Understanding the complexity of sediment residence time in rivers: Application of Fallout Radionuclides (FRNs)

Riverine sediments play an important role in the healthy functioning of river ecosystems as they provide nutrients and a connectivity signal throughout the catchment sediment cascade. However, excess sediment supply to rivers can have several detrimental impacts on water quality, availability and ecology. The application of catchment management practices requires a comprehensive understanding of both spatial and temporal sediment dynamics to tackle point and diffuse river pollution. While the sources, pathways and fate of eroded sediment in river systems have been widely studied, temporal dynamics have received less attention, mainly due to the complexity of the processes and the lack of methods available to assess these dynamics. This contribution reviews the application of Fallout Radionuclides (FRNs) as sediment residence time tracers. We explore their suitability as sediment chronometers in rivers, the relevance of sediment residence time to sediment budgeting, and discuss the current models that have been employed to determine sediment residence time in river systems. Our review also identifies the challenges, opportunities and the future research needs for a comprehensive application of FRNs to evaluate sediment residence time. In evaluating approaches to sediment residence time, we have summarised several pitfalls requiring consideration and identified avenues for further research. For instance, attention should be given to sorption behaviour when using 7Be and 137Cs as residence time tracers in rivers under changing environmental conditions; particle size effects; activity concentration dilution by mixing of newly tagged 7Be sediment with 7Be-poor sediment from older or different sources; source controls on 7Be delivery to rivers; and the influence of direct fallout into stream channels. Finally, further research is needed to assess the influence of environmental and anthropogenic factors on sediment residence time such as land use, topography, flow regimes, soil type, soil erosion measures and climate change, all of which have important implications from a catchment management perspective