Is iron redox cycling in a high altitude watershed photochemically or thermally driven?

Many studies have shown that the concentration of aqueous Fe 21 increases in surface waters during exposure to sunlight and attribute this phenomenon either to photoreductive dissolution of ferric minerals/colloids or to ligand-to-metal charge transfer within organic complexes of Fe3+. In a multi-summer study of iron redox cycling in a relatively high pH stream (Middle Crow Creek, MCC) that drains a mostly-granitic watershed at an altitude of 2400 m, aqueous Fe3+ (not Fe2+) concentrations were correlated with both sunlight and temperature. A steady state model fails to explain the [Fe2+] and [Fe3+] data from this stream. However, Fe2+ concentrations can be explained using a simple kinetic model in which rate constants for oxidation and reduction were obtained by fitting data from in situ oxidation experiments, including first-order thermal (non photochemical) reduction of Fe3+. Rate constants obtained from experiments in the dark result in too much Fe2+ to match the data from illuminated experiments, requiring a net photooxidation process to explain [Fe3+] measured in MCC. The organic content of MCC results in high concentrations of Fe-DOM complexes that not only act as a reservoir contributing to daily changes in [Fe-tot] as measured by our methods, but whose photochemistry may contribute highly oxidizing reactive oxygen species to the stream. In situ studies suggest that photochemical reduction of organically bound Fe3+ occurs, followed by thermal release of Fe2+ to the water column and subsequent rapid re-oxidation. (c) 2009 Elsevier B.V. All rights reserved.