Turbulence-Sediment Synergy Controls Buoyant Microplastic Settling in the Three Gorges Reservoir.

Freshwater microplastic pollution has emerged as a major global environmental challenge. Impoundment structures such as dams alter fluvial hydro-sedimentary conditions, facilitating microplastic settlement and retention within reservoirs. However, the vertical transport mechanisms of positively buoyant microplastics in such systems remain poorly quantified, particularly regarding how hydro-sedimentary dynamics (e.g., turbulence and suspended sediments) drive their downward migration. Here, we provide the first experimental quantification of vertical transport processes for positively buoyant microplastics under varying turbulent shear rates (G) and suspended sediment concentrations (SSCs) in the Three Gorges Reservoir (TGR). Our experiments revealed that under typical TGR hydro-sedimentary conditions, two key mechanisms governed vertical transport: (1) increasing turbulent shear forces facilitated microplastic penetration below the surface layer by overcoming buoyancy through fluid-particle interactions; (2) SSC-mediated heteroaggregation dominated their transport to deeper layers. A critical shear rate threshold was identified (G = 19.94 s^-1), at which both aggregate size and settling efficiency peaked-markedly enhancing microplastic accumulation in deep-bottom layers. The fractal dimension of aggregates exerted a greater influence on settling velocity than aggregate size alone: elevated shear rates promoted denser aggregate structures, accelerating settling; conversely, higher SSCs induced structural loosening during aggregate expansion, reducing settling rates. These findings clarify the pivotal role of hydro-sedimentary dynamics in regulating the vertical distribution of microplastics, providing a mechanistic basis for why the TGR acts as a "settling hotspot" for microplastics. More broadly, the results advance our understanding of how reservoirs trap buoyant microplastics, with implications for assessing microplastic fate in freshwater impoundments globally.