Concentration and specific activity of fallout 137Cs in extracted and particle-size fractions of cultivated soils

Fallout 137Cs and stable Cs in soils were separated with two extractants (1 M CH3COONH4 solution and 0.8 M CH3COONH4 in 5% HNO3 solution after H2O2 oxidization). The residue remaining after removal of the oxidizable organic-bound fraction was separated into the particle-size fractions including clay, silt, fine sand and coarse sand with a sieving and sedimentation method. Then, the concentrations of 137Cs and stable Cs in the extracted fractions and the particle-size fractions were determined. The 137Cs contents in the exchangeable and organic-bound fractions in the soil were approximately 10 and 20%, respectively. The 137Cs content in the strongly bound fraction was about 70%, and the concentration of 137Cs in the clay was the richest among the particle-size fractions. The specific activity of 137Cs (concentration ratio of fallout 137Cs/stable Cs) decreased in the order exchangeable, organic-bound and strongly bound fractions. The data suggest that equilibrium between 137Cs and stable Cs was not reached among those fractions, even though most of the 137Cs that had been deposited on the soil was derived from fallout weapons tests that occurred several decades ago. The concentration of 137Cs among the particle-size fractions in each soil was different, whereas the specific activity of 137Cs in the particle-size fractions had a relatively similar value.