In recent years, concerns about marine pollution have grown significantly, with particular emphasis on the increasing presence of plastic waste, as highlighted by . These synthetic materials, commonly referred to as plastic debris, enter the oceans primarily through rivers due to inadequate waste management systems originating from multiple sources . Studies indicate that a significant portion of the plastic waste discharged through rivers eventually returns to nearby beaches, acting primarily as a source of local pollution rather than long-distance transport agents . This widespread pollution travels through rivers and coasts, eventually reaching seas and oceans, posing a serious threat to marine ecosystems and aquatic species . Plastics can remain afloat in the marine environment for prolonged periods, persisting for several years, and are also distributed across ocean depths . In particular, the issue extends on different scales, from macroplastics (with a diameter greater than 5 mm) to microplastics (with a diameter less than 5 mm) . Microplastic particles are particularly worrying since they harm both marine habitats and human health . As noted by and , tracking these smaller particles presents significant challenges, underscoring the importance of prioritizing preventive measures to reduce macroplastic pollution.

The movement of marine plastics through ocean currents remains poorly understood due to the influence of diverse spatial, temporal, and biogeochemical factors, as well as variations in plastic size and density . According to , there is a pressing need for further studies to clarify these complex dynamics. Research on marine litter employs a variety of methods, including visual surveys, trawl nets, and drift traps, which capture floating debris and enable its categorization by size . Remote sensing tools, such as satellite imagery and drones, have also proven effective for identifying high-density litter zones . In recent years, numerical modeling of plastic dispersion has advanced through the use of hydrodynamic models that simulate the pathways of plastics in oceans and rivers . Plastic transport can be modeled using passive Lagrangian tracers, which account for the rise or sinking of particles due to buoyancy and hydrodynamic forces such as Stokes drift and wind . Windage refers to the impact of surface winds on the parts of floating debris exposed above the ocean surface, and studies have shown that it can significantly influence the paths of large objects . The impact of windage is determined by size, shape, and buoyancy of debris, with significant variability in the coupling strength between local wind conditions and the resulting windage velocity of the object .

The primary objective of this study is to investigate the influence of wind on the accumulation patterns of river-transported plastic debris in an estuarine, Ría de Arousa. Using Lagrangian particle tracking models, this study examines how wind conditions affect the distribution of floating particles, comparing accumulation along the shoreline. Additionally, the study explores the correlation between beached particles and river flow rate, as well as the residence time of debris within the estuary. By leveraging numerical simulations, this work provides insights into the key drivers of plastic retention and dispersion, contributing to a better understanding of pollutant transport and more effective mitigation strategies.

Figure  illustrates the mean accumulation patterns in the Ría de Arousa for winter (DJF) and summer (JJA) seasons, for two different wind drag coefficients: 1 % (left panels) and 5 % (right panels). For both seasons, lower wind drag (1 %) results in significantly higher nearshore and embayment accumulation, especially in the northern part of the estuary due to Coriolis force . This North–South assymetry and its modulation by wind forcing are consistent with the results of a non-stationary 2D hydrodynamic model of the Ría de Arousa, which explicitly incorporates Coriolis effects on residual circulation and demonstrates their coupling with wind and stratification . Then, surface outflow of river water is stronger on the northern margin favouring beaching.

When the wind coefficient increases to 5 %, the overall particle concentration is reduced in the inner region and the accumulation becomes more diffuse, particularly during summer given that the north winds blow the particles out of the estuary. Particles generally drift towards southern regions with increased windage. Beached particles have not been taken into account when creating the concentration maps since they would distort the particle concentration in the sea. These results highlight the significant seasonal and geographical fluctuations in particle retention caused by wind forcing.

Beached particle distribution in the northern (red bars) and southern (blue bars) coasts of the Ría de Arousa for winter (DJF), spring (MAM), summer (JJA), and autumn (SON), for three different wind drag coefficients: 1 %, 3 %, and 5 % is shown in Fig.  as the percentage of beached particles out of all emitted in each season. A somewhat higher relative beaching is the result of higher wind coefficients. Throughout the winter and fall, higher populations are regularly seen along the northern coast. On the other hand, beaching along the southern coast grows and sometimes reaches the level of beaching in the north during the warmer months (JJA).

