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pH dependence of iron photoreduction in a rocky mountain stream affected by acid mine drainage

January 1, 2001

The redox speciation of dissolved iron and the transport of iron in acidic, metal‐enriched streams is controlled by precipitation and dissolution of iron hydroxides, by photoreduction of dissolved ferric iron and hydrous iron oxides, and by oxidation of the resulting dissolved ferrous iron. We examined the pH dependence of these processes in an acidic mine‐drainage stream, St Kevin Gulch, Colorado, by experimentally increasing the pH of the stream from about 4·0 to 6·5 and following the downstream changes in iron species. We used a solute transport model with variable flow to evaluate biogeochemical processes controlling downstream transport. We found that at pH 6·4 there was a rapid and large initial loss of ferrous iron concurrent with the precipitation of aluminium hydroxide. Below this reach, ferrous iron was conservative during the morning but there was a net downstream loss of ferrous iron around noon and in the afternoon. Calculation of net oxidation rates shows that the noontime loss rate was generally much faster than rates for the ferrous iron oxidation at pH 6 predicted by Singer and Stumm (1970. Science 167: 1121). The maintenance of ferrous iron concentrations in the morning is explained by the photoreduction of photoreactive ferric species, which are then depleted by noon. 

Citation Information

Publication Year 2001
Title pH dependence of iron photoreduction in a rocky mountain stream affected by acid mine drainage
DOI 10.1002/hyp.251
Authors Diane M. McKnight, B. A. Kimball, R.L. Runkel
Publication Type Article
Publication Subtype Journal Article
Series Title Hydrological Processes
Series Number
Index ID 70023349
Record Source USGS Publications Warehouse
USGS Organization Toxic Substances Hydrology Program