Microbially Mediated Massive Iron Hydroxide Deposits Associated with Net Alkaline Coal Mine Drainage

Acidic iron- and sulfate-rich water from the abandoned Ocean No. 2 bituminous coal mine in Westmoreland County, Pennsylvania discharged directly into the Youghiogheny River from 1932 until 1968, when mine portals were sealed to restrict oxygen input and reduce pyrite oxidation. Currently net alkaline (~125 mg L^-1 as CaCO₃, pH 6-7), iron-rich (~38 mg L^-1) water travels a short distance before transitioning to a 10 m waterfall over a steep hillside.  Friable red and black deposits, possibly precipitated by CO₂ degassing of the turbulent falling water, exhibit vertical curtains and ripples morphologically similar to those in speleothem flowstones.

Preliminary XRD and SEM-EDAX analyses indicate the deposits consist almost entirely of microcrystalline to amorphous iron oxide (2-line ferrihydrite, poorly crystalline goethite, and non-crystalline hydrous iron oxides). These high surface area materials are effective at sorbing trace metals from solution; the deposits are enriched in Pb, Zn and Mn (about 20, 200 and 500 ppm, respectively).  Dissolved Na (~150 mg/L) in the mine water could result in enhanced sorption of anions such as arsenic (~100 ppm in the deposits). Petrographic examination reveals crenulations and undulatory fabric (cm-scale) as well as sub-mm laminations. Porous, filamentous laminae and irregular interlaminar features (~0.5 mm x 2 mm) resemble the bacterial shrubs and other biogenic textures found in travertine and geothermal acidic spring deposits. PLFA and DGGE analyses and culturing of the microbial population within the deposit indicate the presence of a wide variety of bacterial and fungal groups, including iron-oxidizing and –reducing and sulfur-oxidizing and –reducing bacteria (e.g., Desulfobacter).  Structural similarities between the interior laminations observed here and those attributed to bacterial catalysis in goethite, calcite, pyrolusite and other mineral deposits, suggest the influence of bacteria in near-surface precipitation and diagenetic processes under net alkaline conditions and ambient temperatures.