Vertical distribution of radiocesium in coniferous forest soil after the Fukushima nuclear power plant accident

Abstract

This study deals with the description of the vertical distribution of radiocaesium (137Cs and 134Cs) in a representative coniferous forest soil, investigated 10 months after the Fukushima radioactive fallout. During soil sampling, the forest floor components (understory plants, litter (Ol-) and fermented layers (Of)) were collected and treated separately. The results indicate that radiocesium is concentrated in the forest floor, and high radiocesium transfer factor observed in the undergrowth plants (3.3). This made the forest floor an active exchanging interphase for radiocesium. The raw organic layer (Ol + Of) holds 52% (5.3 kBq m−2) of the Fukushima-derived and 25% (0.7 kBq m−2) of the pre-Fukushima 137Cs at the time of the soil sampling. Including the pre-Fukushima 137Cs, 99% of the total soil inventory was in the upper 10 cm, in which the organic matter (OM) content was greater than 10%, suggesting the subsequent distribution most likely depends on the OM turnover. However, the small fraction of the Fukushima-derived 137Cs at a depth of 16 cm is most likely due to the infiltration of radiocesium-circumscribed rainwater during the fallout before that selective adsorption prevails and reduces the migration of soluble 137Cs. The values of the depth distribution parameters revealed that the distribution of the Fukushima-derived 137Cs was somewhat rapid.