Using a novel tracing-tracking framework to source fine-grained sediment loss to watercourses at sub-catchment scale

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Although traditional sediment tracing approaches provide valuable information for characterising key generic sediment sources, Catchment Officers working as part of the Catchment Sensitive Farming initiative frequently require more detailed evidence to permit improved targeting of mitigation. Accordingly, a novel framework combining conventional sediment source fingerprinting and a dual signature tracking method has recently been used to improve sediment source information for the River Glaven (˜115 km2) priority catchment in eastern England. Conventional geochemical tracing incorporating a number of recent refinements to the mass balance modelling, including the combined use of local and genetic algorithm optimisation, was used to provide information on the average median relative contributions (±95% confidence limits) of generic sediment sources in three sub-catchments, categorised as grassland (1 ± 1%–12 ± 1%) or arable (25 ± 1%–46 ± 1%) surface soils, damaged road verges (2 ± 1%–50 ± 1%) and channel banks/subsurface sources (20 ± 1%–50 ± 1%) and to provide a framework for the spatial extrapolation of tracking data. Particle tracking using fluorescent-magnetic grains was used to provide preliminary sub-catchment scale information on sediment loss from key components of the primary arable topsoil and channel bank generic sources, characterised, respectively, as wheelings (18 ± 1%–33 ± 1%) or inter-wheelings (7 ± 1%–13 ± 1%) and poached (19 ± 1%–47 ± 1%) or fluvially eroded (1 ± 1%–3 ± 1%) channel margins. The insertion of high-strength magnets in watercourses ensured that the tracking component of the novel framework links sediment loss from labelled areas to river channels as opposed to providing information to edge-of-field only. Uncertainty and prior information on source contributions are explicitly recognised by the framework. This study represents the first in the UK to link wheeling sediment losses to river channels. The findings underpin the importance of compacted areas used for trafficking as sources of, and conduits for, sediment delivery in the lowland agricultural landscape of the UK.

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