Lagrangian approach applied to microplastic transport in a Tide dominated highly turbid estuary

– Betty John –

Résumé : 

Tide dominated estuaries are highly dynamic environments where tidal forcing, freshwater discharge, suspended sediment dynamics, and bathymetric and morphological features interact to control the transport and retention of particulate matter. In highly turbid systems, these processes generate strong spatial and temporal variability in particle distribution, including zones of trapping, resuspension, vertical exchange and accumulation. Understanding these physical mechanisms is therefore essential for describing the transport and fate of particles such as microplastics.

This work investigates the physical processes controlling microplastic transport in a tide dominated, highly turbid estuarine system using a coupled hydro-sedimentary-Lagrangian modelling framework. The three- dimensional hydro-sedimentary numerical model resolves the hydrodynamic and suspended sediment fields, which are used to drive a Lagrangian particle tracking model to simulate particle trajectories, transport pathways and retention under varying hydrodynamic and sedimentary conditions. The modelling framework is complemented by in-situ observations of hydrodynamics, turbidity and microplastics.

The results demonstrate that particle transport is strongly coupled to estuarine hydrodynamics and suspended sediment dynamics. Tidal currents and associated convergence–divergence structures, shaped by estuarine morphology govern horizontal particle pathways, while vertical mixing, settling, and resuspension control exchange between water column and bed. Particle density determines distinct transport regimes: buoyant particles are governed by surface convergence and shoreline interactions like beaching–refloating, whereas non-buoyant particles are strongly controlled by turbulent vertical mixing, bed shear stress, and resuspension–settling cycles. Particle-sediment interactions and temporary bed trapping further enhance retention and prolong residence times within the estuary. Lagrangian simulations further reveal that tidal pumping, a key mechanism driving Estuarine Turbidity Maxima (ETM) formation, together with frontal convergence, generate accumulation zones for the particles that evolve over the tidal cycle and shift with changes in river discharge and sediment-induced density gradients. These processes contribute to the formation of persistent microplastic accumulation zones in both the water column and at the surface.

Overall, the study demonstrates that the particle transport in a tide-dominated, highly turbid estuary is governed by the interplay between tidal hydrodynamics, estuarine morphology, and sediment dynamics. The Lagrangian framework provides a powerful approach to resolve particle pathways, identify the physical mechanisms driving accumulation and retention. Microplastics offer a useful application to investigate how particles with contrasting physical properties respond to complex estuarine processes.