Characterizing vertical migration of 137Cs in organic layer and mineral soil in Japanese forests: Four-year observation and model analysis

Because of the Fukushima Dai-ichi Nuclear Power Plant accident, forest ecosystems in wide areas were contaminated with Cs-137. It is important to characterize the behavior of Cs-137 after its deposition onto forest surface environments for evaluating and preventing long-term radiation risks. In the present study, Cs-137 vertical distributions in the soil profile were observed repeatedly at five forest sites with different vegetation types for 4.4 years after the accident in 2011, and Cs-137 migration in the organic layer and mineral soil was analyzed based on a comparison of models and observations. Cesium-137 migration from the organic layer to the underlying mineral soil was represented by a two-component exponential model. Cesium-137 migration from the organic layer was faster than that observed in European forests, suggesting that the mobility and bioavailability of Cs-137 could be suppressed rapidly in Japanese forests. At all sites, Cs-137 transfer in mineral soil could be reproduced by a simple diffusion equation model with continuous Cs-137 supply from the organic layer. The diffusion coefficients of Cs-137 in the mineral soil were estimated to be 0.042-0.55 cm(2) y(-1), which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions using the determined model parameters indicated that 10 years after the accident, more than 70% of the deposited Cs-137 will migrate to the mineral soil but only less than 10% of the total Cs-137 inventory will penetrate deeper than 10 cm in the mineral soil across all sites. The results of the present study suggest that the Cs-137 deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.