Hydrodynamic controls on vertical transport and retention of microplastic particles in subsurface sediments

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.