Fate of Microplastics in Urban Rivers: Hydro-Environmental Trade-offs between Grey and Green Infrastructure

Urban rivers act as primary conduits for transporting land-based microplastics (MPs) to marine environments. However, the impact of river restoration—specifically the transition from engineered concrete channels to naturalized, vegetated streams—on the fate and transport of MPs remains underexplored. This study employs a novel, fully coupled one-dimensional numerical model integrating the Saint-Venant hydrodynamics, Exner equation, and advection-dispersion equations for dissolved pollution and suspended MPs. Two distinct management scenarios were simulated over a ۴۲,۰۰۰-second period: a concrete channel (Grey Infrastructure, n = ۰.۰۱۵) and a restored channel (Green Infrastructure, n = ۰.۰۴). Results indicate a fundamental divergence in pollutant dynamics. The concrete channel functioned as a passive super-highway, exhibiting high flow velocities and shear stresses (Tb > Tce) that prevented deposition, resulting in negligible benthic retention and the rapid, unattenuated flush of the pollution plume downstream. Conversely, the restored scenario demonstrated a significant filtering effect; increased roughness reduced flow velocity and bed shear stress, triggering the deposition of MPs (Mbed peaking at ۰.۰۰۷ g/m²). While this significantly attenuated suspended concentrations, it transformed the channel bed into a transient reservoir. The simulation revealed a secondary source phenomenon, where accumulated MPs migrated slowly as a bed wave long after the cessation of the primary input. These findings suggest that while Nature-based Solutions (NbS) effectively protect downstream ecosystems from shock loads, they necessitate rigorous sediment management strategies to mitigate the legacy risk of benthic accumulation within the restored reach.

کلیدواژه ها:

Coupled Hydro-morphodynamic Modeling ، Microplastics ، Urban River Restorations ، Nature-based Solution (NbS) ، Sediment-pollutant interaction