Effect of surface water-groundwater exchange on heavy metal transport in the vegetated rivers.

Abstract

In river ecological restoration, the mechanical and morphological properties of vegetation are critical modulators of hyporheic exchange, thereby influencing the broader fate of nutrients and contaminants. However, few studies have considered the effect of the dynamic interplay of vegetation flexibility on surface water-groundwater exchange, limiting our ability to predict the coupled dynamics of "flow-vegetation-hyporheic exchange". To address this concern, a coupled model integrating surface water-groundwater exchange and heavy metal transport was developed to investigate the synergistic regulatory mechanisms of river vegetation characteristics on hyporheic exchange at the sediment-water interface, and also the transport of copper ions. The results indicate that: (i) vegetation height significantly influences the surface flow field structure; higher vegetation creates a high-velocity zone above the canopy (approximately 0.35 m/s) and a low-velocity zone within the vegetation, forming a distinct velocity stratification that is considerably more pronounced than that observed with shorter vegetation; (ii) vegetation height is the dominant factor regulating hyporheic exchange, the hyporheic exchange flux under higher vegetation conditions are approximately 123% greater than that of the shorter vegetation, and rigid vegetation enhances flux by about 17% compared to flexible vegetation of the same height; (iii) vegetation height significantly promotes the transport of copper ions within the hyporheic zone; under higher vegetation conditions, the vertical transport depth of copper ions increases by up to 100% within 24 h, whereas the effect of vegetation flexibility on this process is fewer than that of the vegetation height. This study reveals the multi-scale regulatory mechanisms of vegetation mechanics and morphological parameters on hyporheic exchange processes, providing a theoretical basis for the rational configuration of vegetation and the integrated hydrological-ecological design in river ecological restoration.