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Longitudinal Dispersion and Hyporheic Exchange of Neutrally Buoyant Microplastics in the Presence of Waves and Currents
Summary
Laboratory experiments tracked neutrally buoyant microplastics (mimicking polyethylene density) under different water conditions including combined wave-current flows, finding that their movement through the water column closely resembles dissolved tracers — but that microplastics already lodged in a riverbed sediment exchange more slowly than solutes due to density-driven effects over longer timescales. Understanding these transport dynamics is essential for accurate models predicting where microplastics accumulate in river and coastal sediments.
An experimental study was conducted to identify the behaviour of neutrally buoyant microplastics (specific density, 0.94) in different hydrodynamic conditions while focusing on combined wave–current conditions and the mixing across the hyporheic zone. For in-water-column microplastics, it was observed that the streamwise dispersion of neutrally buoyant microplastics is comparable to solute dye in both slow open-channel flow conditions and combined wave–current conditions. However, for in-bed microplastics, when compared to soluble tracers, the longer timespans associated with the hyporheic exchange process allowed the density effects to enhance the vertical exchange when compared to solutes.
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