Effect of bank slope and ambient groundwater discharge on hyporheic transport and biogeochemical reactions in a compound channel

Abstract

As floodplains are inundated during floods in a compound channel, solutes in the surface water column reach the hyporheic zone and react with solutes upwelled from the groundwater. These biogeochemical reactive processes, such as aerobic respiration, nitrification, and denitrification, need more clarification. In this study, a 3D hydrodynamic model combined with a 2D groundwater and biogeochemical model was used to examine the influence of bank slope angle and ambient groundwater discharge on these processes. A denitrification zone was found under the interface between the main channel and the floodplain when the bank slope angle was 90°, while lower angles extended that zone horizontally. In addition, a lower bank angle decreased N entry into the streambed and enhanced nitrogen removal. A decrease in ambient groundwater had a negative impact on both aerobic respiration and denitrification. When the ambient groundwater discharge reached below −0.9 m/d, nitrification was dominant in the model domain, and the hyporheic zone turned into a NO3− source. The greatest removal efficiency, equal to 0.8, was attained at a discharge rate of −0.5 m/d for ambient groundwater and a bank slope angle of 30°. The hyporheic zone should lose its ability to remove N when ambient groundwater discharges exceed 0.25 m/d and removal efficiency fluctuates by 0. In conclusion, our findings indicate that bank slope angle and ambient groundwater discharge have a substantial impact on solute transport and biogeochemical activities in the hyporheic zone of a compound channel.

Integrating ecohydrology and ecohydraulics

“…Similarly, the 3D hydrodynamic model when combined with a 2D groundwater and biogeochemical model have successfully determined the influence of bank slope angle and ambient groundwater discharge on biogeochemical reactive processes, such as aerobic respiration, nitrification and denitrification in the floodplain system. While the paper by Nakayama (2023) provides important insights into the reservoir's contribution to global carbon cycle, the paper by Liu et al (2023) unravels the denitrification zone at the interface between the main channel and the floodplain, where the hyporheic solute transport and biogeochemical activities would occur (Liu et al, 2023).…”