ROS production upon groundwater oxygenation: Implications of oxidative capacity during groundwater abstraction and discharging

Production of reactive oxygen species (ROS), such as hydrogen peroxide (H₂O₂) and hydroxyl radical (•OH), has been increasingly discovered upon oxygenation of reduced species in anoxic soil and sediment pore waters. We therefore hypothesize that H₂O₂ and •OH can be produced during groundwater abstraction and discharging due to oxygenation. To evaluate the capacities of H₂O₂ and •OH accumulation during groundwater oxygenation, here we measured H₂O₂ and •OH accumulations during dark oxygenation of groundwater abstracted from different depths of 11 wells adjacent to the Han River. This abstraction could simulate the scenarios of groundwater abstraction by riverside pumping and also of groundwater discharging in hyporheic zones. Results showed that H₂O₂ and •OH formed during oxygenation of the groundwater with low Eh values (< 63 mV), and only H₂O₂ (< 3.37 μM) formed for the groundwater with high Eh values (63–206.5 mV). Statistical analysis indicated that dissolved Fe(II) was mainly accountable for the production of measurable •OH in the low Eh locations. The aquifer conditions (i.e., aquifer lithology) and external environmental factors (i.e. pumping disturbance) within short-term abstraction process had negligible influence on ROS production. The relationship between ROS production and groundwater chemistry was further explored by multiple linear regression, which deduced the quantitative models for estimating ROS production upon oxygenation of groundwater from different sources with different chemistry. Using the abstracted groundwater in western Bengal basin as an example, the estimated results indicated that ROS production had a great potential to oxidize the typical contaminant of As(III). As ROS, particularly •OH, represents strong oxidants, our findings implicate an overlooked oxidative capacity during groundwater abstraction and discharging in hyporheic zones or other areas, which could lead to the oxidative transformation of substances and the oxidative stress of microbial metabolism and associated biogeochemical processes in the environments suffering from groundwater oxygenation. This oxidative capacity is particularly important when the groundwater with low Eh values, i.e., under strongly reductive conditions, were abstracted and oxygenated in the surface.