Hydrological water exchange and nitrogen transformation in the two consecutive meander-driven hyporheic zones

Hyporheic zones play a critical role in water exchange and nitrogen (N) removal from river ecosystems. However, the hyporheic water exchange and N transformation driven by consecutive meanders have rarely been explored. To better understand how hydrological conditions control N transformation processes in two consecutive meander bends, high-resolution sampling campaigns were carried out in a meandering reach of the Xilin River during the rainy seasons of 2021 (wet year) and 2022 (normal year). Highly spatial and temporal variability of hyporheic water exchange and nitrogen concentrations were found in two consecutive meander bends. The higher hydrological connectivity between river water and shallow groundwater was observed in the inner bank of upstream sharp meander with a sinuosity of 2.01, compared to the consecutive downstream mild bend with a sinuosity of 1.47. Higher connectivity allows river water to provide more adequate nutrients as a reaction substrate, thereby promoting nitrate attenuation in the hyporheic zone of meander A. Although higher nitrate (NO₃^–) and ammonium (NH₄⁺) concentrations were observed in subsurface water during the wet year, NO₃^– removal efficiency in the hyporheic zone increased by 47.8 % relative to normal year, attributed to enhanced subsurface hydrological connectivity in the two consecutive meanders. Rainfall events of varying magnitude resulted in distinct alterations in hyporheic exchange and N distribution patterns. The sharp meander bend exhibited a shielding effect, sustaining the inner-bank lateral hyporheic flow even during storm events. The results suggest hydrogeomorphic characteristics, hyporheic exchange and nitrogen transformation are highly coupled in a consecutive meander system.