Nitrate fate in mixed surface water and groundwater: Role of mixing-dependent denitrification and DNRA in hyporheic zones

Hyporheic zones (HZs), where surface water (SW) and groundwater (GW) mix underneath and adjacent to streams, are known for their inherent ability to attenuate contaminants. Mixing of reactants from SW and GW enables the occurrence of mixing-dependent reactions, mixing-dependent denitrification is commonly regarded as the last defense against groundwater-bone nitrate before it enters to streams. However, the impact of mixing-dependent DNRA on nitrate transformation is often overlooked. In this study, we conducted a flume experiment to generate downwelling of SW with dissolved organic carbon (DOC) into the sediments and create a hyporheic exchange flow (HEF) cell. We added nitrate to anoxic upwelling GW to stimulate mixing-dependent reactions. Hydrodynamics, hydrochemical conditions, microbial community and its biogeochemical function with respect to nitrogen transformation were tested and analyzed. The SW and GW mixing zone was situated along the fringe of HEF cell. The mixing zone represented a transition zone between the HEF cell and deeper GW in microbial community structure, and hosted active mixing-dependent reaction potentials. Both mixing-dependent denitrification and DNRA occurred, with the hotspots for these processes appearing predominantly on the right side (closer to the GW) and the left side (closer to the HEF cell) of the mixing zone, rather than evenly within it. The downstream and upstream movement of the mixing zone enhances the mixing-dependent denitrification and DNRA reactions. The NH₄⁺ produced by mixing-dependent DNRA would undergo further nitrification within the HEF cell because higher concentrations of nitrification functional genes present upstream. Disregarding the mixing-dependent DNRA would lead to an overestimation of HZs’ capacity to attenuate groundwater-borne nitrate. This study enhances our understanding of nitrate processing within HZs and contributes valuable insights for the effective management of watershed contaminants.