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河川中の「目に見えない」プラスチックを定量化する新しい方法を開発(Scientists develop new method for quantifying ’invisible’ plastics in rivers)

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2024-05-09 カーディフ大学

◆現在の川でのプラスチック汚染を数える方法は不十分であり、水面下に沈む微細なプラスチック片を計算に入れていないと、科学者たちが警告しています。国際チームは、川の基本的な汚染レベルを設定し、清掃戦略の成功を評価するために、これらの見えないプラスチックが川でどのように動くかを説明し、それらを数える新しい方法を提案しています。

◆研究チームは、3000個のプラスチック破片を実際の川の条件を模倣した人工水路に投入し、複数のカメラでミリメートル単位で追跡しました。この分析から、異なる形状とサイズのプラスチックが川で多様な方法で輸送されることが明らかになりました。さらに、沈むプラスチックの動きを解明し、沈殿物の方程式を適用してプラスチックの量を予測する新しい方法を提案しました。

<関連情報>

乱流輸送における非浮力プラスチックの垂直構造について On the vertical structure of non-buoyant plastics in turbulent transport

James Lofty, Daniel Valero, Antonio Moreno-Rodenas, Biruk S. Belay, Catherine Wilson, Pablo Ouro, Mário J. Franca

Water Research  Available online:13 February 2024

DOI:https://doi.org/10.1016/j.watres.2024.121306

Highlights

  • 400 settling tests and 3000 plastic samples in river-like transport were tracked.
  • A multimodal distribution of plastic settling velocity was observed.
  • Three plastic transport layers are observed; surfaced, suspended and bed load.
  • Particle-bed dynamics caused concentration profiles to depart from a Rouse profile.
  • New transport theories are derived from the stochastic nature of settling plastic.

Abstract

Plastic pollution is overflowing in rivers. A limited understanding of the physics of plastic transport in rivers hinders monitoring, the prediction of plastic fate and restricts the implementation of effective mitigation strategies. This study investigates two unexplored aspects of plastic transport dynamics across the near-surface, suspended and bed load layers: (i) the complex settling behaviour of plastics and (ii) their influence on plastic transport in river-like flows. Through hundreds of settling tests and thousands of 3D reconstructed plastic transport experiments, our findings show that plastics exhibit unique settling patterns and orientations, due to their geometric anisotropy, revealing a multimodal distribution of settling velocities. In the transport experiments, particle-bed interactions enhanced mixing beyond what established turbulent transport theories (Rouse profile) could predict in low-turbulence conditions, which extends the bed load layer beyond the classic definition of the bed load layer thickness for natural sediments. We propose a new vertical structure of turbulent transport equation that considers the stochastic nature of heterogeneous negatively buoyant plastics and their singularities.

Graphical abstract

Image, graphical abstract

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0904河川砂防及び海岸海洋
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