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

[MAG34-P05] Mechanism of dissolved-form ¹³⁷Cs runoff from forest headwater catchment

*Yutaro Nagata¹, Yuichi Onda², Koichi Sakakibara³, Junko Takahashi², Tsubasa Hiramatsu³ (1.College of Geoscience, School of Life and Environmental Sciences, University of Tsukuba, 2.Center for Research in Radiation, Isotopes, and Earth System Sciences, 3.Department of Environmental Sciences, Faculty of Science, Shinshu University)

Keywords:Fukushima Dai-ici nuclear power plant, Dissolved Cs-137, Stream water, Litter, Forested headwater catchment, Dissolved organic carbon

The accident at the Fukushima Daiichi nuclear power plant released a large amount of radioactive material into the environment, 72% of which was deposited in forests on land. Among these, ¹³⁷Cs has a long half-life of 30 years and is a factor in prolonging the damage. Most of the forests have not been decontaminated and ¹³⁷Cs circulates in the forests by being taken up from the forest floor into the tree belt and being carried out of the forests by rivers. It is known that the concentration of dissolved ¹³⁷Cs discharged from forests increases as water flows out of the forests, and this is thought to be due to leaching from litter caused by saturated surface flows and the discharge of highly concentrated soil water formed through the litter. There have also been several reports that dissolved ¹³⁷Cs in stream water is formed by ion exchange with K⁺ and leaching from litter, and that a positive correlation can be observed between the concentration of dissolved ¹³⁷Cs and K⁺ and DOC concentrations. In this study, stream water, groundwater, water passing through litter, rain passing through tree trunks, and sedimentary litter were sampled in the Seto Hachiyama watershed in the Yamakiya area, Kawamata City, Fukushima Prefecture, to investigate the effect of litter on dissolved ¹³⁷Cs leaching in forest headwater streams based on the concentrations of dissolved ¹³⁷Cs, K⁺, NH₄⁺, and DOC. The concentrations of sedimentary litter varied from 839.8 Bq/kg to 4354.1 Bq/kg. The amount of leaching from litter, determined from 137Cs, K+, NH4+ and DOC concentrations in water passing through the litter, was lower in litter near the river channel, then higher at sites where saturated surface flow occurs during rainfall, and highest from litter on slopes. This suggests that leaching from the litter along the river channel increases due to the occurrence of saturated surface flow. In the groundwater, concentrations of K⁺ were higher at deeper depths and NH4⁺ were higher at shallower depths. During discharge, concentrations of K⁺ and NH₄⁺ were 2 and 3.8 times higher than in normal water in the shallow wells near the basin outlet, respectively. In other wells, concentrations of K⁺ were similar to normal water and NH₄⁺ were lower than normal water. Changes in the hydraulic head of each well during the outflow indicated the occurrence of saturated surface flow and changes in the direction of groundwater flow. Dissolved ¹³⁷Cs, K⁺ and DOC in the stream water showed a tendency to increase during the summer season and were 1.7, 1.6 and 1.4 times higher than those in normal water, respectively. The concentrations of ¹³⁷Cs and DOC were highest in the runoff, more than twice the average of the normal water. On the other hand, K⁺ concentrations were similar to those in normal water at the time of outflow. These results suggest that dissolved ¹³⁷Cs concentrations show seasonal variability with ion exchange and organic matter decomposition, and that the reason for the increase in dissolved ¹³⁷Cs concentrations at runoff is not ion exchange but direct elution from litter.