Microplastic transport in rivers and their hyporheic zone – combining modeling and experiment

Abstract

Microplastic (MP) pollution in the aquatic environment has become a problem of growing concern due to potential adverse effects on aquatic organisms and ecosystems. While MP transport and fate in marine systems has been researched to quite some extent relatively little is known about the transport mechanisms of MP particles in terrestrial surface waters and in saturated porous media like in groundwater or the hyporheic zone (HZ). We investigated the transport and fate of small (1, 3 and 10 μm diameter) polystyrene MP particles in a rippled, sandy stream bed (D50 = 1.04 mm) using CFD simulations calibrated to a set of flume experiments. A novel detection system for fluorescent MP particles (Boos et al. 2021) was used to track and quantify particle movement in the turbulent open water and in the hyporheic sediments in the laboratory flume following a pulse injection of MP particles into the surface water compartment. A new, integrated CFD simulation scheme within the OpenFOAM suite of CFD solvers was implemented for the flume system for a seamless simulation of water flow and particle transport in the open water and in the hyporheic sediments (Dichgans et al. 2023). Additionally we simulated the transport and fate of a range of “virtual” particles in the open water for different channel geometries using a Lagrangian approach. Simulations show that 1 μm MP particles are transported through the HZ like a solute, following the typical hyporheic flow cells below the bedforms. Transport and particle progression through the HZ could be adequately described with an advection-dispersion equation. Larger 10 µm MP particles instead showed retarded transport through the HZ, while retardation increased with travel distance in the sediments. Our results indicate that advective pumping across the streambed interface can transport very small MP particles through the HZ, while larger particles are increasingly retained. Distinct flow structures in the open water are found to be decisive for the fate of MP particles in the river channel.