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Microplastic trapping in sandy bedload: insights from flume experiments
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Researchers conducted flume experiments to investigate the mechanisms controlling microplastic trapping in sandy bedload sediments, examining how particles of different sizes and densities become buried within ripple structures formed by unidirectional tractional flows. The study provided insights into riverine microplastic sedimentation dynamics relevant to understanding transient storage during land-to-ocean transport.
During their transfer from land to oceans, microplastic particles can temporarily be trapped in riverine sediments. So far, only a few studies have considered microplastic distribution in sediments as a function of different depositional conditions and their interaction with inorganic particles in different depositional sub-environments. Flume experiments were carried out to investigate the mechanisms governing microplastic trapping in sand ripples formed by unidirectional tractional flows. A 4 m long and 0.3 m wide flume was used to develop 3D sandy ripple forms in 11 cm deep and 0.28 m/sec flowing water. Ripples were ca. 2-3 cm high and migrated at ca. 0.7 cm/min. Polyamide and polyester 500 µm-long fibres were released into 1000 litres of water (15 and 20 items per litre, respectively) that recirculated within the flume. The experiments developed over three phases. Phase 1: the onset of sediment transport and equilibrium reaching. During this phase, ripples formed and were allowed to migrate to reach their morphological equilibrium. Microplastics circulated through the flume with the water flow. Phase 2: marking the bedform configuration. This was obtained by scanning the flume floor and releasing a tracer (coloured sand) that marked the position of the ripple fronts (i.e. leed side) at the beginning of measurements. Phase 3: sampling and measurements. Sampling included a 5 minutes-lasting, continuous sampling of suspended water and the recovery of sand accumulated over the tracer during the same time. After that, the flume floor was scanned. Comparison between different scans of the flume bottom allowed for quantifying the amount of sediments accumulated during ripple migration over 5 minutes. The concentration of microplastics in water and sediments, measured at the microscope after gravity separation, allowed the definition of a relationship between the amount of microplastics transported as a suspended load and items trapped in the flume bed. Also see: https://micro2024.sciencesconf.org/558691/document
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