Transport et piégeage des microplastiques dans un estuaire fortement turbide et dominé par la marée

Microplastics are an emerging pollutant in aquatic systems, with estuaries acting as key zones for their retention and transformation. However, limited field observations, complex estuarine hydrodynamics and diverse particle properties hinder a comprehensive understanding of microplastic transport and fate, limiting accurate risk assessment and evaluation of environmental impacts. The objective of this work is to better understand the physical processes governing the transport and trapping of microplastics in macrotidal turbid estuaries, using the Gironde estuary (SW France) as a case study. The methodology of this work is mainly based on a hydro-sedimentary numerical model coupled with a Lagrangian particle tracking model. This approach is complemented with in-situ observation data. A comprehensive review of process-based modelling approaches used to study microplastic dynamics in estuaries was first conducted to assess various parameterization strategies, identify key challenges, and offer recommendations and future directions to advance microplastic modelling strategies in estuaries. Building on insights from this review, the relative influence of estuarine physical processes on microplastic transport was examined through sensitivity scenarios using different release configurations. The results identify the shoreline interaction by beaching– refloating dynamics as a key process for buoyant particles, while resuspension and vertical mixing modulate the transport and vertical distribution of non-buoyant particles. Microplastic-sediment interactions, such as flocculation and temporary trapping in bottom sediments, play an important role in enhancing particle retention within the estuary. Model results also show that hydrodynamic processes alone can significantly trap microplastics, with seasonal variability modulating the intensity and location of trapping. Elevated river discharge during the spring season enhances seaward transport, particularly for buoyant particles, whereas in summer, microplastics are more likely to be retained, with denser particles accumulating near the estuarine turbidity maxima (ETM) in the tidal rivers. This accumulation forms a water-column estuarine microplastic maximum (EMPM), sustained by net upstream transport driven by tidal pumping. In-situ observations in the water column support these findings, confirming the presence of strong near-bottom microplastic concentrations in summer in the Garonne tidal river, particularly during strong flood and ebb current velocities, with a dominance of high-density fibrous particles. Model simulations also indicate that floating particles are consistently trapped along a frontal line near the main channel, generating a surface EMPM. In the upper estuary, this line of particle accumulation follows the primary convergence zone produced by the combined effects of tidal currents and estuarine bathymetry. However, in the middle estuary, the accumulation line shifts to a secondary convergence zone due to the combined effect of morphological features and the alternance between convergence and divergence patterns over the tidal cycle. Sensitivity tests confirm that baroclinic effects play a significant role in shaping frontal convergence, with sediment-induced water density modulating its strength. Overall, the results highlight that tide-dominated, highly turbid estuaries act as efficient microplastic retention zones due to the combined influence of tidal hydrodynamics, sediment-microplastic interactions, and morphological features.