U and Th speciation in river sediments

A study of the distribution of some natural radionuclides in different chemical fractions of river bottom sediments has been carried out. The study has shown that the majority of the total U in sediments is located in the non-residual fractions, while Th is more likely to be present in the residual ones. This difference between U and Th reflects largely a much higher mobility of U relative to Th in surface and near-surface environments. Coprecipitation with amorphous ferromanganese oxyhydroxides is the main process of U and Th incorporation from the water column into the soil particles. Moreover, the distribution of the radionuclides and the analysis of the Th/U mass ratios in different chemical fractions of sediments has made an unequivocal connection between the enhanced U content in river sediments and the wastes discharged into the rivers by the operation of fertilizer industries.