Hydrogen peroxide cycling in surface geothermal waters of Yellowstone National Park

Hydrogen peroxide (H2O2), iron, and sulfide ion were measured every 4 h ove r 40-h periods at four hot springs in Yellowstone National Park, WY: an alk aline spring (Black Sand Pool); the sulfur-rich, near-neutral Roadside Spri ng near Nymph Lake; and two iron-rich, acidic springs (Chocolate Pots and t he iron-rich Roadside Spring near Nymph Lake). Hydrogen peroxide concentrat ions 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 sugge st that photo chemically mediated processes were responsible far its format ion. Photochemical reactions with DOC are likely the primary pathway respon sible for H2O2 formation in geothermal waters. Although microbially mediate d processes are important in limiting the buildup of H2O2, the inverse rela tionship between H2O2 and sulfide ion suggests that H2O2 decay may also occ ur via chemically mediated processes in the sulfur-rich waters.