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The Citarum River flows through different characteristic of terrestrials with 297 km length and become one of the largest rivers in West Java. It potentially transfers debris from land into the sea. This research aimed to define the Marine Debris (MD) trajectories based on seasonal monsoon. The method used was numerical analysis combined with artificial debris pathways. The simulation controlled by ocean currents, tide, wind pattern, and bathymetry conditions. The MD observations were conducted in four mouth of estuaries across the Muara Gembong areas. These simulations with specific time during two main monsoons (the northwest and southeast Monsoon) period. The results showed that the debris trajectory patterns vary in the two monsoons. The macro debris trajectory showed the waste patterns similar to oceanographic condition, especially the ocean currents pattern. The trajectories of waste from two estuaries flow towards the south and southwest follow the coastal contours. Specifically, in Northwest Monsoon, MD spread to the south and was stranded in the surrounding coast areas. In Southeast Monsoon, MD was forced to the central of Jakarta bay and surrounding islands in the western and southern side of the estuaries. Compared to the Bendera estuary, the MD that comes from Jaya estuary affects the surrounding areas, including in the northern side and southern side.

The complex circulation for both atmospheric and ocean affects the pathways of floating marine debris (MD). The aim of this research was to define the MD trajectories in the Indonesian boundary seas. The Lagrangian particle method was applied with oceanographic factors and debris classes based on 1 to 3% windage class. The simulation was applied during the Northwest and Southeast Monsoon period. On the basis of the obtained findings, the debris trajectory patterns were indicated to vary in the two monsoons. In the northern part of Indonesia, all neighbouring countries were affected, especially those near the boundaries. In the southern part of Indonesia, most of the debris headed west towards the Indian Ocean. MD was mostly transported by the ocean currents and monsoonal wind and ended up in the coastal area. Some debris was also still floating in the open ocean after three months. This study concluded that marine litter not only polluted Indonesia water, but also the neighbouring countries.