Rivers of plastic: A socio-economic and topographic approach to modeling plastic transport from catchment to sea

Marine litter caused by discharge of mismanaged plastic waste is considered to be one of the major environmental challenges by the international society. With the annual increase of plastic production, a growing number of plastic products are being used in people's daily lives. A large number of these plastics end up as waste emitted into rivers and subsequently into oceans through the effects of downpours or wind, posing a threat to the marine ecosystem. In this study, we developed a riverine plastic transport model based on catchment topography and social-economic factors. By applying reasonable compromise on the complexity of the model, this compromised simplified process-based model has the innovative capability to estimate plastic emissions effectively under the current conditions of limited data availability for model inputs. Compared to existing models, this novel model can also resolve challenges related to the contributions of various land use types and transport stages to plastic emissions into the oceans. To further explore the applicability of our results on a global scale, certain input parameter such as the proportion of mismanaged waste is crucial for users to acquire. Here, taking the S river catchment as our study area, the tourism-driven seasonal variation of land-based plastic emissions was quantified. According to our estimation, the emission flux in S river catchment in 2020 was 68 to 280 tons. 62.4% of riverine plastics reached the ocean. Although urban areas are the predominant contributors to the total emission flux, the contributions from other land use types such as forests and cultivated areas are also unignorable. For instance, forests and cultivated areas contribute 25.7% and 6.3%, respectively, even surpassing the contributions from high tourist activity (5.8%). Stricter waste collection legislations are imperatively needed particularly in these regions.