Post-deposition early-phase migration and retention behavior of radiocesium in a litter-mineral soil system in a Japanese deciduous forest affected by the Fukushima nuclear accident

(T)he fate of radiocesium (Cs-137) derived from the Fukushima nuclear accident and associated radiation risks are largely dependent on its migration and retention behavior in the litter soil system of Japanese forest ecosystems. However, this behavior has not been well quantified. We established field lysimeters in a Japanese deciduous broad-leaved forest soon after the Fukushima nuclear accident to continuously monitor the downward transfer of Cs-137 at three depths: the litter mineral soil boundary and depths of 5 cm and 10 cm in the mineral soil. Observations were conducted at two sites within the forest from May 2011 to May 2015. Results revealed similar temporal and depth-wise variations in Cs-137 downward fluxes for both sites. The Cs-137 downward fluxes generally decreased year by year at all depths, indicating that Cs-137 was rapidly leached from the forest-floor litter layer and was then immobilized in the upper (0-5 cm) mineral soil layer through its interaction with clay minerals. The Cs-137 fluxes also showed seasonal variation, which was in accordance with variations in the throughfall and soil temperature at the sites. There was no detectable Cs-137 flux at a depth of 10 cm in the mineral soil in the third and fourth years after the accident. The decreased inventory of mobile (or bioavailable) Cs-137 observed during early stages after deposition indicates that the litter soil system in the Japanese deciduous forest provides only a temporary source for Cs-137 recycling in plants. (C) 2016 Elsevier Ltd. All rights reserved.