The Freitas, Maria Eugenia Fernandes creator Thammarat Koottatep Chairperson Chongrak Polprasert Examination Committee Harari, Joseph Examination committee Somchai Chonwattana Examination committee Government of Japan Scholarship Donor text series technical report th Pathum Thani, Thailand Asian Institute of Technology 2022 continuing eng 61 leaves : ill. Marine plastic waste is a transboundary, cross-cutting, and global issue. The Asian continent is responsible for 86% of plastic pollution input to the oceans. Thailand is the second-largest per capita plastic consumer among the Asian countries and the sixth country with the highest plastic waste mismanagement rates. This scenario contributes to input plastic marine debris in the Andaman Sea and the Gulf of Thailand. One of the knowledge gaps for effectively combating plastic pollution is identifying sources and trajectories of debris for coastal and marine ecosystems. However, among the scientific publications on marine plastic pollution in Thailand, there are no works related to pathways, plastic accumulation, and marine plastics hotspots. To fill this research gap, this work was a pioneer in approaching numerical modelling to identify plastic trajectories in the Gulf of Thailand, where they accumulate, and possible source identification for considering islands the seasonal variation of particles. For this purpose, numerical hydrodynamic and particle dispersion models were used, using the techniques of forward and backward vectors. The hydrodynamic simulation was performed using the Delft3D modelling software, and the particle dispersion model was processed in MATLAB for December 2020 and July 2021 to identify if there were differences during the monsoon season. The validation process demonstrates that the hydrodynamic modelling results are satisfactory, having skill values for elevations above 0.98. The particle trajectories were interpreted in joint analysis with the average wind speeds and surface currents results. Many potential plastic debris sources have been analyzed, such as the Chao Phraya River can be considered a potential source for Chonburi province and other provinces in the upper Gulf of Thailand in both scenarios. The possible sources of plastic debris also changed due to seasonal variation. December proved to be the month with the greatest contribution to the spatial dispersion of particles. The results presented in this work allow decision-makers to consider them for efficient management and fight against the plastic solid waste that affects beaches, coastal zones, mangroves, estuaries, and islands in the Gulf of Thailand. Decision-makers must know the origins of marine litter that affects the Gulf, especially in marine protected areas, fish stock renewal areas, and tourism interest places. Thus, preventive strategies and practical and integrated solutions can be proposed to mitigate and eliminate this plastic waste, whose presence conflicts with the conservation objectives. A research study submitted in partial fulfillment of the requirements for the degree of Master in Marine Plastics Abatement Research studies project report (M.) - Asian Institute of Technology, 2022 Mappings (Mathematics) Plastic marine debris Environmental aspects Gulf of Thailand Asian Institute of Technology. http://203.159.5.9/ait-thesis/detail.php?q=B17482 http://203.159.5.9/ait-thesis/detail.php?q=B17482 221005 20260819085846.0