Chang Ge

In situ chemical oxidation (ISCO) is widely used for groundwater remediation, whereas the influence of aquifer heterogeneity, that is, the difference in physical structure and chemical compositions, on oxidant retention and consumption remains poorly understood. In low-permeability lens-containing sand columns, we found that the retention and consumption of oxidant (KMnO4) were largely influenced by the chemical reactivity of the lens besides the well-known permeability. Inert lens (represented by kaolin) mainly altered the transport pathways of oxidant, whereas chemically reactive lenses (represented by real sediments with different electron-donating capacities) further enhanced the retention and consumption within the lenses. A higher electron-donating capacity led to more oxidant retention and consumption. Moreover, the reduced components released from the reactive lens could also increase the oxidant consumption. This study highlights the importance of chemical reactivity besides the physical structure of lenses in heterogeneous aquifers for oxidant transport, retention, and consumption and also implicates the consideration of chemical reactivity for designing the injection dosages and position of oxidants in ISCO remediation.

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.