Hydrogen Peroxide Cycling in Surface Geothermal Waters of Yellowstone National Park

Hydrogen peroxide (H2O2), iron, and sulfide ion were measured every 4 h over 40-h periods at four hot springs in Yellowstone National Park, WY: an alkaline spring (Black Sand Pool); the sulfur-rich, near-neutral Roadside Spring near Nymph Lake; and two iron-rich, acidic springs (Chocolate Pots and the iron-rich Roadside Spring near Nymph Lake). Hydrogen peroxide concentrations reached 200−600 nM by late afternoon and decreased, in most cases, to less than 50 nM during the night. Diel changes in H2O2 concentrations suggest that photochemically mediated processes were responsible for its formation. Photochemical reactions with DOC are likely the primary pathway responsible for H2O2 formation in geothermal waters. Although microbially mediated processes are important in limiting the buildup of H2O2, the inverse relationship between H2O2 and sulfide ion suggests that H2O2 decay may also occur via chemically mediated processes in the sulfur-rich waters.