Particle-tracking assessment of potential groundwater pathways for radionuclide transport across the Futaba Fault, coastal Fukushima, Japan

Radiocesium deposited after the Fukushima Daiichi Nuclear Power Plant accident remains a long-term environmental concern, but its groundwater pathways toward inhabited coastal lowlands are poorly constrained. This study evaluates conservative groundwater pathways potentially relevant to radionuclide transport near the Futaba Fault in coastal Fukushima by coupling a steady-state MODFLOW-2005 model with MODPATH 7. One thousand realizations varied hydraulic conductivity and the porosity components used to derive effective porosity for the fault zone, granitic basement, mudstone, and three sandstone units using bounded PERT and log-PERT distributions. Fixed particles were released separately on each side of the fault. Of 2,000,000 western endpoint records, 93.24% terminated west of the fault, 6.33% reached eastern non-ocean cells, and 0.43% reached the ocean boundary. Western paths were concentrated in local circulation cells and valley-directed discharge zones. Eastern non-ocean outcomes originated from 465 of 2000 western release locations and were spatially clustered rather than controlled by fault distance alone. Of 6,000,000 eastern endpoint records, 89.02% remained within the eastern sedimentary domain and 10.98% reached the ocean boundary; none returned to the western block. Within the adopted barrier-dominated conceptual model and specified hydraulic-property distributions, endpoint maps, travel-time-distance densities, ensemble cross sections, and representative-realization visualizations show that the limited west-to-east pathways remain spatially restricted. Because sorption and decay were omitted, the results represent a conservative, retardation-free pathway assessment rather than a concentration forecast. Even under this formulation, west-derived pathways into the eastern lowland were spatially restricted, indicating limited potential for widespread groundwater-mediated radionuclide transport under the modeled conditions.