Hydrogen Peroxide in Groundwater at Rifle, Colorado

Abstract:

Hydrogen peroxide (H₂O₂), as a reactive transient presenting ubiquitously in natural surface waters, can react with a large suite of biologically important and redox-sensitive trace elements. The dominant source of H₂O₂ in natural waters has long been thought to be photo-oxidation of chromophoric dissolved organic matter by molecular oxygen to produce superoxide radical, which then proceeds via dismutation to generate H₂O₂. However, recent studies have indicated that dark production of H₂O₂ in deep seawater, principally by biological production, is potentially on par with photochemical generation. Here, we present evidence for abiotic dark generation of H₂O₂ in groundwater in an alluvial aquifer adjacent to the Colorado River near Rifle, CO. Background H₂O₂ concentrations were determined in situ using a sensitive chemiluminescence-based method. Our results suggest H₂O₂ concentrations ranged from lower than the detection limit (1 nM) to 54 nM in different monitoring wells at the site, and the concentrations exhibited close correlations with profiles of dissolved oxygen and iron concentrations in the wells, indicating a possible metal redox cycling mechanism. In addition, dissolved natural organic matter, which could potentially coordinate the interconversion of ferric and ferrous species, might also play an important role in H₂O₂ formation. While biologically mediated activities have been recognized as the major sink of H₂O₂, the detected H₂O₂ pattern in groundwater suggests the existence of a balance between H₂O₂ source and decay, which potentially involves a cascade of biogeochemically significant processes, including the interconversion of ferrous/ferric species, the generation of more reactive oxygen species, such as hydroxyl radical, the depletion of dissolved oxygen and further transformation of natural organic matter and other chemical pollutants.