Fine sediment-associated contaminants in gravel bed rivers: Evaluating storage times and turnover using fallout radionuclides (FRNs).

Excess fine sediment supply and its associated contaminants can have detrimental effects on water quality and river ecology with sediment deposition on, and subsequent infiltration in, streambeds impacting riverine habitats. Fallout radionuclides (FRNs) are used as tracers in aquatic systems, and the Be/Pb ratio is a useful indicator for sediment residence/storage time. Suspended and submerged mid-channel bar sediments were collected during five surveys within a 5 km reach of a typical temperate lowland agricultural river system. Solids were analysed by gamma (Be and Pb) and inductively coupled plasma (ICP, trace metals and phosphorus) spectrometry, and analysed for total nitrogen and organic carbon, to assess sediment dynamics and associated contaminant and nutrient storage. Significant spatial and temporal variation in Be/Pb activity ratios was observed, indicating changes in sediment sources closely related to contaminant inputs from legacy mining and agriculture. Storage times and the proportion of recently deposited sediment (RDS) varied between sampling sites and seasons in response to local channel characteristics and floods, which also influenced particulate contaminant distributions. This study demonstrates that FRN technology offers improved understanding of fine sediment and contaminant storage and turnover in river channel systems, which is vital to aid sediment management, river restoration and to tackle the global challenge of siltation and associated pollution in riverine habitats.