Geochemistry and stable isotope investigation of acid mine drainage associated with abandoned coal mines in central Montana,USA |
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| Authors: | Christopher H Gammons Terence E Duaime Stephen R Parker Simon R Poulson Patrick Kennelly |
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| Institution: | 1. Institut für Bergbau und Spezialtiefbau, Technische Universität Bergakademie Freiberg, 09596 Freiberg, Germany;2. School of Resources and Geosciences, China University of Mining and Technology, 221116 Xuzhou, PR China;1. Graduate School of Horticulture, Chiba University, Matsudo, 271-8510 Chiba, Japan;2. College of Resources and Environmental Engineering, Guizhou University, Huaxi, 550004 Guiyang, Guizhou, China;3. Environmental Engineering Program, Center for Watershed Research & Service, Saint Francis University, 117 Evergreen Drive, Loretto, PA 15940, USA;1. IGG-CNR, via Moruzzi 1, I-56124 Pisa, Italy;2. Dipartimento di Scienze Ambientali, Informatica e Statistica, Università Ca'' Foscari Venezia, via Torino 155, 30172 Venezia, Italy;3. Dipartimento di Scienze della Terra, Università di Pisa, via S. Maria 53, I-56126 Pisa, Italy;4. IDPA-CNR, via Torino 155, 30172 Venezia, Italy |
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| Abstract: | The Great Falls-Lewistown Coal Field (GFLCF) in central Montana contains over 400 abandoned underground coal mines, many of which are discharging acidic water with serious environmental consequences. Areas of the mines that are completely submerged by groundwater have circum-neutral pH and relatively low concentrations of metals, whereas areas that are only partially flooded or freely draining have acidic pH (< 3) and high concentrations of metals. The pH of the mine drains either decreases or increases after discharging to the surface, depending on the initial ratio of acidity (mainly Al and Fe2+) to alkalinity (mainly HCO3?). In acidic, Fe-rich waters, oxidation of Fe2+ after exposure to air is microbially catalyzed and follows zero-order kinetics, with computed rate constants falling in the range of 0.97 to 1.25 mmol L? 1 h? 1. In contrast, Fe2+ oxidation in near-neutral pH waters appears to be first-order with respect to Fe2+ concentration, although insufficient data were collected to constrain the rate law expression. Rates of Fe2+ oxidation in the field are dependent on temperature such that lower Fe2+ concentrations were measured in down-gradient waters during the day, and higher concentrations at night. Diel cycles in dissolved concentrations of Zn and other trace metals (Mn, Ni) were also noted for down-gradient waters that were net alkaline, but not in the acidic drains.The coal seams of the GFLCF and overlying Cretaceous sandstones form a perched aquifer that lies ~ 50 m above the regional water table situated in the underlying Madison Limestone. The δD and δ18O values of flooded mine waters suggest local derivation from meteoric water that has been partially evaporated in agricultural soils overlying the coal mines. The S and O isotopic composition of dissolved sulfate in the low pH mine drains is consistent with oxidation of biogenic pyrite in coal under aerated conditions. A clear distinction exists between the isotopic composition of sulfate in the acid mine waters and sulfate in the adjacent sedimentary aquifers, making it theoretically possible to determine if acid drainage from the coal mines has leaked into the underlying Madison aquifer. |
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