[M-AG34] Radioisotope migration: New Development of Environmental Dynamics Research on the 1FNPS Accident

[MAG34-02] Effects of shallow groundwater runoff on Cs-137 discharge flux from forest headwater catchment

*Hiroaki Kato¹, Yuma Niwano¹, Yuichi Onda¹ (1.Center for Research in Radiation, Isotopes, and Earth System Sciences, University of Tsukuba)

Keywords:Fukushima Dai-ichi Nuclear Power Plant accident, Cs-137, Forest catchment, Shallow groundwater

In the aftermath of the Fukushima Daiichi Nuclear Power Plant disaster, radiocesium (Cs-137) dispersion through forest ecosystems into aquatic systems has raised environmental concerns. While prior investigations have delineated the augmentation of Cs-137 in surface waters, attributing this to the deposited litter leaching, the role of groundwater runoff on Cs-137 discharge remains underexplored. This research aims to elucidate the role of shallow groundwater, characterized by diverse residence times and pathways on forested slopes, in modulating Cs-137 concentrations within surface waters. Employing detailed hydrological monitoring at a forest hillslope, we discerned elevated Cs-137 levels in shallow groundwater compared to adjacent surface waters. Notably, surface water Cs-137 concentrations surged in conjunction with rainfall events, accompanied by pronounced groundwater flow toward river channels. A significant correlation between the groundwater hydraulic gradient and surface water flow rate was identified, underscoring the pivotal contribution of shallow groundwater discharge to both the enhanced flow and Cs-137 levels in surface waters. This study quantifies shallow groundwater's impact on Cs-137 surface water concentrations, leveraging detailed hydrological data and Cs-137 measurements. Our findings contribute to a deeper understanding of radioactive contaminant dynamics in forested watersheds, highlighting the need for integrated water management strategies post-nuclear accidents.