Interpreting the deposition and vertical migration characteristics of 137Cs in forest soil after the Fukushima Dai-ichi Nuclear Power Plant accident.

We investigated the deposition and depth distributions of radiocesium in the Takizawa Research Forest, Iwate Prefecture, in order to understand the behavior of radionuclides released from the Fukushima Dai-ichi Nuclear Power Plant. The deposition distribution and vertical depth distribution of radiocesium in the soil were compared between topographically distinct parts of the forest where two different tree species grow. The results for all investigated locations show that almost 85% of the radiocesium has accumulated in the region of soil from the topmost organic layer to a soil depth of 0-4 cm. However, no activity was detected at depths greater than 20 cm. Analysis of the radiocesium deposition patterns in forest locations dominated by either coniferous or deciduous tree species suggests that radiocesium was sequestered and retained in higher concentrations in coniferous areas. The deposition data showed large spatial variability, reflecting the differences in tree species and topography. The variations in the measured 137Cs concentrations reflected the variability in the characteristics of the forest floor environment and the heterogeneity of the initial ground-deposition of the Fukushima fallout. Sequential extraction experiments showed that most of the 137Cs was present in an un-exchangeable form with weak mobility. Nevertheless, the post-vertical distribution of 137Cs is expected to be governed by the percentage of exchangeable 137Cs in the organic layer and the organic-rich upper soil horizons.