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

Abstract

Water and acetate extractable radiocesium (¹³⁷Cs) concentrations were monitored for >400 days in soils that were amended with aqueous ¹³⁷Cs or solid organic sources of ¹³⁷Cs (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 ¹³⁷Cs, the water extractable ¹³⁷Cs fraction was very low (<1%) and decreased over time while it was below the detection limit in soils amended with solid organic sources of ¹³⁷Cs. The acetate extractable ¹³⁷Cs in soil amended with aqueous ¹³⁷Cs also exhibited an exponential decrease over time (∼55%-30%) but, remained higher than in soils amended by solid organic sources of ¹³⁷Cs which remained stable (ranging from 2% to 15%). These results collectively indicate that: (1) drying and wetting cycles have little impact on ¹³⁷Cs availability, possibly due to the relatively short observation period; (2) ¹³⁷Cs ageing (increased binding to soil) was apparent only when ¹³⁷Cs was applied in the aqueous form; and (3) both the water and acetate-extractable ¹³⁷Cs fractions were greater for aqueous amended than for solid organic amended soils. More acetate extractable ¹³⁷Cs 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 ¹³⁷Cs availability with time from different contamination sources.

Keywords: Ageing; Degradation; Drying and wetting; Organic matter; Radiocesium.