Water Table Fluctuations Regulate Hydrogen Peroxide Production and Distribution in Unconfined Aquifers.

Significance of reactive oxygen species (ROS) in subsurface has been increasingly documented in recent years, whereas the mechanisms controlling ROS production and distribution in subsurface remain poorly understood. Here we show that water table fluctuations regulate the dynamics of hydrogen peroxide (H2O2) production and distribution in unconfined aquifers. In one hydrological year, we measured the dynamics of H2O2 distribution in an unconfined aquifer impacted by a 14 m water level fluctuation in the adjacent Yangtze River. H2O2 concentrations in groundwater attained up to 123 nM at rising water table stage in summer, but were low or even below the detection limit at the other stages of stable and falling water table. Lab experiments and kinetic models revealed that abiotic reactions between dissolved O2 and reduced species (i.e., Fe(II) and organic matter) were responsible for H2O2 production in the aquifers. Both field observations and reactive transport models unveiled that a rising water table developed a thermodynamically unstable banded zone in the unconfined aquifer in which elevated coexisting dissolved O2 and reduced species favored abiotic H2O2 production. Our findings provide fundamentals for understanding and predicting ROS distribution in unconfined aquifers, and constrain the significance of ROS in aquifers to specific temporal and spatial domains.