The role of estuarine axial convergent fronts in microplastic dispersal.

Microplastic transport through estuaries represents a critical and poorly understood pathway from catchments and wastewater sources to coastal zones. Microplastics can host microbial communities including pathogenic microorganisms that pose serious public health risks for coastal activities. This study investigated the role of estuarine hydrodynamics on microplastic dispersal in the Conwy estuary, UK, characterised by density-driven secondary flows that can retain microplastics, interspersed with riverine floods that export microplastics seaward. Using the Delft3D hydrodynamic model, parameterised and validated with observational data, we simulated the Axial Convergent Front (ACF) under a range of tidal regimes and river inputs. The ACF formed regularly, lasting between 45 minutes and 2 hours, in the mid- and upper-estuary (where salinity was greater than 3 ppt) depending on the saline intrusion that controls the lateral density gradients driving the front. Its strength varied, with cross-channel velocities of between 3.6 (during low river flows) and 4.7 cm s$^{-1}$ (during high river flows), i.e., 10-20\% of the instantaneous longitudinal flow, but tended to not form on channel meanders. Coupling Delft3D to a Lagrangian particle tracking model, we showed that the ACF acts as an additional retention process for microplastics within estuaries, contributing to estuarine pollution sinks. However, high river flow events disrupted ACF formation, and promoted microplastics transport offshore. Limited sorting of microplastics of different densities was simulated because the estuary is shallow and well-mixed with retention times of several days. These results emphasise the necessity of fine-scale three-dimensional modelling, comprehensive microplastic sampling, and improved knowledge of microbial aggregation processes, to accurately represent microplastic dispersal through estuaries. This study advances our understanding of estuarine processes and marine pollution to inform risk assessments and mitigation for microplastic pollution and associated health risks.