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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.
Microplastics (MPs) enter into the soil-water systems through various pathways, including atmospheric deposition, runoff, and the direct use of plastic-containing products such as mulches, fertilizers, and composts in agricultural practices. While surface soils are known to be primary sinks for MPs, their vertical movement in the subsurface remains poorly understood due to a lack of comprehensive data. In this study, we conducted column experiments to explore the transport and retention of MPs in subsurface sediments. Two types of pre-stained MPs (median size 50.4 µm), one with a density greater than water (polystyrene (PS)) and one with a lower density (polyethylene (PE)), were introduced to the top of 110 cm wet-packed fine gravel columns, mimicking typical riverine subsurface gravel. The MP concentration used was 50,000 particles per kilogram of sediment. Different rainfall scenarios were simulated, including continuous rain, wet-dry cycles, and a single rain event followed by drying. Water samples were periodically collected from ports at depths of 30, 50, and 70 cm, with continuous effluent collection at 90 cm to maintain a controlled flow rate. Gravel samples were taken from specific sediment layers (5-10 cm, 10-20 cm, 20-30 cm, 30-40 cm, 40-50 cm, 50-60 cm, 60-70 cm, 70-80 cm and 80-90 cm) to measure MP retention. The results indicated that PS-MPs, due to their higher density, showed greater transport potential compared to PE-MPs under continuous flow. Increased rainfall reduced MP concentrations in the source layer and significantly increased them in deeper layers. Wet-dry cycles further enhanced MP penetration depth. These findings suggest that sediment can serve both as a sink and a pathway for MPs into subsurface environments, affecting subterranean fauna and aquifers. The study highlights the critical impact of hydrological conditions on MP transport in soil-water systems, emphasizing the need to understand these effects for environmental assessments. Also see: https://micro2024.sciencesconf.org/559680/document
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Subsurface transport of microplastic particles in gravel columns: Impacts of different rain events and particle characteristics
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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.
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