Uranium series disequilibrium in river sediments and waters: the significance of anomalous activity ratios

Samples of river water, suspended and bottom sediments from the Fenland catchment system draining into the Wash, U.K., have been analysed for the longer-lived U/Th series radionuclides ( 238U, 234U, 230Th and 232Th). The aim of this work was to assess the effects of open-system phase-transfer on the U series disequilibria observed in the river waters and sediments and to investigate the controls exerted on this phase-transfer by the hydrological regime. Two important aspects of the U series radionuclide mobilisation in this modern river system have been observed: (1) U series radionuclides are in a continuous state of flux between river sediments and river water; and (2) U sorption on sediment particles is seasonably variable. Sequential extraction of whole sediments revealed that the U-series isotopic characteristics of the ion-exchangeable, organic-bound, carbonate, amorphous Fe Mn oxyhydroxides and crystalline Fe Mn oxides are controlled to a significant degree by the acquisition of U isotopes from the aqueous phase. During summer when surface run off and river discharges are generally low, the U isotope acquisition by sediment particles results in “anomalously” high 234U/ 238U activity ratios (>1). This is of importance for studies which rely upon the isotopic and elemental separation of U series radionuclides in the weathering system i.e. mass balance, weathering rate and sediment yield assessments.