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Below the surface: A laboratorial research to the vertical distribution of buoyant plastics in rivers
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Researchers conducted laboratory experiments measuring the vertical distribution of buoyant plastic litter in flowing water, finding that significant amounts of plastic are transported at depth rather than only at the surface. This challenges surface-only monitoring approaches and suggests conventional plastic flux estimates in rivers may undercount actual loads.
Rivers are identified as main sources of plastic litter in oceans. About 65% of the plastic litter is buoyant in fresh waters, meaning it has the capability to float, making transport over rivers relatively easy. A better understanding of how plastic litter is transported via rivers is crucial. Both for quantification and mitigation of the plastic problem. Most research on quantification of the plastic flux is based on surface-measurements only, up till about 50 cm water-depth. Thereby, most cleaning strategies focus on skimming only the surface. This research investigates the distribution of buoyant plastic litter over the water depth in rivers. Given the wide use of marginal buoyant plastics, it is hypothesized that a significant share of the plastic in rivers is transported below the first 50 cm surface-water, due to the mixing ability of turbulent flow. In such case, a great share of the plastic litter is overlooked in both flux estimates and riverine removal strategies. The question arises: How is plastic distributed over the water depth in rivers and how is this distribution related to prevailing flow conditions? The research is based on four pillars: Knowledge on the hydraulic plastic parameters (1), in combination with experimental observations (2), might lead to an explanation of the distribution with a theoretical approximation (3), based on existing literature from neighboring research fields. Lastly, manipulation (4) of this plastic distribution by hydraulic interventions is investigated. With this research, a first insight is created on the vertical behavior of plastic in relation to the flow conditions. This study shows that marginal buoyant plastics can be sensitive to turbulent motions in flow and a significant amount of plastic might be transported below the surface. In order to create a complete picture of the behavior of different kinds of plastic in stream flows, more extensive research is needed.
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