Physical drivers of transport, dispersion and trapping of microplastics in a macrotidal, hyper-turbid fluvial–estuarine system: A modelling approach

Estuaries serve as both reservoirs and gateways for microplastics, yet the processes that control particle trapping, release, and redistribution are still insufficiently understood. This study assesses the relative importance of key physical processes in a macrotidal, hyperturbid Gironde estuary, influencing the transport and trapping of floating and settling microplastics - namely, interactions with shorelines, vertical mixing, effect of water density on particle buoyancy, interactions with bottom sediments, and microplastic-sediment flocculation and examines seasonal transport trends. Results show that floating microplastics are mainly exported seaward, although a fraction remains trapped as a longitudinal line in the middle estuary. The intensity of microplastic export and accumulation varies seasonally with river discharge. Shoreline interactions via beaching-refloating dynamics, along with flocculation with fine sediments, represent the dominant processes modulating the distribution and fate of floating microplastics. In contrast, settling microplastics exhibit limited export to the ocean and remain confined within the estuary. Although this retention is primarily governed by hydrodynamics, interactions with bottom sediments through (temporary or permanent) deposition and resuspension, as well as intra-tidal variability in vertical diffusivity, have a relatively stronger influence on the transport and distribution of settling microplastics. Two distinct accumulation zones, or Estuarine Microplastic Maxima (EMPM), are identified: a surface EMPM, formed by low-density floating particles associated with surface convergence, and a water-column EMPM of denser particles in suspension near the Estuarine turbidity maxima (ETM) linked to tidal pumping. Overall, macrotidal estuaries act as significant sinks for microplastics, where hydro-sedimentary dynamics govern particle retention and redistribution.