Impact of drying and wetting cycles on 137Cs ageing in forest soils contaminated with different input forms

Water and acetate extractable radiocesium (Cs-137) concentrations were monitored for > 400 days in soils that were amended with aqueous Cs-137 or solid organic sources of Cs-137 (plant litter or fragmented organic materials) and subjected to a series of wet-dry cycles. The soils were collected from broadleaf and cedar forests in Fukushima, Japan. In soils amended with aqueous Cs-137, the water extractable Cs-137 fraction was very low (< 1%) and decreased over time while it was below the detection limit in soils amended with solid organic sources of Cs-137. The acetate extractable Cs-137 in soil amended with aqueous Cs-137 also exhibited an exponential decrease over time (similar to 55%-30%) but, remained higher than in soils amended by solid organic sources of 137 Cs which remained stable (ranging from 2% to 15%). These results collectively indicate that: (1) drying and wetting cycles have little impact on Cs-137 availability, possibly due to the relatively short observation period; (2) Cs-137 ageing (increased binding to soil) was apparent only when Cs-137 was applied in the aqueous form; and (3) both the water and acetate-extractable Cs-137 fractions were greater for aqueous amended than for solid organic amended soils. More acetate extractable Cs-137 was observed in soils contaminated with broadleaf materials compared to their cedar counterparts, which may be linked to the nature of the organic material itself. For natural conditions, such kind of information is useful to improve our understanding of the evolution of Cs-137 availability with time from different contamination sources.