N2O Production and Consumption Processes in a Salinity‐Impacted Hyporheic Zone

Abstract Hyporheic zone plays a key role in the release of greenhouse gas nitrous oxide (N 2 O) to the stream water, and the variable‐density flow induced by salinity from stream water can increase the hyporheic exchange flux. However, the nitrogen dynamics in a salinity‐impacted hyporheic zone under different saltwater concentrations still remain unclear. In this study, a numerical model coupling the hyporheic flow, salt transport and nitrogen transformations is proposed to investigate how density gradients affect the N 2 O production and consumption processes in the streambed. The results show that density gradients between infiltrating saltwater and ambient groundwater create gravitational instabilities that significantly accelerate the downward flow of pore water. Such density gradients enhance the species (e.g., dissolved organic carbon and NO 3 − ) mixing efficiency and increase the reaction area, leading to the formation of N 2 O fingers and promoting the N 2 O production‐consumption rate. The variable‐density flow also drags more reactants infiltrating deeper area with longer residence times and increase the frequency of the Damköhler number Da > 1 in the streambed, facilitating the conversion of NO 3 − to the final product N 2 through stepwise denitrification processes. Therefore, the NO 3 − removal efficiency and N 2 O yield are increased by 24% and reduced by 3% at low‐density gradients (Δ ρ / ρ f = 7 × 10 −4 ), respectively. The possible implications and applications about the effects of hydrodynamic changes due to the density effect are also presented. However, further biogeochemical experiments and river restoration strategies are needed to validate the results of this study.