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

Abstract

The fate of radiocesium (¹³⁷Cs) 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 ¹³⁷Cs 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 ¹³⁷Cs downward fluxes for both sites. The ¹³⁷Cs downward fluxes generally decreased year by year at all depths, indicating that ¹³⁷Cs 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 ¹³⁷Cs fluxes also showed seasonal variation, which was in accordance with variations in the throughfall and soil temperature at the sites. There was no detectable ¹³⁷Cs 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) ¹³⁷Cs observed during early stages after deposition indicates that the litter-soil system in the Japanese deciduous forest provides only a temporary source for ¹³⁷Cs recycling in plants.

Keywords: Fukushima Daiichi nuclear power plant accident; Litter layer; Lysimeter; Radiocesium; Soil water; Vertical migration.