Infiltration and retention of micro/nanoplastics in the hyporheic zone of rivers

In recent years, the ecological impact of microplastics (MPs) in surface water and sediment of river systems has attracted substantial research interest. However, the understanding of MP infiltration in the hyporheic zone remains limited. The gap arises from a lack of consideration for the interactions among hydrodynamics, sediment characteristics, and MP properties. This study investigates the infiltration pathways and retention patterns of MPs in the hyporheic zone through a combination of experiments and numerical simulations. Results reveal that hyporheic exchange, along with settling and retention processes, plays a crucial role in MP infiltration. Most MPs accumulate within the surface layers of the riverbed, with hydrodynamic conditions and particle properties critically regulating vertical distribution. MPs with larger particle sizes and higher densities promoted surface accumulation. While increased settling velocities enhanced deeper infiltration, particularly for denser MPs. It is demonstrated that the mass transfer rate of MPs between surface water and riverbed is proportional to the settling velocity and retention rate coefficient. These findings advance the prediction of MP distribution in the hyporheic zone and provide a scientific basis for effective management of MP pollution in aquatic environments.