137Cs transport flux to surface water due to shallow groundwater discharge from forest hillslope

Groundwater systems and surface water can interact in a complex manner that influences catchment discharge, which then becomes more complex in forest slopes. A large amount of Radioactive cesium (¹³⁷Cs) deposited on forests due to the Fukushima Daiichi Nuclear Power Plant accident remains in terrestrial environments and is transported downstream as suspended or dissolved forms by surface water. Generally, the concentration of dissolved ¹³⁷Cs in surface water increases especially during runoff. While the leaching behavior of ¹³⁷Cs from contaminated forest materials and soils to surface water has been heavily studied, the influence of ¹³⁷Cs concentration in shallow groundwater systems in forest slopes have not been investigated. Therefore, detailed hydrological observations of groundwater on a forest hillslope will enable quantitative analysis of the influence of groundwater flow on the formation of dissolved ¹³⁷Cs concentrations in surface water during base flow and during runoff. Our results showed that the dissolved ¹³⁷Cs concentration in surface water increases during water discharge. The average concentration of dissolved ¹³⁷Cs in shallow groundwater was 0.64 Bq/L, which was higher than that in surface water (average 0.10 Bq/L). Furthermore, it was also observed that a part of the shallow groundwater on the slope moves toward the river channel at the time of water runoff. This suggests that shallow groundwater may have flowed into the surface water during the outflow and contributed to the increase of ¹³⁷Cs in the surface water. In this study, the contribution of groundwater in forest slopes to the dissolved ¹³⁷Cs concentration in surface water was estimated using the hydrodynamic gradient distribution of groundwater in forest slopes and the measured dissolved ¹³⁷Cs concentration in groundwater.