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

Water and acetate extractable radiocesium (137 Cs) concentrations were monitored for >400 days in soils that were amended with aqueous 137 Cs or solid organic sources of 137 Cs (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 137 Cs, the water extractable 137 Cs fraction was very low (<1%) and decreased over time while it was below the detection limit in soils amended with solid organic sources of 137 Cs. The acetate extractable 137 Cs in soil amended with aqueous 137 Cs also exhibited an exponential decrease over time (∼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 137 Cs availability, possibly due to the relatively short observation period; (2) 137 Cs ageing (increased binding to soil) was apparent only when 137 Cs was applied in the aqueous form; and (3) both the water and acetate-extractable 137 Cs fractions were greater for aqueous amended than for solid organic amended soils. More acetate extractable 137 Cs 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 137 Cs availability with time from different contamination sources.