Desenvolvimento de modelo matemático para simulação de transporte de resíduo plástico em estuário amazônico.

Abstract

The increasing use of plastics has raised environmental concerns due to the persistence of these materials in marine ecosystems. It is estimated that trillions of pieces of plastic debris are currently in the oceans, with millions of additional tons entering annually through rivers and other terrestrial sources. There is a gap between models of plastic input to the oceans and field observations, indicating the need for more accurate approaches and simulations based on in-situ data. The Amazon Coastal Zone (ACZ) emerges as a hotspot for the accumulation of plastic waste, especially in vegetated areas close to large urban centers. Although several numerical models have been developed for the ACZ, none have addressed the transport of plastic waste to date. This pioneering study implemented the first mathematical simulation for plastic transport in an urban estuary on the Amazon coast, specifically in the Guajará Bay, using the urban river (Tamandaré river) as a release site due to the availability of in-situ data for plastic parameterization. The hydrodynamic model (D-Flow), validated by the LAPMAR environmental modeling group, and the water quality module (D-Waq), were implemented using the Delft-3D software. The results show that Guajará Bay is more sensitive to plastic contamination during the dry period, characterized by slow transport of water masses and a reduced capacity for renewal. In contrast, during the rainy season, the tendency is to export plastics, due to the system's greater capacity for renewal. The residence time of the waters of the Tamandaré River in the bay varies from approximately 8.28 days in the period of greatest discharge to 31.76 days in the period of lowest discharge, allowing us to estimate that the waste released by the Tamandaré can reach the Tropical Atlantic Ocean in a range of 36 to 63 days, depending on the seasonal season. The study concludes that Amazon estuaries dominated by positive asymmetric tides favor the import of waste during periods of lower flow and increase exports during periods of higher flow. Temporary plastic retention zones were observed in both seasonal periods for marginal regions of low energy and atypical morphology, such as the coastline close to the port of Belém (BG1) and the mouth of the Guamá River (RG). These retention zones are intensified in the dry season and attenuated in the rainy season, depending on local hydrodynamic variability. Although the study analyzed the dynamics resulting from the input of plastics in a single ebb tide, it did not consider the continuous flow and potential accumulation of plastics over time. This work represents an important initial step towards improving the understanding and methodology of plastics transport in the ACZ.