Modelling the dispersal of microplastics in estuaries

Abstract

Microplastic pollution has become an emerging contaminant since its
discovery, however it is notoriously difficult to identify, monitor and mitigate.
Due to the difficulty of in-situ measurements and paucity of observations,
numerical modelling has substantially increased our knowledge on the
distribution of marine litter and microplastic in the marine environment.
Estuaries are a major pathway for plastic pollution. They connect terrestrial
with marine environments and are therefore an important consideration
when investigating the movement of microplastic along the source-pathwaysink
continuum. Oceanic and coastal fronts are well-documented as
accumulators of microplastic debris, however, the impact of estuarine fronts,
such as the axial convergent front, and their associated secondary flows on
microplastic concentrations are less well-known. An investigation into the
dynamics of microplastic behaviour within estuarine systems will allow for a
greater understanding of plastic retention and export to coastal and offshore
environments. The aim of this thesis is to investigate the physical controls on
microplastic transport within well-mixed estuaries through using a mixture of
hydrodynamic and particle tracking modelling.

In the first modelling chapter, a hydrodynamic model (Delft3D Flexible Mesh)
is used to simulate a well-mixed estuary and investigate the secondary
flows associated with an axial convergent front. Different scenarios, both
idealized and realistic, are modelled to test how different tidal and hydrological
conditions impact the frontogenesis and frontolysis of an axial convergent
front. This chapter successfully models an axial convergent front in a
realistic estuary model for the first time. The results show that these fronts
are highly dynamic and dependent on past and present estuary state and
geomorphology.

In the second modelling chapter a bespoke particle tracking model has been developed to track the transport of microplastic particles throughout an axial
convergent front and larger three-dimensional estuary simulations. Various
hydrological scenarios are tested with different plastic densities to investigate
their concentrations, transport throughout the estuary, and residence time
within the system. It is shown that axial convergent fronts increase retention of
microplastic in estuaries. This chapter also shows that transport in the Conwy
estuary is highly dependent on the hydrological regime of the river Conwy,
and moderately dependent on the tidal regime. Ultimately, not only does this
chapter clarify the role of estuarine fronts with respect to microplastic transport
but also provides a method to run particle tracking models on flexible model
grids.

The model results of this thesis suggest that secondary flows and similar
physical processes in well-mixed estuaries can increase the retention of
microplastic. These results imply that current popular sampling strategies
may lead to large error margins in regional and global budgets and that
greater consideration of small-scale physical processes is needed to correctly
parameterise microplastic emissions into our oceans.

Date of Award	19 Jun 2024
Original language	English
Awarding Institution	Bangor University
Supervisor	Simon Neill (Supervisor) & Peter Robins (Supervisor)