First Measurements of Dark Reactive Oxygen Species in a Groundwater Aquifer
First measurement of the presence of hydrogen peroxide concentrations in groundwater establishes importance of reactive oxygen species in the subsurface.
Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) are very short-lived intermediate molecules generated during the one-electron reduction of oxygen to water through photochemical oxidation or through a “dark” process involving microorganisms. To date, ROS have been found in the deep ocean, sediments, and fresh waters. Now, a team of scientists has demonstrated a dark biological process that generates hydrogen peroxide in groundwater from an alluvial aquifer.
Hydrogen peroxide and an associated class of compounds called ROS have long been known to be important drivers of biogeochemical cycling and contaminant decomposition in surface water (oceans, rivers, and lakes). By demonstrating that hydrogen peroxide and therefore the associated group of ROS were widely distributed in the groundwater of a uranium-contaminated alluvial floodplain, scientists have established that ROS likely are important to the chemistry and functioning of biogeochemical cycles in this floodplain and other groundwater systems. The presence of ROS in some groundwater systems may help explain the apparent non-equilibrium conditions in these systems, as well as potential organic matter oxidation pathways.
The commonly held assumption that photodependent processes dominate H2O2 production in natural waters has recently been questioned. This paper demonstrated for the unrecognized and light-independent generation of H2O2 in groundwater of an alluvial aquifer adjacent to the Colorado River near Rifle, Colorado.
Using a sensitive chemiluminescent method to detect H2O2 along vertical profiles at various locations across an alluvial aquifer of the Colorado River, a team of scientists from Lawrence Berkeley National Laboratory (LBNL), Peking University, and the University of New South Wales found that H2O2 concentrations ranged from lower than the detection limit (<1 nM) to 54 nM. The data also suggest dark formation of H2O2 is more likely to occur in transitional redox environments where reduced elements [e.g., reduced metals and natural organic matter (NOM)] meet oxygen, such as oxic-anoxic interfaces. A simplified kinetic model involving interactions among iron, reduced NOM, and oxygen was able to reproduce roughly many, but not all, of the features in the detected H2O2 profiles. This suggests there likely are other minor biological and/or chemical controls on H2O2 steady-state concentrations in such an aquifer. Because of its transient nature, the widespread presence of H2O2 in groundwater indicates the existence of a balance between H2O2 sources and sinks, potentially involving a cascade of various biogeochemically important processes that could have significant impacts on metal or nutrient cycling in groundwater-dependent ecosystems, such as wetlands and springs. More importantly, these results demonstrate that ROS are not only widespread in oceanic and atmospheric systems, but also are present in the subsurface domain, possibly the least understood component of the Earth system, yet critical for understanding a wide variety of biogeochemical cycles.
Peter S. Nico
Lawrence Berkeley National Laboratory
Support for the Subsurface Biogeochemical Research (SBR) Watershed Function Scientific Focus Area (SFA) at Lawrence Berkeley National Laboratory is by the SBR program of the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science.
Yuan, X., P. S. Nico, X. Huang, T. Liu, C. Ulrich, K. H. Williams, and J. A. Davis. “Production of hydrogen peroxide in groundwater at Rifle, Colorado.” Environmental Science & Technology 51(14), 7881–91 (2017). [DOI:10.1021/acs.est.6b04803].