| Two field-scale bioreactors, which were built to treat acid mine drainage from a mine in Lacy, Indiana, were studied in an effort to quantify autochthonous mineral production. Both bioreactors were deployed in June 2009, and one was decommissioned in August 2011 due to evidence of declining performance. Chemical data from water samples, which were collected seasonally from twelve internal sampling ports in each bioreactor between June 2009 and June 2012, were subjected to equilibrium-chemistry analysis in order to determine saturation indices for sixteen minerals. Seventeen samples of reactive substrate material were removed from the bioreactors and analyzed for total iron, aluminum and sulfur content and speciation. delta34S values were obtained for the extracted sulfur. Selected samples were also subjected to X-ray diffraction analysis. Hydrobasaluminite, felsobanyaite, gibbsite, goethite, sulfate-bearing green rust, gypsum, pyrite, mackinawite and elemental sulfur were identified as the minerals likely to have formed within the Lacy bioreactors. Pore volume loss due to the formation of autochthonous minerals was determined to be insufficient to negatively impact the hydraulic conductivity of the bioreactor substrate. Sulfur isotope fractionations between sulfate and reduced sulfur species indicated that dissimilatory sulfate reduction was the dominant microbial process within the bioreactors. Furthermore, organically bound sulfur compounds were found to be a significant sink for sulfur within the Lacy bioreactors. |
