Simulating the impact of estuarine fronts on microplastic concentrations in well-mixed estuaries

Oceanic and coastal fronts are well-documented as accumulators of microplastic debris; however the impact of estuarine fronts 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 exportation to coastal and offshore environments. This study combines high resolution modelling of estuarine processes with realistically parameterized microplastic particles to determine local exposure levels, residence times and temporal variability. We present a validated, three-dimensional, D-Flow Flexible mesh (D-Flow FM) model of a well-mixed estuary (Conwy Estuary, Wales, UK), demonstrating the regular development of an axial convergent front following high tide. A Lagrangian particle tracking model has been applied to simulate the behaviour of microplastic in these frontal systems and analyse how this behaviour may change as a response to various river discharge levels and tidal phases. The results of the ocean model and Lagrangian particle tracking model will be presented. Understanding how estuarine fronts impact microplastic concentration and dispersal within estuaries will increase the accuracy of modelling and in-situ estuarine microplastic studies alike, helping to quantify the contribution of well-mixed estuaries to regional and global microplastic budgets, and bridging the gap between terrestrial and marine environments.