Entrapped Gas Bubbles as O₂ Sink and Source for Reactive Oxygen Species Production in Surface Water and Groundwater...

Surface water-groundwater interaction zones are recognized as hotspots for reactive oxygen species (ROS) production. Entrapped gas bubbles formed due to water table fluctuations are important sinks and sources of dissolved oxygen (DO), whereas their influence on ROS production remains unknown. Here, we use column experiments to simulate surface water-groundwater interactions and quantitatively investigate the role of entrapped gas bubbles in the spatiotemporal distributions of DO and H₂O₂. During surface water recharge, gas bubbles act as O₂ sinks, which uptake O₂ from DO in the infiltrated surface water and thus restrict O₂ penetration and the resultant H₂O₂ production. For subsequent groundwater discharge, the O₂ stored in gas bubbles serves as a DO source for groundwater, which promotes H₂O₂ production. Cycles of groundwater discharge and surface water recharge experiments support the fact that the restriction and promotion effects induced by gas bubbles persist across recharge-discharge cycles. Reactive transport modeling reveals that the peak concentration and distribution area of H₂O₂ during surface water recharge decrease with the increase in the degree of gas saturation, but they increase during groundwater discharge. These findings highlight the previously overlooked dual roles of entrapped gas bubbles in ROS production and distribution under fluctuating hydrological regimes.