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

Effect of intense rainfall on the distribution of Fukushima-accident-derived Cs-137 in soil was examined. Inventories and vertical distributions of Cs-137 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 Cs-137 inventory levels scarcely changed between points in time spanning the first rainy season after the accident. Moreover, the majority of Cs-137 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 Cs-137 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 Cs-137 distribution in the soil, indicating the very low mobility. Chemical extraction of Cs-137 from selected soil samples indicated that Cs-137 in the soil was barely water soluble, and even the fraction extracted with I M ammonium acetate was only approximately 10%. This further supports the low mobility of Cs-137 in our soils. Soil mineralogical analyses, which included the identification of clay minerals, suggested that smectite and mica could lower the exchangeable fraction of Cs-137. However, no direct relationship was obtained between mineral composition and Cs-137 retention in the upper soil layer. In contrast, positive correlations were observed between Cs-137 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 Cs-137 on soil constituents may be masked by the non-specific adsorption offered by the physicochemical properties of the soils. (C) 2013 Elsevier B.V. All rights reserved.