Particle-size control of radionuclide enrichment and environmental risk pathways in slurry from the Wandan mud volcano.

Mud volcanism acts as a natural conduit for transporting deep geological materials, saline fluids, and naturally occurring radionuclides to the surface, potentially generating complex environmental hazards. Despite extensive research on the geochemistry of mud volcanoes, the role of particle-size-dependent processes in controlling radionuclide mobility and exposure risk remains insufficiently understood. This study presents an integrated physicochemical and radiological investigation of slurry samples collected from the Wandan mud volcano in southern Taiwan. Particle-size analysis demonstrates that the slurry is dominated by fine-grained materials that are highly susceptible to aeolian and hydrological transport, thereby serving as primary vectors for contaminant dispersion. Further, the slurry is highly alkaline (pH 9.81 ± 0.28) with elevated electrical conductivity (4.10 ± 0.35 mS cm-1), indicating a chemically hostile environment capable of inducing soil salinization and agricultural stress. Radiological measurements reveal a pronounced inverse correlation between particle size and radionuclide activity, with the finest fraction (<0.061 mm) showing substantially higher activities of natural radionuclides of 40K, 232Th, and 226Ra compared with coarser sediments. This enrichment is attributed to surface-controlled adsorption mechanisms, whereby fine silt-clay particles with high specific surface areas, efficiently scavenge radionuclide cations during slurry ascent. Although bulk hazard indices remain below international safety thresholds, particle-specific lifetime cancer risk exceeds the global average, revealing a critical exposure pathway that conventional bulk assessments may overlook. Collectively, these findings demonstrate that fine particles function not only as the most mobile fraction, but also as dominant carriers of radiological risk. This study highlights the necessity of incorporating particle-size-resolved analyses into environmental risk frameworks to improve hazard prediction and land management strategies in mud-volcanic regions.