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Subsurface transport of microplastics in riverine sediment: Impacts of different rain events and particle density
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Microplastics added to the surface of riverbed gravel columns gradually migrate deeper into the sediment as rain events accumulate, especially through repeated wet-dry cycles. Both polystyrene (denser than water) and polyethylene (less dense) particles traveled deeper over time, with smaller, less hydrophobic particles moving farthest. This shows that riverine sediment is not just a permanent sink for microplastics but can also funnel them downward into groundwater aquifers and subsurface habitats.
Microplastics (MPs) have emerged as a growing concern, posing potential risks to both marine and terrestrial environments. While surface soils are recognised as a primary sink for these particles, the vertical mobility of MPs in the subsurface remains uncertain due to a lack of comprehensive scientific data. Here, we conducted column experiments to study the transport behaviour of MPs through and retention in subsurface sediment. Two types of pre-stained MPs (median size 50.4 µm) with densities greater than (polystyrene) and smaller than (polyethylene) water were added to the top of large (110 cm) wet-packed fine gravel columns - the most common gravel found in the subsurface zone of the riverine environment. The concentration of deposited MPs was 50,000 particles per kilogram of sediment, derived from an extensive literature survey of polluted sites. Various scenarios, including continuous rain, wet-dry cycles, and dry conditions (characterised by a single rain event followed by a subsequent drying period), were implemented to simulate diverse rain events. 20 mL of water samples were systematically collected at specified intervals from different ports of the column at depths of 30, 50 and 70 cm. Additionally, continuous effluent collection took place at the bottom port (90 cm), which was connected to a pump that maintained a controlled flux at around 4.6 mL/min. At the end of the experiment, gravel samples were methodically collected from discrete sediment layers within the columns (0–5 cm (top of the source layer), 5–10 cm (source layer), 10–30 cm, 30–50 cm, 50–70 cm, 70–90 cm) to quantify the MP mass retained in the column. Results showed that the smallest PS-MPs with a continuous flow system exhibit the highest potential for transport due to higher density and less hydrophobicity compared to PE. With increasing rain events, MPs in the source sediment layer decreased, while MPs concentrations in deeper column layers increased significantly. Furthermore, an intriguing observation indicates that as these MPs undergo more wet-dry cycles, their penetration depth substantially increases. The results indicate that sediment may not only act as a sink for MPs but also as a possible entry point to subsurface receptors such as subterranean fauna and aquifers. This research underscores the intricate dynamics of MPs in sediment and raises awareness regarding the potential environmental consequences. Keywords: Microplastics, Transport, Raining events, Density, Hydrophobicity
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Subsurface transport of microplastic particles in gravel columns: Impacts of different rain events and particle characteristics
AI summary Read the abstract
Researchers conducted column experiments using 110 cm wet-packed fine gravel columns to examine subsurface transport of two microplastic types — polystyrene (denser than water) and polyethylene (less dense than water) at 50 µm median size — under different simulated rainfall scenarios including continuous rain, wet-dry cycles, and single events followed by drying. They found that particle density, rainfall pattern, and subsurface heterogeneity all influenced microplastic vertical transport and retention depth in gravel sediments.
Subsurface transport of microplastic particles in gravel columns: Impacts of different rain events and particle characteristics
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Researchers conducted column experiments using pre-stained microplastic particles of two density types in gravel sediment to investigate how different rainfall intensities and land-use scenarios influence the vertical transport and retention of microplastics in subsurface environments. The study found that both particle density and rainfall event characteristics significantly affected microplastic mobility through subsurface sediments, informing models of microplastic fate in soil-water systems.
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