Radiocesium distribution in aggregate-size fractions of cropland and forest soils affected by the Fukushima nuclear accident

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium (Cs-137) contamination in soils in a range of terrestrial ecosystems. It is well documented that the interaction of Cs-137 with soil constituents, particularly clay minerals, in surface soil layers exerts strong control on the behavior of this radionuclide in the environment; however, there is little understanding of how soil aggregation the binding of soil particles together into aggregates can affect the mobility and bioavailability of Cs-137 in soils. To explore this, soil samples were collected at seven sites under different land-use conditions in Fukushima and were separated into four aggregate-size fractions: clay-sized (<2 mu m); silt-sized (2 20 mu m); sand-sized (20-212 mu m); and macroaggregates (212-2000 mu m). The fractions were then analyzed for Cs-137 content and extractability and mineral composition. In forest soils, aggregate formation was significant, and 69%-83% of Cs-137 was associated with macroaggregates and sand-sized aggregates. In contrast, there was less aggregation in agricultural field soils, and approximately 80% of Cs-137 was in the clay- and silt-sized fractions. Across all sites, the Cs-137 extractability was higher in the sand sized aggregate fractions than in the clay-sized fractions. Mineralogical analysis showed that, in most soils, clay minerals (vermiculite and kaolinite) were present even in the larger-sized aggregate fractions. These results demonstrate that larger-sized aggregates are a significant reservoir of potentially mobile and bioavailable Cs-137 in organic-rich (forest and orchard) soils. Our study suggests that soil aggregation reduces the mobility of particle-associated Cs-137 through erosion and resuspension and also enhances the bioavailability of Cs-137 in soils. (C) 2018 Elsevier Ltd. All rights reserved.