Production of Hydrogen Peroxide in Groundwater at Rifle, Colorado

The commonly held assumption that photodependent processes dominate H₂O₂ production in natural waters has been recently questioned. Here, we present evidence for the unrecognized and light-independent generation of H₂O₂ in groundwater of an alluvial aquifer adjacent to the Colorado River near Rifle, CO. In situ detection using a sensitive chemiluminescent method suggests H₂O₂ concentrations ranging from lower than the detection limit (<1 nM) to 54 nM along the vertical profiles obtained at various locations across the aquifer. Our results also suggest dark formation of H₂O₂ is more likely to occur in transitional redox environments where reduced elements (e.g., reduced metals and 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 our detected H₂O₂ profiles, and therefore there are other minor biological and/or chemical controls on H₂O₂ steady-state concentrations in such aquifer. Because of its transient nature, the widespread presence of H₂O₂ in groundwater suggests the existence of a balance between H₂O₂ sources and sinks, which potentially involves a cascade of various biogeochemically important processes that could have significant impacts on metal/nutrient cycling in groundwater-dependent ecosystems, such as wetlands and springs. More importantly, our results demonstrate that reactive oxygen species are not only widespread in oceanic and atmospheric systems but also in the subsurface domain, possibly the least understood component of biogeochemical cycles.