Microfracture Surface Geochemistry and Adherent Microbial Population Metabolism in TCE-Contaminated Competent Bedrock


A TCE-contaminated competent bedrock site in Portsmouth, NH was used to determine if a relation existed between microfracture surface geochemistry and the ecology and metabolic activity of attached microbes relative to terminal electron accepting processes (TEAPs) and TCE biodegradation. The bedrock is a metasandstone and metashale of the Silurian Kittery Formation. Eleven microfractures (MF 01-11) were extracted from cores of competent rock from 2 boreholes (BBC5 and BBC6) at depths > 21.3 m below ground. The host rock had 3 nominal pore width sizes (131.1, 1.136, and 0.109 μ m), a porosity of 0.8%, and a permeability of < 1 μ d. Microfracture surface precipitates were polycrystalline with grain sizes ranging from 10 to 100 μ m. Petrography and XRD revealed that carbonates and quartz were the dominant microfracture surface precipitates. Mineral distribution was heterogeneous at the 10 μ m scale. Oxidized and reduced iron species were identified with XPS on the microfracture precipitate surfaces. Carbon functional groups characteristic of NOM were also identified. SIMS mass fragment fingerprints suggested that TCE, PCE and/or VC were possibly adsorbed to NOM on the microfracture surfaces. Packer waters were alkaline (131–190 mg/L as CaCO3, pH 8.8 to 9.6), mildly reducing (Eh of −208 to 160 mV, DO of 0.4 to 2.5 mg/L), with low NPDOC values (0.8–1.7 mg/L), and measurable Fe (II) (0.1 mg/L) and Fe (III) (0.02 to 0.3 mg/L). Sulfate was the dominant anion in the packer sample water (110–120 mg/L). No sulfide was detected. H 2 was present in a number of the BBC wells at the site (2.2–7.3 nM). Amplification with specific primer sets of seven microfractures from BBC5 showed the presence of bacteria, Archaea, anaerobic dehalorespirers (Dehalococcoides sp.), sulfate reducing bacteria, and iron reducing bacteria (Geobacteraceae). Redox zonation may exist relative to spatial distance from within the microfracture network to the open fracture system. The microfracture surface precipitates, frequently spatially complex and comprised of a variety of C-, Fe- and S-containing minerals, may be another region for redox zonation. Fe was the dominant microfracture surface element and active Fe cycling is suspected. However, the primer data suggest that the microfracture network may have been more reducing than the open fracture system. In this case, the microfracture network may constitute a zone where more reductive metabolic processes occur, making this system similar to biogeochemical redox zones found in other environments.

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Geomicrobiology Journal


Taylor and Francis

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