Microplastic fate driven by river water turbidity

Microplastic fibres are widespread in rivers, yet the link between their transfer from suspension in the water column to deposition in the riverbed remains unclear. We conducted flume experiments to quantify the transfer of microplastic fibres from flowing water into a sandy ripple bed under both clear-water and suspended-sediment conditions. In clear water, fibres remained evenly distributed in the water column and were scarcely accumulated in the bed, indicating efficient downstream transport. When suspended sediment was added, fibres concentrated near the base of the water column and were readily incorporated into the bed. We also showed that, under turbid conditions, curlier and denser fibres move closer to the bed and interact more frequently with suspended and saltating sediment, promoting entrapment. These results demonstrate that suspended sediment strongly influences fibre deposition, with clear-water flows supporting downstream transport towards the sea and turbid conditions enhancing microplastic fibre trapping in riverbeds.