Variability of Fallout Radionuclides in River Channels: Implications for Sediment Residence Time Estimations

Fine sediment plays an important role in the healthy functioning of river ecosystems providing nutrients and contributing to habitat functioning. However, excessive sediment supply into rivers has several detrimental impacts on water quality and it causes sedimentation in river channels, reservoirs and estuaries. In addition, silts and clays are geochemically active and consequently are responsible for the transport of contaminants, including trace metals, phosphorus, pesticides and radionuclides among others which have high sorptive affinity for fine-grained particles. Hence, quantifying the timescales of sediment transfer throughout a river system is critical for understanding river basin sediment dynamics and the fate of their associated pollutants.

Fallout radionuclides (⁷Be, ²¹⁰Pb_ex and ¹³⁷Cs) have been used to assess sediment travel distances, sediment age and sediment residence times in a variety of landscapes. An advantage of using these radionuclides as sediment chronometers is their half-lives which can be used to model sediment residence time from days to decades in different catchment compartments.

The River Avon (Devon, UK) is a 40 km long gravel-bed river, draining rough moorland and with a catchment area of 110 km². The mean annual flow is 3.7 m³ s^-1 and is moderated by managed discharges from a reservoir upstream. Suspended and channel bed sediments were sampled in a 5 km section of the river during four seasonal surveys (January, March, July and November 2022) and suspended sediments during a stormflow event were also sampled.

Radionuclide activity concentrations of channel deposited sediments varied substantially within and between river bars and seasonally. Suspended sediment activity concentrations varied within the stormflow hydrograph and seasonally. Relationships between radionuclide activity concentrations and sediment storage, particle size, total organic carbon and C:N ratios were also evaluated. Channel sediment residence times obtained using ⁷Be/²¹⁰Pb_ex activity ratios ranged between 0 to 110 days, reproducing the high variability found in activity concentrations. Future research will assess the influence of sediment sources on ⁷Be/²¹⁰Pb_ex ratios and the relationship between sediment storage dynamics and sediment-bound contaminants. Sediment residence time modelling will allow an improved understanding of sediment dynamics in gravel-bed rivers which is essential to inform management decisions and prediction of the timescales of transfer and fate of associated contaminants.