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Comparison of concentration, shape, and polymer composition between microplastics and mesoplastics in Japanese river waters
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Researchers conducted field surveys across 147 Japanese rivers to compare the concentration, shape, and polymer composition of microplastics and mesoplastics in river water. They found microplastics at 99% of sampling stations and mesoplastics at 74%, with the gap between the two size categories widening as overall contamination increased. The study argues that monitoring only microplastics or macroplastics while ignoring mesoplastics can lead to significant underestimation of plastic pollution in rivers.
While plastics are classified by size as microplastics (<5 mm), mesoplastics (5-25 mm), and macroplastics (>25 mm), research in rivers has centered on microplastics, followed by macroplastics, with limited research on mesoplastics (research gap). This study aims to clarify the concentration, shape, and polymer composition of microplastics and mesoplastics in Japanese river water. We conducted field surveys for microplastics and mesoplastics in 147 rivers and at 185 measurement stations. The novelty of this study is in the use of a large number of field data to minimize the effect of the spatial difference in the microplastics and mesoplastics on the data analysis. Microplastics and mesoplastics were found at 183 (99 %) and 136 (74 %) stations, respectively. The difference between the concentration of microplastics and mesoplastics increased significantly with the increase in the concentration of microplastics, showing that the concentration of both microplastics and mesoplastics should be monitored to prevent an underestimation of plastic pollution in rivers with the appropriate sampling. A 2-stage size classification with microplastics (<5 mm) and macroplastics (>5 mm) is not suitable because the mesoplastics may be substantially overlooked. The regression slopes between microplastics and mesoplastics concentrations significant decrease in variance with increasing data number, suggesting the necessity of the large number of samples used in this study. The predominant shapes and polymer types of microplastics and mesoplastics were found to be fragment and fiber and polyethylene and polyethylene terephthalate (PET), respectively, which were affected appreciably by many fiber clusters. The fiber and PET ratios were dominant at stations with small population densities and urban ratios and can be attributed to atmospheric deposition and the underdeveloped rate of wastewater treatment plants (WWTPs). Therefore, it is necessary to monitor the dynamics and fate of fiber clusters inside and outside the basin.
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