none 10.1144/geoenergy2023-047 Geological Society of London Geological Society of London 1881 162364957 513106 2025031009443900120 10.1144 2025-03-10T16:48:19Z 2024-07-04T07:50:18Z 4 Geoenergy Geoenergy 2755-1725 01 08 2024 12 31 2024 2 1 Hirokazu Ohno Horonobe Underground Research Center, Japan Atomic Energy Agency Horonobe-cho, Japan http://orcid.org/0000-0002-4769-0358 Eiichi Ishii Horonobe Underground Research Center, Japan Atomic Energy Agency Horonobe-cho, Japan http://orcid.org/0000-0002-1103-2855 Masaki Takeda Horonobe Underground Research Center, Japan Atomic Energy Agency Horonobe-cho, Japan http://orcid.org/0009-0005-2623-9575 Faults in some deep mudstones have poor hydraulic connectivity owing to high normal stress on the fault planes. Designing a method for modelling solute transport pathways in such faults/fractures using available data is a critical issue vis-à-vis the safety assessment of radioactive waste disposal. In this study, faults in deep siliceous mudstones with low swelling capacity are investigated using cross-hole hydraulic and tracer tests between two boreholes. The water pressures observed during the hydraulic test are reproduced via hydraulic simulations, assuming a one-dimensional (1D) flow channel with a length 8–80 times the linear distance (4.5 m) between the two borehole test sections. This flow channel length indicates a tortuosity remarkably higher than that previously reported in discrete fracture network simulations and laboratory experiments using a single fracture (i.e. 1–5). Advection–dispersion simulations using a pipe model with a tortuosity of 8–80 reproduces a time series of tracer concentrations observed during the tracer test. The radius of the pipe simulated in the tracer test was 7–39 times the flow channel radius estimated via the hydraulic test, which is in agreement with the ratio of 5–20 previously reported based on in situ tracer and hydraulic tests of faulted/fractured rocks. The simulated longitudinal dispersivity is 1/9–1/20 of the estimated flow path lengths, in agreement with the laboratory and field tracer study values. 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