To further understand this dynamic, we investigate the link between beached particles and the mean flow rate of the Ulla River. Figure  depicts the relationship between particle beaching on one of the most contaminated beaches (Ladeira beach) in the inner region and the river outflow. Note the strong correlation between beaching and flow rate, mostly during raining seasons. For this beach, nearly no time lag was observed between flooding and beaching peaks due to its proximity to the river mouth

The time lag distribution of beached particles for two wind factors is displayed in Fig. . As anticipated for Lagrangian particles carried by wind and surface currents, the data fit a Weibull distribution quite well. As the wind coefficient decreases, the distribution gets wider. The correlation coefficient and the most common time lag between river discharge and particle beaching for three windage coefficients are displayed in Table . Because of the river plume effect, beaches O Chazo (ID = 8) and Nabal (ID = 10) have shorter time delays than beaches nearer the river mouth. As previously mentioned, northern beaches are more polluted during the winter and fall rainy seasons, which intensifies the river plume. The significance of geography and coastal currents is demonstrated by the fact that other beaches around those two have the biggest temporal lags. Conversely, as anticipated, time delays along the coast decrease by increased windage. Time lags for two beaches in the outside region are also displayed for comparison. Following up on our earlier statement, we should observe that while Touro Beach is situated on the northern coast and is directly affected by the river plume, its time lag (ID = 12) is lower than Carreiro Beach's (ID = 11). It is also of crucial importance to remind that the number of emitted particles is directly correlated with the riverine discharge.

Coastal segments located in the Rianxo area, situated on the northern side near the river mouth, consistently exhibit higher correlation coefficients between river discharge and particle accumulation. This behavior can be attributed to the proximity of these segments to the mouth of the river, which leads to earlier beaching of particles, reducing the extent to which they disperse throughout the estuary and thus preserving a stronger correlation with riverine input. In contrast, segments located farther from the river mouth generally show lower correlation coefficients, likely due to increased particle dispersion before reaching the shore, which weakens the direct influence of the river discharge. Overall, the northern part of the study area tends to exhibit stronger correlations. In contrast, at Punta Remuíño (ID = 2), located on the southern coast of the estuary, correlations are consistently lower. This behavior may be explained by the prevailing northeasterly winds, which tend to impede uniform particle deposition in this area.

Residence time is an important parameter for evaluating estuary environmental water quality. Figure  depicts the monthly mean residence times for the three examined regions (inner, intermediate, and external) throughout time, as well as the three windage factors studied. Residence time is calculated by taking the average amount of time a particle spends in one of the defined zones. If a particle returns, after exiting, to one of these regions, a new counting is established. Similarly to what was proposed for the concentration maps in Fig. , beached particles have not been taken into account for this calculation, as their residence time keeps increasing after being beached. Also, particles that have exited the domain have not been considered either. This method to calculate the residence time differs from that in as in our case particles are continuously released from the river mouth. Increasing windage leads to a decrease of residence times as plastics are expelled faster resulting in reduced particle accumulation in all locations. A consistent residence time increase is observed in the second half of the year. This behavior can be explained by changes in the primary wind direction, as northerly breezes are gradually replaced by southwestern winds, which help to accumulate surface water in the estuary. River flow rate increases at the end of the year, when the rainy season begins, struggling with winds expelling water from the estuary. This results in the observed peak in the time series, which is delayed as the location is farther from the river mouth. The geographical nature of Ría de Arousa results in longer dwell durations in places closer to the release point. In addition to being closer to the emission source, the inner and intermediate zones experience more geographical features. The Coriolis effect causes particles from the river to enter the outer zone from the north side (see to Fig. ) , allowing for faster ejection from the estuary as that zone is free of geographical mishaps.

