II experiment, particularly the sharp gradients within a few millimeters from the injection hole, which were attributed to BDZ. The derived D_e and K_d values for sorbing tracers (Na⁺, Cs⁺, and Ba²⁺) were consistent between laboratory and in-situ conditions for transport distances up to several centimeters, confirming the reliability of the parameter derivation approach for sorbing species over short distances. In contrast, non-sorbing tracers (HTO and ³⁶Cl^-) exhibited depth-dependent concentration variations extending ∼50 cm from the injection hole. The D_e values of non-sorbing tracers were two to three times higher than those obtained in laboratory tests, yet remained within the same order of magnitude. These variations of non-sorbing tracers could be qualitatively explained by anisotropic transport along foliation and minor advection over several tens of centimeters, highlighting the importance of accounting for local structural and hydraulic variations when interpreting in-situ diffusion data. These results demonstrate that integrating in-situ diffusion data with laboratory verification and spatial profiling provides a methodology for deriving diffusion and sorption parameters representative of in-situ conditions, and clarifies the practical limitations and applicable range of transport modeling in crystalline rocks.