Synergy between plastic pollution andsediments within river systems

Between 2019 and 2060 global plastic production is expected to triple, leading to mounting challenges associated with plastic pollution and its leakage into the environment. River environments are major sinks and conveyors of plastic pollution from land-based sources to the ocean, however, our current description of how plastic pollution behaves and is transported in these systems is poorly understood. To address these unknowns, this thesis aims to provide fundamental descriptions of microplastic (MP) (plastics < 5mm in size) and macroplastic (MaP) (plastics > 5mm in size) transport in river flows, as well as to evaluate the applicability of sediment transport theories for plastic transport. This includes conducting a systematic metaanalysis of existing field-based MP studies (Chapter 2), as well as laboratory-based (Chapters 3, 4 and 5) and field-based experiments (Chapter 6) that focus on understanding the transport mechanisms of plastic particles in bed load, suspended load and vertical transport. Results of this thesis show that bed load saltation trajectory characteristics of spherical MPs and amber (used as a proxy for natural sediment) were statistically similar and analogously described by the Rouse number, which was derived for the transport of sediment particles, suggesting that sediment transport theory could be directly applied as a foundation for spherical MPs transport (Chapter 3). For MaP, negatively buoyant polystyrene cups and fragments exhibited unique settling orientations, due to their geometric anisotropy, which resulted in a multimodal settling velocity, differing from theories for the vertical transport of sediment grains (Chapter 4). The vertical concentration profile of these plastic cups in turbulent riverlike flows was predicted by the Rouse profile for suspended sediments, within an accuracy of 10% in high turbulence conditions, relative to the particle’s settling velocity, which provided a potential method to predict plastic concentration in rivers (Chapter 4). It was also shown that vi biofilm colonisation significantly altered the settling dynamics of MaP plates, with biofouled plates exhibiting more chaotic trajectories, larger horizontal dispersion, and higher oscillatory frequencies, compared to their pristine counterparts (Chapter 5). Finally, a field-based experiment described the variability of MP concentration in a rural river (Taff Bargoed, Wales, UK), sourced from the largest opencast coal mine in the UK (Chapter 6). Significant relationships between MP concentration, antecedent rainfall, river discharge and total suspended solids were observed, showing that instream MP concentration was correlated with the hydrological and hydraulic properties of the river. The findings from this thesis provide some fundamental transport processes for plastics in rivers, which can improve global predictions of riverine plastic budgets, aid in implementing effective mitigation policies, and inform the design of instream plastic clean-up strategies.