Decadal stability of radiocesium inventories and soil to tree transfer in forests affected by the Fukushima nuclear accident

Abstract Understanding the long-term dynamics of radiocesium ( 137 Cs) in forests contaminated by the Fukushima nuclear accident requires determining when these dynamics reach a quasi-equilibrium state, in which the flux of 137 Cs between soil and trees becomes approximately balanced. In this study, we analyzed time-series variations from 2011 to 2020 in the total 137 Cs inventory and 137 Cs distribution in aboveground compartments (needles/leaves, branches, bark, sapwood, and heartwood). Time-series analysis using a dynamic linear model indicated that the decay-corrected total 137 Cs inventory (as of September 1, 2020) and its distribution in the aboveground compartments remained stable from 2017 onward, suggesting that a quasi-equilibrium state had been reached. Given this stability in both inventory and distribution, the aggregated transfer factor ( T ag )—defined as the 137 Cs activity concentration in aboveground tree tissues divided by the total 137 Cs inventory in soil—measured approximately 6 years after the Fukushima nuclear accident can be considered representative of the long-term transfer of 137 Cs from forest soil to trees. These findings on the stability of 137 Cs in forests provide valuable insights for validating the accuracy of models that predict long-term 137 Cs activity concentration in stem wood.