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.