Modelling the conveyance of plastic pollution in rivers: Is storage within fluvial systems the key to the missing ocean plastic conundrum

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Overview:Plastic pollution of the environment is a major global concern, generating negative impacts on wildlife, their habitats and environmental aesthetics (Welden, 2020). Rivers are widely recognised as major corridors for the transfer of discarded plastic waste within terrestrial landscapes. Estimates of the global riverine flux to oceans range from 1.15-2.41 (Lebreton et al., 2017) and 0.41-4 Mt yr-1 (Schmidt et al., 2017). Relatively little, however, is known about how plastic moves through river systems. Recent research suggests that rivers act as 'jerky conveyor belts' for plastic conveyance, just as they do for sediment (Ferguson, 1981). For example, macroplastics (> 5 mm) can be trapped by vegetation and other channel obstructions (Schreyer et al., 2021; Newbould et al., 2021) and micro- (1 m - 5 mm) and nano-plastics (< 1 m) can accumulate and be temporarily stored in channel bed sediments (Hurley et al., 2018). This suggests that the transport of plastic debris in rivers can be conceptualised as a series of discrete 'steps' between sites of temporary storage, the dynamics of which will exhibit complex interdependencies between river geomorphology and hydraulics, catchment hydrology, riparian vegetation, and plastic characteristics (e.g. size, morphology, buoyancy, density, biofouling). Newbould et al., (2021) demonstrated that the transfer of buoyant macroplastic litter through a 1 km reach can be modelled probabilistically with stranding and release events at trapping points described by a stochastic process. The model was calibrated using field tracer data and offers a potential approach for modelling plastic conveyance through river channel networks that explicitly accounts for the transport behaviours of different types of plastic waste.The aim of this study is to develop, calibrate and test a more general probabilistic catchment scale model of plastic conveyance. This will increase our understanding of plastic fluxes and stores in river systems leading to improved predictions of plastic behaviour. Ultimately, this improved understanding could be the key to closing the gap on the disparity between predicted emissions of plastics to the environment and the estimated stock of plastic in the world's oceans (the so-called "missing plastic" question: Cózar et al., 2014; Weiss et al., 2021).