Variability of fallout radionuclides (FRNs) in river channels: implications for sediment tracing

Purpose: The aims of this research were to assess the sources of variability of particulate FRNs ( ⁷Be, ²¹⁰Pb _ex and ¹³⁷Cs) in river channels, the influence of sediment properties such as particle size distribution (PSD) and organic matter (OM) on FRN distributions, and to discuss the implications for sediment tracing in rivers. Methods: Suspended and channel bed sediment samples were collected in the River Avon (Devon, UK) during five strategic surveys involving a wide variation of river flows, including flood conditions. Particulate matter was analysed for ⁷Be, ²¹⁰Pb _ex and ¹³⁷Cs by gamma spectrometry, PSD by laser diffraction, and organic constituents, total organic carbon (TOC) and total nitrogen (TN) by elemental analysis. Results: FRNs activity concentrations vary significantly both spatially, characterised by changes in activity concentrations within and between locations, and temporally, with changes during the storm hydrograph and between seasons. Variability was attributed to changes in sediment sources and, on some occasions, to significant correlation of activity concentrations with sediment properties. The results also highlighted the influence of changes in channel characteristics and the magnitude and frequency of floods on FRN distributions. Conclusion: In the context of sediment tracing, attention should be given to seasonal changes in riverine conditions that have the potential to affect ⁷Be and ¹³⁷Cs conservativeness. Application of FRNs in sediment fingerprinting studies should be accompanied by appropriate temporal characterisation of potential sediment sources. Finally, potential contribution of ⁷Be-depleted sediment from channel resuspension to the suspended load should also be considered.