[M-AG34] Radioisotope migration: Development from Environmental Dynamics Research on the F1NPS Accident

[MAG34-02] Dissolved Cs-137 discharge and the effect of hillslope groundwater in the headwater forest watershed

*Hiroaki Kato¹, Haruki Nakada¹, Hao Wang¹, Yuichi Onda¹ (1.Center for Research in Radiation, Isotopes and Earth System Sciences, University of Tsukuba)

Keywords:Fukushima Daiichi Nuclear Power Plant accident, Forest catchment, Dissolved radiocesium concentration, hillslope groundwater

A significant portion of the ¹³⁷Cs released due to the Fukushima Daiichi Nuclear Power Plant accident was deposited in forested areas, where it continues circulating within the ecosystem. Although the amount of dissolved ¹³⁷Cs is smaller than that of particulate ¹³⁷Cs, it is more readily absorbed by crops and ecosystems, making it crucial to identify its sources and transport pathways. In this study, we investigated the influence of shallow and deep groundwater on the concentration of dissolved ¹³⁷Cs in stream water within a forest headwater catchment in Namie Town, Fukushima Prefecture. Groundwater flow and water quality were observed around springwater discharge points, and stream water samples were collected during runoff events to measure ¹³⁷Cs concentrations. The results showed that shallow groundwater exhibited higher dissolved ¹³⁷Cs concentrations than stream water, with a strong correlation between dissolved ¹³⁷Cs and dissolved organic carbon (DOC) concentrations. The dissolved ¹³⁷Cs, DOC, and K⁺ concentrations differed among stream water, soil infiltration water at different depths and groundwater. During runoff events, dissolved ¹³⁷Cs, DOC, and K⁺ concentrations in stream water increased, displaying hysteresis-type changes. These findings suggest shallow and deep groundwater influences stream water before and after peak discharge. Furthermore, dissolved ¹³⁷Cs concentrations may provide a quantitative means to assess groundwater contributions to stream water dynamics.