Exposing estuarine regions as microplastic traps: modeling transport pathways and accumulation hotspots in the Minho Estuary (Portugal-Spain)

The ubiquitous presence of microplastics (MPs) in the marine environment has become an increasingly evident crisis. Yet, fundamental questions regarding their origin, transport pathways and fate remain largely unanswered, particularly in estuaries. While international policy frameworks to reduce plastic pollution at their source are negotiated, targeted interventions are critically needed to prevent MPs from entering the ocean. Wastewater treatment plants (WWTPs) serve as essential infrastructures for improving freshwater quality, however, their efficiency in removing MPs is limited. Many WWTPs discharge their effluents into freshwater tributaries, making estuaries important sinks of MPs’ pollution, but the MPs’ dynamics in these systems remains poorly investigated and highly depends on the type of estuary. This study evaluates how varying river discharges and tidal conditions influence the spatial distribution, transport mechanisms and retention of MPs in estuarine environments. Integrating field observations with a Lagrangian particle tracking model coupled to a depth-averaged 2DH hydrodynamic model, the Minho Estuary (Portugal) is used as a case study to observe how contrasting hydrodynamic conditions affected the dynamics of MPs. The numerical models explicitly resolved the distribution of MPs across the lower estuary, revealing key mechanisms controlling MPs’ transport, determining whether the estuary functions as source or sink, and identifying concentration hotspots persistent across all conditions. While the study provides further evidence of the extensive presence of MPs in these systems, the findings demonstrate that turning this environmental crisis into targeted solutions represents scalable intervention strategies with direct, meaningful impacts on the reduction of marine MPs in the environment.