Speciation and mobility of uranium isotopes in the Shu River: impacts for river to sea transfer

Fresh water outflows to sea are a source of marine contamination by radionuclides, most originating from nuclear industry operations. Usually, these lead to small amounts of radionuclides entering the sea, for example, because of the discharge of cooling water. However, under accident conditions large amounts of radionuclides may enter river systems. The extent of radionuclide transport along freshwater systems and their subsequent dispersal in the ocean depends upon their speciation. This manuscript examines the speciation of uranium, with particular reference to sulphate complexes, along the Shu River in Central Asia with a view to predict its transport along such rivers to receiving seas. The speciation of uranium isotopes in the Shu River (at the border of Kazakhstan and Kyrgyzstan) was determined. Calculations were based on the measured concentrations of Ca2+, Mg2+, Na++K+, HCO3 −, Cl−, SO4 2−, and of uranium isotopes. These calculations were determined by either titrimetric methods, gravimetric methods or, for uranium isotopes, by alpha-particle spectrometry. Almost all uranium was present as one of three soluble complexes, either [UO2OH]+, [UO2(CO3)2(H2O)2]2− or [UO2(CO3)3]4−. However, despite the significant concentration of sulphate-ions in the water, especially for the Lower Shu in Kazakhstan, the probability of uranium being present as of sulphate complexes in the analyzed water was very low. The presence of soluble ions is consistent with their mobility along freshwater systems and their transfer to the marine environment. No evidence of an effect from influxes of sulphate was identified.