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 ( ¹³⁷ Cs) contamination in soils in a range of terrestrial ecosystems. It is well documented that the interaction of ¹³⁷ Cs 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 ¹³⁷ Cs 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 μm); silt-sized (2-20 μm); sand-sized (20-212 μm); and macroaggregates (212-2000 μm). The fractions were then analyzed for ¹³⁷ Cs content and extractability and mineral composition. In forest soils, aggregate formation was significant, and 69%-83% of ¹³⁷ Cs was associated with macroaggregates and sand-sized aggregates. In contrast, there was less aggregation in agricultural field soils, and approximately 80% of ¹³⁷ Cs was in the clay- and silt-sized fractions. Across all sites, the ¹³⁷ Cs 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 ¹³⁷ Cs in organic-rich (forest and orchard) soils. Our study suggests that soil aggregation reduces the mobility of particle-associated ¹³⁷ Cs through erosion and resuspension and also enhances the bioavailability of ¹³⁷ Cs in soils.
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