Assessment of radiocesium cycle equilibrium in a deciduous broad-leaved forest using stable cesium: A pilot study after the Fukushima accident

To accurately predict long-term ¹³⁷Cs activity concentrations in stem wood, it is crucial to assess whether the ¹³⁷Cs cycle in forests has reached quasi-equilibrium between trees and forest soil. A method has been proposed to evaluate this equilibrium based on the concentration ratio of ¹³⁷Cs to stable cesium (¹³³Cs) within the tree compartments. However, this assessment has not yet been conducted in the recent situation following the Fukushima accident. In this study, we investigated the relationship between the concentration ratios of ¹³⁷Cs and ¹³³Cs in tree leaves, inner bark, stem wood, and stemflow in a deciduous broad-leaved forest at Kawauchi Village, the Fukushima Prefecture, as of 2022–2023. Our comparisons between ¹³⁷Cs and ¹³³Cs concentrations revealed that ¹³⁷Cs/¹³³Cs concentration ratios were constant in the above-ground compartment and stemflow, suggesting that the cycle of ¹³⁷Cs in the Fukushima forest has reached quasi-equilibrium 11–12 years after the accident. This concentration ratio-based assessment for quasi-equilibrium may be applicable to forest sites where long-term monitoring is not conducted.