GEOMICROBIOLOGICAL AND MINERALOGICAL CHARACTERISTICS OF IRON-OXIDE SLUDGE ASSOCIATED WITH ABANDONED MINE DRAINAGE FROM THE OAK HILL BOREHOLES IN EASTERN PENNSYLVANIA

Runoff and groundwater discharged from abandoned mines commonly contain dissolved metals such as iron, manganese, aluminum, copper, arsenic, nickel, and zinc. The dissolved metals can be elevated at acid mine drainage (AMD) sites because of high solubility of metal oxides at low pH, but also at circumneutral pH under suboxic conditions because of high solubility of Fe(II) and Mn(II) oxides and carbonates. Because of the tendency for the gradual oxidation of Fe(II) and attenuation of trace metals by Fe(III) precipitates in rivers, wetlands, and engineered treatments systems, we are interested in factors affecting the rates of Fe(III) mineral formation and their associated chemical characteristics and stabilities.

With a goal of evaluating remediation strategies for circumneutral AMD, we are investigating the hydrology, chemistry, and microbiology of the Oak Hill boreholes in eastern Pennsylvania, which discharge 2,830 gal/min of water with pH 5.8 to 7 and high concentrations of Fe (12-21 mg/L) and Mn (3.1-4.5 mg/L) into the West Branch Schuylkill River. One objective is to document geomicrobial interactions and mineralogical characteristics of Fe(III) oxide sludge at the site to better understand the role of microbes in Fe(II) oxidation and sludge formation and how such microbes may be involved in possible treatment approaches to remove dissolved Fe from AMD. A visible- to near-infrared spectrometer was used to identify the mineralogical composition of Fe(III) oxide samples taken from the Oak Hill boreholes discharge channel and similar sites nearby by comparing their reflectance to USGS standard reference samples. Of the 17 samples analyzed, 16 were identified as ferrihydrite and one as goethite. Light microscopy was used to identify the presence of iron- and sulfur-oxidizing bacteria within the sludge. Both water/sludge samples taken from the field sites and glass slides suspended in the discharge were examined using a 40X-2000X oil immersion microscope. Bacteria in the samples were identified as Gallionella ferruginea, with subordinate Leptothrix ochracea and Toxothrix trichogenes. Studies of the genetic phylogeny of the microbes and the tendency for Fe(III) oxidation in the presence and absence of native microbes are ongoing.