Wind-tide coupling as a key driver of microplastic pathways in the Ria de Vigo (NW Spain)

The accumulation and dispersal of marine plastic debris represent significant challenges for coastal systems, where estuarine hydrodynamics, strongly influenced by tides and wind, play a crucial role in their transport and retention. This study aims to investigate the hydrodynamic mechanisms that control the transport, retention, and export of microplastic particles discharged from the main wastewater treatment plant (WWTP) in the Ría de Vigo (NW Spain). The methodology followed comprises the use of the Delft3D modelling system to simulate a set of wind and tidal scenarios designed to identify the key processes governing the dispersion of microplastics in this estuarine system. Results show that floating microplastic particles are rapidly exported from the estuary within 24 h after release, emphasizing the strong influence of estuarine hydrodynamics on particle residence time and transport efficiency. Upwelling favorable winds and neap tide conditions promote the most efficient export of microplastic particles toward the adjacent continental shelf, while downwelling conditions enhance particle retention within the estuary. Additionally, vertical stratification modulates transport efficiency: stronger stratification enhances surface advection, while weaker stratification increases mixing and retention. These findings are essential to improve wastewater discharge management and to mitigate microplastic pollution from WWTPs in estuarine environments, providing a scientific basis for developing more effective strategies to reduce microplastic export from estuarine systems to the adjacent coastal ocean. • Wind-tide coupling controls microplastic retention and export in the Ria de Vigo. • Upwelling with neap tides promotes the most efficient offshore microplastic export. • Spring tides enhance short-term near-field retention near WWTP discharge points. • Lateral velocity gradients drive microplastic accumulation along estuarine margins.