Production of hydrogen peroxide in an intra-meander hyporheic zone at East River, Colorado

The traditionally held assumption that photo-dependent processes are the predominant source of H₂O₂ in natural waters has been recently questioned by an increrasing body of evidence showing the ubiquitiousness of H₂O₂ in dark water bodies and in groundwater. In this study, we conducted field measurement of H₂O₂ in an intra-meander hyporheic zone and in surface water at East River, CO. On-site detection using a sensitive chemiluminescence method suggests H₂O₂ concentrations in groundwater ranging from 6 nM (at the most reduced region) to ~ 80 nM (in a locally oxygen-rich area) along the intra-meander transect with a maxima of 186 nM detected in the surface water in an early afternoon, lagging the maximum solar irradiance by ∼ 1.5 h. Our results suggest that the dark profile of H₂O₂ in the hyporheic zone is closely correlated to local redox gradients, indicating that interactions between various redox sensitive elements could play an essential role. Due to its transient nature, the widespread presence of H₂O₂ in the hyporheic zone indicates the existence of a sustained balance between H₂O₂ production and consumption, which potentially involves a relatively rapid succession of various biogeochemically important processes (such as organic matter turnover, metal cycling and contaminant mobilization). More importantly, this study confirmed the occurrence of reactive oxygen species at a subsurface redox transition zone and further support our understanding of redox boundaries on reactive oxygen species generation and as key locations of biogeochemical activity.