The flux and fate of plastic in the world's major rivers: Modelling spatial and temporal variability

Over the past few decades, environmental contamination from plastics has received considerable attention from scientists, policymakers, and the public. Although some models successfully simulated the transport and fate of plastic debris in freshwater systems, a complete model of the dynamics of plastics in rivers on a global scale has yet to be elucidated. Recently, two process-based eco-hydrology models, NICE and NICE-BGC, were applied to evaluate biogeochemical cycling in a range of river basins on scales ranging from local/regional to continental/global. This effort provided insights into the quantification of the role of inland waters on global biogeochemical cycles, but was limited in its treatment of plastic dynamics. Here, we linked NICE-BGC to a plastic debris model that accounts for both the transport and fate of plastic debris (advection, dispersion, diffusion, settling, dissolution and biochemical degradation by light and temperature). We evaluated spatio-temporal variations of plastic debris in the world's major rivers (325 rivers) by simulating the amount of plastic that flows from land into rivers and finally into the ocean. Our NICE-BGC simulations show how diffuse sources of mismanaged plastic waste (MPW) and point sources of tyres, personal care products (PCPs), dust, and laundry in these 325 rivers are transported from land to rivers, and finally to the ocean. We compare continental-scale simulated plastic transport from agricultural and urban land uses to previous studies. Our model results confirm previous assessments that flood events have a great impact on the mobilisation of plastic and lead to high interannual variability in fluxes. The use of “untreated wastewater” and the simulation of a two-dimensional diffusion model for macro-plastic transport helped to decrease the uncertainty of the model beyond previous studies that included only sewerage connectivity, removal efficiency, and transport probability. Uncertainty and sensitivity analyses using Monte Carlo model simulations also clarified that most plastic fluxes originate in 20 global rivers, largely in Asia, and are greatly influenced by the effects of the summer monsoon. The new model presented here may be valuable for to detecting and predicting the dynamics of plastic in large rivers and to the development of efficient measures to reduce plastic input to the ocean on a global scale.