Suspended load effects on microplastic transfer from the water column to the riverbed

Rivers act as major pathways for microplastic pollution, yet the mechanisms governing the transfer of suspended microplastics from the water column to riverbeds remain poorly understood. We conducted controlled flume experiments under steady, unidirectional flow to quantify the deposition of microplastic fibres within a sandy ripple bed. Polyamide and polyester fibres, introduced at environmentally relevant concentrations, were tracked under clear-water conditions and in flows containing suspended kaolin to simulate enhanced fine-sediment loads. In clear water, fibres remained widely mixed throughout the water column and were only rarely incorporated into sand, indicating efficient downstream transport and limited short-term sequestration. In contrast, the presence of suspended kaolin induced pronounced and elevated near-bed fibre concentrations and substantially increased the incorporation of fibres into the sand bed. Fibre properties influenced this process: higher-density fibres exhibited greater settling tendencies, while curled fibres experienced increased drag and more frequent interactions with both saltating sand grains and suspended particles, promoting their entrapment within the bed. These results demonstrate that suspended fine sediments can markedly enhance microplastic deposition in riverbeds by altering near-bed transport dynamics and promoting physical entrapment within bedforms. Such conditions create accumulation zones that may influence the short-term accumulation and long-term distribution of microplastics in river systems.