In situ tracer experiments and modeling of solute transport in a fault zone of siliceous mudstone at the Horonobe Underground Research Laboratory, Japan

Fault zones in low-swelling argillaceous host rocks may act as preferential pathways for radionuclide transport despite the low permeability and high sorption capacity of intact rock. However, field-scale evidence remains limited. In this study, in situ cross-hole tracer experiments were conducted in a fault zone in siliceous mudstone of the Wakkanai Formation at the Horonobe Underground Research Laboratory (URL), Japan. Tracers with contrasting transport properties (uranine, Mo, Cs) were injected using both pulse and constant-concentration injection modes. Tracer breakthrough curves were monitored, and post-test overcoring was performed to characterize fracture distributions and tracer concentrations on individual fracture surfaces. Breakthrough behavior was consistently governed by tracer properties under both injection modes. Combined analyses of breakthrough curves and fracture-scale observations revealed transport heterogeneity, including tracer transport through multiple hydraulically active fractures and spatial variability of tracer concentrations on individual fracture surfaces. These results indicate that solute transport in the tested fault zone cannot be adequately interpreted with models that assume a single flow path. Instead, the observations were reproduced using a transport model incorporating multiple pathways and the coexistence of mobile and immobile zones within fractures. The optimized parameters are consistent with previous studies, including contrasting behaviors of non-sorbing and sorbing tracers and trends related to anion exclusion and cation-enhanced diffusion. The results demonstrate that integrating in situ tracer experiments, fracture-scale observations, and transport modeling provides a robust framework for deriving reliable transport models and parameters in complex fault zones relevant to the geological disposal of radioactive waste.