Geomicrobiology

Iron-rich wetland resulting from weathering of sulfide minerals from nearby country rock near Silverton, Colorado. Photo credit: Mark R. Stanton, USGS Image Gallery

Several tasks in the Mineral Resources Program (MRP) are concerned with the microbial aspects of acid mine drainage (AMD) generation and mitigation. AMD is characterized by acidic, metal-rich waters and red-orange iron-bearing solids formed by the weathering of pyrite (FeS2) and subsequent oxidation of soluble ferrous iron (Fe[II]) to insoluble ferric iron (Fe[III]). Iron-oxidizing bacteria present in AMD environments can accelerate the rate of ferrous oxidation by almost 10^6 compared to inorganic mechanisms. Thus, iron-oxidizing bacteria can be responsible for nearly continual generation of AMD. In contrast, sulfate-reducing bacteria in areas near AMD sites are responsible for trace metal sequestration in authigenic sulfides such as FeS. In the course of oxidizing organic matter, these bacteria produce sulfide that reacts with metals, and bicarbonate that acts to raise the local pH. These studies employ classic microbiological methods, such as isolation and culture of bacteria from solid and aqueous samples, determination of cell size and morphology, staining and biochemical properties, and when possible, identification based on these characteristics. Bacterial and abiological experiments have been performed to compare to metal release rates, such as the release of iron and zinc from the acid dissolution of sphalerite (ZnS). Current tasks are examining the role of bacteria in the geochemical stabilization (reduced acid production) of mine waste rock piles via carbon fixation and sequestration, and soil formation. Additionally, enumeration of major microbial groups overlying an unmined sulfide ore body are in progress to determine pre-mining geochemical and microbiological conditions for comparison to weathered mine waste rockpiles. These studies will employ newer molecular biological methods for microbial analysis. Together, the results should provide a better understanding of how bacteria influence or control AMD development.

Relevant Publications:

Stanton, M.R., Yager, D.B., Fey, D.L., and Wright, W.G., 2007, Formation and geochemical significance of iron bog deposits, Chapter E14 in Integrated Investigations of Environmental Effects of Historical Mining in the Animas River Watershed, San Juan County, Colorado. Church, S.E., von Guerard, Paul, and Finger, S.E., eds., U.S. Geological Survey Professional Paper 1651, p. 689-720. (online paper, PDF 31.2 MB )

Stanton, M.R., Fey, D.L., Church, S.E., and Holmes, C.W., 2007, Processes affecting the geochemical composition of wetland sediment, Chapter E25 in Integrated Investigations of Environmental Effects of Historical Mining in the Animas River Watershed, San Juan County, Colorado. Church, S.E., von Guerard, Paul, and Finger, S.E., eds., U.S. Geological Survey Professional Paper 1651, p. 1029-1064. (online paper, PDF 744 KB )

Mark R. Stanton, 2008, Bacterial processes can influence the movement of metal contaminants in wetlands that have been affected by acid drainage, Chapter D in Verplanck, P.L., ed., U.S. Geological Survey Circular 1328, p. 18-23. (online circular)

Mark R. Stanton, Pamela A. Gemery-Hill, Wayne C. Shanks III, and Cliff D. Taylor, 2008, Rates of zinc and trace metal release from dissolving sphalerite at pH 2.0 - 4.0. Applied Geochemistry, v. 23, p. 136-147.

For more information contact Mark R. Stanton, Crustal Imaging and Characterization Team.

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