Comparison of the vertical distributions of Fukushima nuclear accident radiocesium in soil before and after the first rainy season, with physicochemical and mineralogical interpretations Original Research Article

Effect of intense rainfall on the distribution of Fukushima-accident-derived 137Cs in soil was examined. Inventories and vertical distributions of 137Cs in soils were determined at 15 locations (including croplands, grasslands, and forests) in Fukushima city in the post-rainy season, approximately 4.5 months after the accident, and were compared with those in the pre-rainy season determined in our former study. The 137Cs inventory levels scarcely changed between points in time spanning the first rainy season after the accident. Moreover, the majority of 137Cs remained stored in the aboveground vegetation and in the upper 5 cm of soil layer at undisturbed locations in the post-rainy season. A more quantitative analysis with the characterization of the vertical profile of 137Cs using the relaxation length confirmed that the vertical profile was almost unchanged at most locations. Accordingly, it is concluded that rainfall during the rainy season had a limited effect on 137Cs distribution in the soil, indicating the very low mobility. Chemical extraction of 137Cs from selected soil samples indicated that 137Cs in the soil was barely water soluble, and even the fraction extracted with 1 M ammonium acetate was only approximately 10%. This further supports the low mobility of 137Cs in our soils. Soil mineralogical analyses, which included the identification of clay minerals, suggested that smectite and mica could lower the exchangeable fraction of 137Cs. However, no direct relationship was obtained between mineral composition and 137Cs retention in the upper soil layer. In contrast, positive correlations were observed between 137Cs extractability and soil properties such as pH, organic matter content, finer-sized particle content, and cation-exchange capacity. These results suggest that the mineralogical effect on the firm fixation of 137Cs on soil constituents may be masked by the non-specific adsorption offered by the physicochemical properties of the soils.