MARINE PLASTIC DRIFT FROM THE MEKONG RIVER TO SOUTHEAST ASIA
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- Master theses 
Plastic waste has been identified as a major worldwide environmental issue (Sun et al., 2021; Zhang et al., 2020). Jambeck et al. (2015) estimated that between 4.8 and 12.7 million tons of plastic waste enters the ocean every year. A major share of this amount originates in the countries of Southeast Asia (Harris et al., 2021; Sun et al., 2021). The South China Sea, the biggest sea in Southeast Asia, is surrounded by six out of the ten biggest marine plastic contributors. Mekong is the greatest river in the South China Sea. Therefore, it is important to examine how plastics drift from the Mekong river to Southeast Asia, and potential factors (stranding, rivers, wind drift, vertical mixing and biofouling) that can affect the trajectory of plastic particles. There are several findings. Firstly, the seasonal drifts are influenced by the monsoon systems that during the summer the plastic particles drift mainly to the northeast, and in the winter they drift to the southwest. Secondly, in the long term, 15 months, the particles drift across Southeast Asia, and some of them leak to the Pacific and Indian oceans, and the Philippines is most vulnerable to plastic pollution. Thirdly, if considering the stranding of plastics, most of the plastics (97%) are stranded after 15 months, the average traveling time is 3.7 months, and again the Philippines is most vulnerable to plastic pollution. Next, rivers play a role in dispersing plastics; however, their influence is mainly around the rivers and for a short time. In addition, wind drift and vertical mixing can have combined effects on the trajectory of marine plastics. Wind drift plays an important role in pushing water particles right at the surface, while vertical mixing is particularly important in the vertical distribution of the particles. Lastly, biofouling simulations with different terminal velocities show that plastic particles mostly deposit in the southern continental shelf of the South China Sea where the waters are shallow, and many are suspended in the deep waters in the middle of the South China Sea.