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Hydrodynamic controls on vertical transport and retention of microplastic particles in subsurface sediments
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Lab columns filled with gravel tested how two plastic types (polyethylene and polystyrene) move through sediment under different water flow patterns. Steady and pulsed water flow carried microplastics deeper and faster than single bursts, with smaller particles moving earlier and over half of particles staying trapped in the top 10 centimeters of sediment.
Microplastic (MP) pollution represents an emerging threat to subsurface environments, yet the mechanisms governing their vertical mobility in coarse sediments remain poorly understood. This study systematically investigates the transport and retention behaviour of polyethylene (PE; ρ = 0.92 g cm⁻³) and polystyrene (PS; ρ = 1.10 g cm⁻³) MPs through fine gravel under controlled hydrodynamic regimes using six large-scale columns (110 cm × 24 cm). Continuous, pulsed, and single-pulse infiltration conditions were applied to simulate contrasting recharge scenarios. Effluent and pore-water MP were collected over 503 h at multiple depths (20-80 cm), and sediment subsets were analysed post-experiment to quantify MP retention and recovery. Results demonstrated strong flow-dependent MP transport. Continuous infiltration produced earlier breakthrough, whereas pulsed flow generated episodic MP concentration peaks associated with rewetting events and enhanced pore-water velocities. Pulsed infiltration also increased the first temporal moment of both polymer types, indicating sustained particle transport and prolonged recovery at the column outlet. Contrarily, single-pulse infiltration resulted in limited MP downward transport. PS-MPs showed earlier breakthrough, while polymer-specific transport differences generally decreased with increasing transport depth. MPs <45 µm preferentially showed early transport, while particles >50 µm were mobilised later. MP recoveries ranged from 104% to 126% with 51% to 67% of the particles retained within the upper 10 cm of the sediment profile. Our findings demonstrate that hydrodynamic conditions, particle size, and polymer-specific properties influence MP transport and retention in subsurface sediments, highlighting the importance of transient conditions in regulating MP mobility and potential groundwater contamination.
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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 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.
Transport and retention of micro-polystyrene in coarse riverbed sediments: effects of flow velocity, particle and sediment sizes
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Researchers conducted column experiments to investigate how polystyrene microplastic fragments are transported and retained in coarse riverbed sediments under different flow conditions. They found that most particles were captured in the upper 15-20 centimeters of sediment, but smaller fragments between 100-500 micrometers could penetrate to depths of at least 50 centimeters. The study suggests that riverbeds can act as both temporary sinks and long-term retention sites for microplastics, slowing their transport from streams to oceans.
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.
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.
SiO2 and microparticle transport in a saturated porous medium: effects of particle size and flow rate
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Column experiments tracking the movement of polystyrene microplastic particles and silica particles through saturated gravel showed that larger particles are retained more strongly, but higher water flow rates push both types deeper into the porous medium. At the same flow rate, 10-micrometer polystyrene particles were retained 46% more effectively than 2-micrometer particles, illustrating how particle size and water velocity interact to control microplastic transport through subsurface environments. Understanding these dynamics is important for predicting how microplastics reach groundwater and spread through aquifer systems.
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