posted on 2020-04-01, 19:07authored byNa Zhang, Xiaochuang Bu, Yiming Li, Yanting Zhang, Songhu Yuan, Zhang Wen, Man Tong, Li Lin
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.