Macroplastic fate and transport in an Amazonian Estuarine System: A Lagrangian Modelling Approach

Plastic pollution is an increasing environmental problem, with rivers acting as key pathways for transporting urban waste to the ocean. Despite this, the transport dynamics of macroplastics in fluvial and estuarine systems remains poorly understood, such as rivers in the Amazon region, characterized by intense river discharge, macrotidal regimes, and urban centers with high rates of waste disposal. This study investigates the transport and fate of macroplastics released from the city of Belém (northern Brazil) into the Guajará Bay–Pará River estuarine system. A two-dimensional hydrodynamic model (Delft3D-FM) was implemented and validated using observed water level and river discharge data. Macroplastic transport was simulated with the TrackMPD model, incorporating the beaching and refloating processes available. 3481 particles representing the most common macroplastic types in the study area were released from three urban discharge points. Eight scenarios were simulated, combining periods of high and low river discharge with spring and neap tides during flood and ebb phases. River discharge emerged as the primary control on macroplastic export. During high-discharge periods, macroplastics were quickly transported downstream. In contrast, low-discharge conditions promoted increased retention within the Guajará Bay and adjacent rivers. Tidal dynamics controlled dispersion patterns and influenced transport direction depending on the tidal phase at release. Persistent accumulation hotspots were identified along the right margin of Guajará Bay near Belém, indicating high environmental risk. These results highlight the importance of river discharge and tidal processes in controlling macroplastic fate in amazonian estuarine systems and provide insights to support targeted pollution management strategies.