Trapping Efficiency of Non-Buoyant Microplastics by River Groynes

Microplastics (MPs) are an emerging environmental concern driven by extensive plastic use and inadequate waste management. As rivers act as major transport pathways and temporary sinks, reducing MPs loads within them can limit their transfer into the food chain. This study examines the ability of groynes, structures originally designed for flow and erosion control, to trap non-buoyant MPs. Ten configurations formed from pairwise combinations of four common groyne geometries were tested using MPs (of equivalent diameter deq = 2.29 mm) of two densities (ρs = 1.08 and 1.11 g/cm³) under subcritical turbulent flow in a recirculating flume. A particle tracking model was applied to derive particle trajectories, trapping efficiencies, and retention zones. It was observed that MPs entered the groyne field from the downstream end and formed a major gyre. All scenarios retained > 6% of particles during the test period. When both groynes shared the same shape, upstream-facing inclined groynes exhibited the greatest trapping potential, achieving retention rates up to 20.7 ± 3.7%. Straight and T-shaped groynes had lower entrainment but higher stability, with less than 3% escape. In cases where the upstream groyne was straight and the downstream geometry was varied, particle escape was negligible, with downstream repelling groynes consistently trapping more MPs than other configurations. Particle density had minimal effect on entrainment, though lighter MPs exhibited lower escape and smaller retention zones, reflecting sensitivity to vertical flow dynamics within the groyne field. The study highlights groynes’ potential role in mitigating riverine plastic pollution.