Numerical simulation of overbank hyporheic transport and biogeochemical reactions in a compound channel

Transverse hyporheic transport and biogeochemical reactions in a compound channel remain poorly understood. As floodplains are submerged, solutes in the surface water enter the hyporheic zone of the compound channel and react with the solutes upwelling below the floodplain. Aerobic respiration (AR) and denitrification (DN) processes in the hyporheic zone of the compound channel still need to be clarified. In this paper, a 3D hydrodynamic model coupled to a 2D groundwater flow and biogeochemical model was applied to investigate such processes. The model results were verified using laboratory experimental measurements. The effects of bank slope angle, ambient groundwater flow, and ambient groundwater solute concentration on aerobic and anaerobic biogeochemical processes were studied. The denitrification zone was found below the interface between the main channel and the floodplain. The lower bank slope angle favours nitrogen removal. Small losing groundwater discharges promote aerobic respiration and denitrification reactions. As the ambient groundwater oxygen concentration rises, O-2 consumption increases while N removal decreases. As the NO3 concentration in ambient groundwater increases, NO3 consumption increases and proportionally NO3 consumption decreases. These findings not only provide a better understanding of biogeochemical reactions in a compound channel but can also be applied to river restoration to efficiently remove nitrate by modifying bank slope angles.