Modeling the potential source routes of marine macroplastics on a beach adjacent to the Patos lagoon estuary, South Brazil

Cassino Beach, on the southernmost coast of Brazil, is especially vulnerable to plastic pollution due to its 220 km-long coastline and proximity to the Patos Lagoon Estuary, which is influenced by urbanization, industrial activities, and intense coastal fishing. This study investigates the aquatic origin (lagoon or ocean) of stranded macroplastics during the summer and winter of 2016 at two sites: one located 3–5 km from the estuary mouth and another approximately 40 km away. A backward particle tracking approach was applied, combining the TELEMAC-3D hydrodynamic model with the TrackMPD model. Model validation using a buoy experiment showed a maximum difference of <500 m between real and virtual particle trajectories. A total of 100 virtual particles (0.8 m LDPE sheets, ρ = 0.94 g·cm⁻³) were released per site and season. At the near-estuary site (S1), most particles (75 % in summer, 68 % in winter) originated from the central region of Patos Lagoon, while an additional 6 % (summer) to 25 % (winter) came from the São Gonçalo Channel. At the remote site (S4), particles mainly originated from marine regions: 98 % (winter) and 43 % (summer: 30 % SW + 13 % NE coastal domain) Simulated transport times ranged from 4.7 to 83 days, with shorter durations during winter due to stronger coastal currents. These results highlight the dominant role of estuarine outflow in transporting macroplastics to areas near the Patos Lagoon mouth, whereas remote areas are influenced by long-distance marine transport, particularly from fishing zones. The integration of numerical modeling with field data is the main contribution of this study to improve our understanding of debris sources and support more effective management strategies.