Distribution and mobility of radiocesium in relation to the clay fraction mineralogy and soil properties in the Iput River floodplain

The role of the mineralogy of the clay fraction and the physicochemical properties of alluvial soils in the floodplain of the Iput River and its tributary the Buldynka River (in the region of the settlement of Starye Bobovichi in Bryansk oblast) in the distribution and immobilization of radioactive isotope 137Cs from the atmospheric fallout after the Chernobyl accident was studied. The soils had a sandy texture; a significant variation in the content of amorphous iron oxides (0.1–0.77%) and labile manganese (11.2–193 mg/kg), the cation exchange capacity (6.1–54.2 meq/100 g soil), and the base saturation (29–100%) was common; an appreciable content of X-ray amorphous mineral substances in the clay fraction (<1 μm) enriched with organic carbon (7.7–13.1%); the predominance of trioctahedral hydromicas (Me=50%) in the clay fraction; and the presence of fine-disperse quartz and lepidocrocite. The specific activity of the 137Cs in the clay fraction of the moderately and strongly contaminated layers increased with the increasing portion of smectite formations and (or) hydromicas. On the whole, the presence of the clay fraction favored a decrease in the 137Cs mobility (the correlation between its content and that of exchangeable cesium was r=−0.608, n=17). However, the portion of exchangeable radiocesium (extracted with 1 M CH3COONH4, 1:10) had a tendency toward an increase with increasing content of hydromicas in the clay fraction. Thus, the minerals of this group were a potential source of exchangeable 137Cs in the soils. The significant role of amorphous and mobile iron forms in the immobilization and migration of radiocesium in the secondary contaminated horizons of the alluvial soils was revealed.