Impact of Settling and Resuspension on Plastic Dynamics During Extreme Flow and Their Seasonality in Global Major Rivers

Abstract

Environmental contamination by plastics has been receiving considerable attention from scientists, policymakers and the public over the last few decades. Though some of the models have succeeded in simulating the transport and fate of plastic debris in freshwater systems, a complete model is now being developed to clarify the dynamic characteristics of the plastic budget on a continental scale. Recently, the author linked two process-based eco-hydrology models, NICE (National Integrated Catchment-based Eco-hydrology) and NICE-BGC (BioGeochemical Cycle), 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), and applied this new model on a regional scale and also for global major rivers. The present study newly incorporated resuspension and bedload transport by extending the author's previous investigations. The simulated results showed that large-sized micro-plastics were distributed more in riverbeds than in river water. Although small-sized micro-plastics are suspended in the water and large-sized micro-plastics settle in the riverbed under normal flow, floods disturb this equilibrium completely and resuspend large-sized micro-plastics in the water. Because the percentage of exported micro-plastic load stored in the riverbed during flood periods is relatively high in some global major rivers, the amount of plastic deposited in riverbeds might be smaller than in lakes and dams. The riverine plastic transport to the ocean revised in the present study was 1.218 ± 0.393 Tg/yr, with macro-plastic flux 0.793 ± 0.305 Tg/yr and micro-plastic flux 0.426 ± 0.248 Tg/yr, being within the range of previous values, that is, 0.41–4.0 Tg/yr. These results aid the development of solutions and measures for the reduction of plastic input to the ocean, and help to quantify the magnitude of plastic transport under climate change.