Modeling surface water and groundwater mixing and mixing-dependent denitrification with bedform dynamics

Abstract. The hyporheic zone (HZ), where surface water (SW) and groundwater (GW) interact and mix, acts as a critical interface that attenuates contaminants through enhanced biogeochemical cycling. While bedform migration significantly influences hyporheic exchange and non-mixing-driven reactions of solutes from upstream SW, the effects of bedform migration on SW-GW mixing dynamics and mixing-triggered biogeochemical reactions – particularly under gaining stream conditions – remain poorly understood. Establishing a coupled hydrodynamic and reactive transport model that incorporates bedform migration, this paper systematically examines nitrogen processing for scenarios of variable sediment grain size, stream velocities, and upwelling GW fluxes. Results of this study reveal that SW-GW mixing and mixing-triggered denitrification zones progressively transition from crescent shapes into uniform band-like configurations as bedforms migrate. Both hyporheic exchange flux and mixing flux increase with increasing stream velocity and associated bedform celerity. The mixing proportion and mixing zone size increase at the start of migration, while they reach approximately stable when turnover becomes the dominant hyporheic exchange mechanism. Slow to moderate migrated bedforms with enhanced mixing dynamics facilitate mixing-triggered denitrification, whereas fast stream flows and migrating bedforms shorten solute residence timescales and limit denitrification potential. Consequently, in fine to medium sandy sediments, groundwater-borne nitrate removal efficiency declines significantly with bedform migration. The self-purification capacity of the HZ, and particularly its functioning as a natural barrier against GW contamination, is hindered under such dynamic bedform conditions. These findings highlight the need to maintain stable bedform conditions in restoration projects to enhance the capacity of HZ contaminant attenuation.