Defining the controls on microplastic settling in river systems to predict areas of ecological risk to microplastic pollution

Abstract:

The majority of marine plastic pollution originates from land-based sources with the dominant transport agent being riverine. Despite the widespread recognition that rivers dominate the global flux of plastics to the ocean, there is a key knowledge gap regarding the nature of the flux, the behaviour of microplastics (<5mm) in transport and its pathways from rivers into the ocean. To predict transport, fate and biological interactions of microplastics in aquatic environments at a global scale, the factors that control these processes must be identified and understood. Currently, there remains a large knowledge gap around prediction of microplastic transport in rivers, especially in regards to how biofilm formation influence particle settling velocities. This prevents progress in understanding microplastic fate and hotspot formation, as well as curtailing the evolution of effective mitigation and policy measures.

A settling experiment was therefore undertaken to understand how different factors, including salinity, suspended sediment concentration and biofilm formation influence microplastic particle settling velocity. The results presented herein explore the role of biofilms on the generation of microplastic flocs and the impact on buoyancy and settling velocities. Five different polymers were tested and compared including fragments and fibres. Settling velocities were then combined with observed flow velocity data from the Mekong River, one of the top global contributors to marine plastic pollution, allowing predictions of areas of microplastic fallout and hotspots. The results highlight potential areas of highest ecological risk related to the dispersal and distribution of microplastics across the river-delta-coast system including the Tonle Sap Lake. Future work involves supporting predicted hotspots with aligned fieldwork from the Mekong River that details the particulate flux and transport of microplastic, throughout the vertical velocity profile.