Development of a process-based eco-hydrology model for evaluating the spatio-temporal dynamics of macro- and micro-plastics for the whole of Japan

Abstract

Plastic contamination has been receiving considerable attention during the last few decades. Although some models could simulate transport and fate of plastic debris in freshwater systems, complete model for the whole picture of plastic dynamics on basin scale has yet to be established. In the present study, the authors extended process-based eco-hydrology models, NICE (National Integrated Catchment-based Eco-hydrology) and NICE-BGC (BioGeochemical Cycle), to link them with plastic debris model, and applied to all of the 109 first-class (class A) river basins throughout Japan. New model included advection, dispersion, diffusion, settling, dissolution and deterioration due to light and temperature. These processes could help to evaluate effect of mismanaged plastic waste (MPW) and point sources (tyres, personal care products, dust, and laundry) on spatio-temporal dynamics of macro- and micro-plastics there. The model showed simulated hydrologic cycle was generally in good agreement with observed one. In contrast, simulated value of annual-averaged nutrient transports was overestimated in comparison with observed value averaged over non-flood periods. Sensitivity analysis of micro-plastic transport to various factors also implied effects of degradation would not be negligible under some conditions. Generally, large amounts of plastic flowed out of some limited rivers (Tone, Kiso, Yodo, and Ara Rivers) with a large proportion of macro-plastic flux in comparison with micro-plastic flux. Further, scenario analysis quantified the total plastic flux varied according to efficiency of micro-plastic removal in wastewater treatment plants (WWTP) and density of plastic, and was estimated as within the range 1,100 – 3,500 ton/yr, relatively similar to that of existing values. It was also clarified only a limited proportion of plastics discharged onto land flow out into the ocean intensively during rainfall seasons. These results help to quantify impacts of plastic waste on biosphere, and may aid development of solutions and measures to reduce plastic input to the ocean.