The results of this study underscore the critical role of wind forcing in modulating particle transport dynamics within semi-enclosed coastal systems such as the Ría de Arousa. One of the most significant findings is that even small increases in the wind drag coefficients (from 1 % to 5 %) result in markedly reduced particle accumulation along the coast, with both spatial extent and intensity of accumulation diminished. This clearly demonstrates that wind not only disperses particles more widely but also accelerates their removal from the estuary, effectively shortening their residence times.

The northern part of the estuary consistently shows higher particle retention, especially under low wind conditions and during colder months, suggesting a natural tendency for pollutants and floating debris to accumulate in impounded and sheltered regions. This asymmetry in distribution is further influenced by estuarine morphology and prevailing wind patterns, which may favor or hinder particle retention depending on location. In contrast to ), who focused on large-scale coastal accumulation driven primarily by regional circulation, our results highlight the importance of local-scale processes within a semi-enclosed estuarine system. In particular, we show that the release location and estuarine morphology play a key role in shaping beaching and retention patterns, leading to markedly different outcomes even when using the same Lagrangian modeling framework.

Furthermore, the clear correlation between fluvial discharge and particle accumulation in the shore segments near the mouth reveals the dominant role of hydrological inputs when wind pressure is low. However, this correlation weakens with stronger winds, indicating a decoupling between fluvial inputs and deposition at the coast when wind becomes the main driver of transport. In this context, prevailing northeasterly winds appear to inhibit deposition at specific southern locations, such as Punta Remuíño, further highlighting the spatial complexity of wind-particle interactions. Similar results were observed in terms of beaching time lags.

The observed seasonal change, with greater accumulation in the southern part during the warmer months, implies a dynamic interaction between seasonal hydroclimatic variability and estuarine circulation patterns. This suggests that management and monitoring strategies should take into account temporal variability, especially when dealing with pollution episodes or designing cleanup measures.

Differences in particle residence times between regions reflect how geographic features, such as bathymetry, bays, and proximity to the emission source, interact with physical forcings to shape retention patterns. Inner and intermediate regions, characterized by more complex coastal geometry, retain particulate matter longer than the outer zone, reinforcing the idea that physical geography can substantially modulate the risk of exposure to pollutants.

Taken together, these results highlight the importance of incorporating realistic wind forcing into predictive models of marine debris and pollutant dispersion. They also point to the need for spatially targeted mitigation strategies, particularly in vulnerable areas with the greatest potential for accumulation.

Notwithstanding the insightful information this study offers, one of its primary drawbacks is the oversimplified beaching scheme that was used, which ignores the various coastline morphologies along the Ria. A more realistic representation of the dynamics of floating and stranded particles might be possible with a more advanced stranding model that could represent diverse coastal features in a single simulation. Moreover, wind-induced vertical mixing processes, which have been shown to significantly alter the vertical distribution of buoyant plastics , are not explicitly represented and may influence both particle availability for beaching and estimated retention patterns. This would provide us more precise estimates of retention and deposition patterns and enable us to move beyond the worst-case accumulation scenario as it was applied here. Furthermore, the study's use of plastic emissions following river flow is based on the best available assumptions, which may not correctly reflect the temporal or spatial diversity of actual discharge volumes. Even though our work does not primarily focus on quantifying plastic inputs, future research could benefit from incorporating more realistic emission estimates, perhaps using regression models or connectivity matrices that link the characteristics of watersheds to the production of pollutants. The incorporation of these data may improve the accuracy of source identification as well as our understanding of the relative contributions of different catchment regions.

On the other hand, increasing the hydrodynamic horizontal resolution and that of the wind model may resolve nearshore processes, many occurring within a few meters of the coast. Regarding dimensionality, the used model is two-dimensional and does not explicitly include vertical processes such as stratification or mixing. Although a 3D approach could refine residence times and pathways, the surface-oriented nature of the particles suggests that the main conclusions would remain qualitatively consistent.

The proposed methodology is transferable to other coastal and semi-enclosed systems – such as estuaries, bays, and rias – where circulation patterns and coastline geometry strongly influence particle accumulation. Its applicability primarily depends on the availability of sufficiently high-resolution hydrodynamic data to ensure that the beaching parameterization remains a realistic representation of coastal interactions.