Effects of hydrodynamics on the cross-sectional distribution and transport of plastic in an urban coastal river.

The mechanisms of plastic transport in rivers remain an important knowledge gap in global plastic pollution research and management. We investigated how river flows and plastics' properties affect transport with a five-point cross-sectional field study in the Hillsborough River in Tampa (Florida, USA) using a 500-µm Neuston net and an Acoustic Doppler Current Profiler. We conducted in-depth analysis of water velocity profiles as well as plastics' concentrations and properties, determining advective, vertical, and lateral transport fluxes. Under calm flow conditions, advective fluxes were two orders of magnitude higher than lateral and vertical fluxes. Under turbulent conditions, enhanced particle exchange in the cross-section resulted in a three to tenfold increase in lateral and vertical plastic fluxes. The impact of turbulence on plastic particles depended on properties such as size, shape, and composition. This study presents a unique assessment of flow conditions driving plastic pollution in an urban coastal river setting. PRACTITIONERS POINTS: Multipoint, cross-sectional sampling and onsite flow profile collection should be adopted as a common practice for plastic field data collection to reduce uncertainty. Varying flow conditions affect the drivers of plastic transport in rivers. Advective surface fluxes govern plastic transport under calm flow conditions, while turbulent flow conditions enhance cross-sectional mixing and particle exchange. Larger and more irregular-shaped plastics are more affected by turbulence.