Reactive Oxygen Species Production in Riparian Zones Governed by a Flow-Induced Chromatographic Separation Process.

Riparian zones are natural hotspots for reactive oxygen species (ROS) generation, yet the spatiotemporal dynamics of ROS within these zones remain poorly understood. In this study, we combine results from a flume experiment and reactive transport modeling to show that H2O2 production is governed by a "chromatographic" separation process in which water flow modulates the interplay of O2 and reductants. The riparian aquifer matrix acts as the stationary phase hosting various mobile and immobile reductants, while water flow serves as the mobile phase supplying oxidants, here dissolved oxygen (DO) and nitrate, during surface water inflow and flushing out of the mobile reductant species during flow reversal. The preferential consumption of DO by the reductants near the up-gradient boundary during surface water intrusion generates H2O2, whereas less reactive oxidants like nitrate are transported further into the riparian aquifer, where they consume the reductants that have not reacted with DO. Although the inflow of nitrate reduces the overall ROS production capacity, it enables deeper DO penetration, hence expanding the ROS production area. The resulting coupling between hydrodynamic solute transport and biogeochemical redox reactions regulates the spatial separation and temporal evolution of H2O2 across the simulated riparian aquifer. Overall, our study advances the mechanistic understanding of ROS dynamics in riparian zones with implications for redox-mediated contaminant attenuation and carbon cycling at the groundwater-river interface.