137Cs vertical migration in a deciduous forest soil following the Fukushima Dai-ichi Nuclear Power Plant accident

The large amount of Cs-137 deposited on the forest floor because of the Fukushima Dai-ichi Nuclear Power Plant accident represents a major potential long-term source for mobile Cs-137. To investigate Cs-137 mobility in forest soils, we investigated the vertical migration of Cs-137 through seepage water, using a lysimetric method. The study was conducted in a deciduous forest soil over a period spanning 2 month to 2 y after the Fukushima nuclear accident. Our observations demonstrated that the major part of Cs-137 in the litter layer moved into the mineral soil within one year after the accident. On the other hand, the topsoil prevented migration of Cs-137, and only 2% of Cs-137 in the leachate from litter and humus layer penetrated below a 10 cm depth. The annual migration below a 10 cm depth accounted for 0.1% of the total Cs-137 inventory. Therefore, the migration of Cs-137 by seepage water comprised only a very small part of the total Cs-137 inventory in the mineral soil, which was undetectable from the vertical distribution of Cs-137 in the soil profile. In the present and immediate future, most of the Cs-137 deposited on the forest floor will probably remain in the topsoil successively, although a small but certain amount of bioavailable Cs-137 exists in forest surface soil. (C) 2013 Elsevier Ltd. All rights reserved.