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Redistribution of microplastics within sediment after remobilization: Experimental study
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Laboratory experiments showed that when sediment is disturbed by currents or bioturbation, microplastics buried within it can be resuspended and redistributed, preventing permanent burial. This remobilization means that sediments previously thought to be final sinks for microplastics may actually continue releasing particles back into the water column. The findings complicate predictions of long-term microplastic accumulation and exposure in benthic ecosystems.
Microplastics (MPs) have been observed almost everywhere researchers looked for them. While the number of observations has exponentially increased since the last decade, the sources, transfers, and fates of MPs are still imperfectly known. This is peculiarly valid at the water-sediment interface, where the mechanisms behind flow and transport processes of particles are complex. There, the vertical and horizontal distributions are influenced by both water flow and particle interactions. Recent studies indicate that basic laws from sediment movement imperfectly estimate MPs behavior, highlighting the importance of proper experimental study with MP particles. In this study, the vertical redistribution of virgin high and low density MPs, with different sizes and shapes, was investigated within 11 sediment size classes. MPs were spiked into sediments in a 1 m height column fill to the top with water. The column was turned upside-down several times to simulate a physical perturbation of the sediment and to study the following deposition pattern. As expected, low-density polymers were mainly recovered in the water column while high-density polymers were found within all sections of the sediments, shifting according to the sediment size classes. Unexpectedly, a significant part of low density was also retrieved at the surface of the sediment and the MP distributions change, not only with size, but also with particle shape. The observed distributions were compared to the expected distributions based on vertical velocity formulas. Both distributions match fairly well, but MPs were observed slightly above their predicted levels. Thereby, vertical velocity formulas could be used at a first approximation, but still have to be enhanced. In conclusion, this study highlight the importance of considering MPs as a multi-dimension particles, and provide some clues to understanding theirs sources, transports, and fates. Also see: https://micro2022.sciencesconf.org/425852/document
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A mesocosm experiment with Baltic Sea benthic invertebrates found that bioturbation caused negligible upward transport of buried microplastics from 2 cm and 5 cm depth, with <2% of particles returning to the surface. The results indicate that once microplastics are deposited in marine sediments, bioturbation is unlikely to re-introduce them to the water column, supporting sediments as effective long-term sinks.
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Scientists found that microplastics settled in riverbeds and lakebeds can get stirred back up into the water when currents reach a certain speed, and how easily this happens depends on the plastic's type, shape, and the kind of sediment it's buried in. This matters because it means microplastics we thought were "trapped" in waterway sediments can re-enter the water we drink, fish from, and rely on for ecosystems, making pollution harder to predict and contain than previously thought.
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Researchers modeled microplastic transport, deposition, and burial in river bed sediments under varying hydrological conditions. River bed sediments were found to act as long-term reservoirs for microplastics, with periodic high-flow events temporarily resuspending and redistributing particles.
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