Hyporheic exchange processes of pore-scale microplastics

The transport of microplastics in the hyporheic zone remains poorly understood with few studies attempting to quantify microplastic hyporheic exchange processes. A laboratory scale erosimeter was utilized in combination with fluorometric techniques to experimentally quantify the dispersion of 3D pore-scale microplastics across the hyporheic zone. Rhodamine WT dye, Polypropylene (PP), polyethylene (PE), and polymethyl methacrylate (PMMA) were well-mixed within the riverbed and individually tested using solute transport theory for three sediment diameters and five bed shear velocities (u_∗) common in the natural environment. Effective dispersion coefficients for solutes significantly differed from that of PE and PMMA in most cases, where their critical sinking velocity within sediment pore water was observed and a method for predicting polymer dispersion was proposed. When u_∗ ≥ 0.0304 m/s, PMMA followed similar pathways to solutes and the effective dispersion scaling model was successfully implemented to predict its fate. PP near the riverbed interface ascended to the surface but was immobilized deeper in the riverbed, likely due to aggregation and flocculation processes. When polymer buoyancy became the dominant process, high concentrations of lighter than water microplastics ascended into the water column and high concentrations of denser than water microplastics descended through pore water, which is concerning for real-world groundwater systems. These findings provide valuable insights to guide future policy and mitigation strategies of microplastic contamination in fluvial systems by advancing our understanding of microplastic transport. Further data collection will enhance our ability to accurately quantify these transport processes and strengthen mitigation efforts, especially within high permeability sediments.