Plastic pollution impacts riverbed sand transport processes

Rivers are the primary link between terrestrial and ocean environments, crosscutting the landscape whilst providing fresh water, nutrients, and sediment to diverse ecosystems. However, over the past 50 years, rivers have become increasingly significant vectors for plastic pollution. On a riverbed, sediment migrates downstream as bedforms, such as dunes, via well-understood morphodynamic processes, yet the impact of plastic on sediment transport behaviours is unknown and so has been widely assumed as passive, whereby the sediment buries plastic between flood events and is otherwise unaffected. Here we find, through undertaking studies using an experimental recirculating flume tank, that when plastic particles are introduced to riverbed sand dunes, even at relatively low concentrations, novel morphologies and altered morphodynamic processes emerge, including irregular stoss-side erosion and dune wash out. We detail new mechanisms of plastic sequestration and transport to outline how plastic particles interacting with riverbed dunes fundamentally influence sediment transport processes, and the resulting deposits. We find that: i) plastic is not a passive component on riverbeds as it significantly speeds up morphological transformations, affecting bed topography and increasing dune erosion rates, which at present has unknown consequences for the wider landscape; ii) plastic inclusion locally changes the ratio of suspended load to bedload material as plastics create a local and temporal shift towards more sediment in suspension, thereby causing the river to develop more conduit-like than storage-like properties with unknown consequences for overall sediment transport fluxes and increased local turbidity; and iii) inclusion of plastic in the sediment layer creates heterogeneous deposits that propagates the disruption of sedimentary processes and forms irregular distribution of plastic on the riverbed that will affect the possibilities of representative sampling. Such insights shed light onto a new branch of environmental consequences of plastic in the environment that requires further research, as a new branch of sedimentology: plastic and sediment interactions. With plastic being continually added to our environments globally, this new field is set to be increasingly relevant amongst emerging challenges of the Anthropocene.