Anaerobic biogeochemical transformation of trichloroethene associated with low permeability source zones




Berns, Erin Christine

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Biogeochemical reactions promoted by reactive mineral species and bacteria in sediment and groundwater influence the fate of environmental contaminants. Enhanced reductive transformations of chlorinated ethenes, a class of persistent contaminants, have been observed in biologically active systems relative to analogous abiotic systems. While some reactions mediated by bacteria are more efficient than abiotic reactions, biological dechlorination of chlorinated ethenes can yield byproducts that are no less toxic than the original contaminant. Chlorinated ethenes are remarkably challenging to remediate in aquifers with low permeability zones (LPZs), such as silts and clays. This dissertation explores the biotic and abiotic contributions to trichloroethene (TCE) transformation during back diffusion from LPZs into adjacent high permeability zones (HPZs). These contributions are quantified with flow cell experiments that represent the LPZ/HPZ interface using clay and sand from a field site. Flow cell data informed development of a numerical diffusion-reaction model, and back diffusion during biotic and abiotic TCE transformation was simulated. Both pathways occurred simultaneously in the flow cell, with biotic processes transforming more TCE mass than abiotic processes in the presence of electron donor. Simulations constrained to abiotic reactions demonstrated that reactive minerals can decrease TCE flux from LPZs by 2-53% over two years, depending on the mineral type. TCE attenuation of this magnitude highlights the potential for abiotic transformations in some contaminated aquifers. Other biologically mediated reductive reactions promote formation of minerals that abiotically transform TCE to innocuous compounds. The role of both sulfate and iron reducing bacteria in forming and maintaining reactive iron sulfide minerals was evaluated. Reduction potential significantly influenced TCE transformation kinetics, with more negative potentials correlating with more iron sulfide precipitation and higher TCE transformation rates. Coprecipitation of other mineral species at less negative potentials contributed to diminished TCE transformation. XPS and XRD data paired with MINTEQ calculations informed conclusions about experimental precipitate reactivity. Prior to this work, no study had shown that mineral-promoted abiotic reactions could attenuate TCE in LPZs. It is also the first time that correlations between redox potential, mineral stability, and TCE transformation kinetics have been evaluated for biogenic iron sulfides with varied iron concentrations


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