Ten-year trends in vertical distribution of radiocesium in Fukushima forest soils, Japan

To elucidate interannual changes in the vertical distribution of ¹³⁷Cs in forest ecosystems contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, we investigated ¹³⁷Cs inventories in forest soils (both organic and mineral soil horizons) at 10 sampling plots with different ¹³⁷Cs deposition levels and dominant species for up to 10 years after the accident. We examined the temporal variation of the ¹³⁷Cs inventories by depth with exponential regression models (assuming that the transition and partitioning of ¹³⁷Cs are still active) and exponential offset regression models (assuming a shift to a stable ¹³⁷Cs distribution, defined as the "quasi-equilibrium steady-state" in the Chernobyl accident). In the organic horizon, the ¹³⁷Cs inventories were exponentially decreasing, and it might take more time to converge in the quasi-equilibrium steady-state at most plots. In the mineral soil horizon, most of ¹³⁷Cs was found in the surface layer of the mineral soil horizon (0-5 cm). In this layer, the inventories first increased and then become relatively constant, and the exponential offset model was selected at most plots, suggesting entry into the quasi-equilibrium steady-state over the observation period. Although we also observed exponentially increasing trends in a lower layer (5-10 cm) of the mineral soil horizon, there was no clear increasing or decreasing trend of ¹³⁷Cs inventory in the deeper mineral soil layers (10-15 and 15-20 cm). Our calculation of the relaxation depth and migration center revealed that downward migration of ¹³⁷Cs is not significant in terms of the overall ¹³⁷Cs distribution in the mineral soil horizon over 10 years.