Hydrogen Peroxide Cycling in Surface Geothermal Waters of Yellowstone National Park

Hydrogen peroxide (H 2 O 2 ), 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 H 2 O 2 concentrations suggest that photochemically mediated processes were responsible for its formation. Photochemical reactions with DOC are likely the primary pathway responsible for H 2 O 2 formation in geothermal waters. Although microbially mediated processes are important in limiting the buildup of H 2 O 2, the inverse relationship between H 2 O 2 and sulfide ion suggests that H 2 O 2 decay may also occur via chemically mediated processes in the sulfur-rich waters.