Competitive Exclusion of Sulfate Reduction by Fe(lll)‐Reducing Bacteria: A Mechanism for Producing Discrete Zones of High‐Iron Ground Water

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Abstract
The Middendorf aquifer of South Carolina exhibits a 40‐kilometer‐wide zone where dissolved ferrous iron concentrations commonly exceed 1 mg/I. Downgradient of this zone, dissolved iron concentrations decrease to less than 0.05 mg/1. Geochemical and microbiologie evidence indicates that this zonation reflects the competitive exclusion of sulfate‐reducing activity by Fe(IH)‐reducing bacteria in the high‐iron zone and the emergence of sulfate reduction as the predominant process in the low‐iron zone. Viable Fe(III)‐ and sulfate‐reducing bacteria coexist throughout the aquifer. However, the observed linear relationship between dissolved iron and dissolved inorganic carbon as well as the lack of sulfate consumption indicates that sulfate‐reducing bacteria are much less active than Fe(III)‐reducing bacteria in the high‐iron zone. Fe(III)‐reducing bacteria appear to exclude sulfate‐reducing activity by maintaining dissolved hydrogen (˜1.0 nM), formate (˜2.0 μM), and acetate (˜1.0 μM) concentrations at levels lower than thresholds required by sulfate‐reducing bacteria. Downgradient of the high‐iron zone, Fe(III)‐reducing activity becomes limited by a lack of Fe(III) oxyhydroxides as Middendorf sediments become progressively more marine in origin. Hydrogen, formate, and acetate concentrations then increase to levels (˜3.0 nM, ˜10.9, and 2.5 μM, respectively) that allow sulfate‐reducing bacteria to become active. Increased sulfide production strips ferrous iron from solution by precipitating ferrous sulfides, and dissolved iron concentrations decrease. The observed high‐iron zonation is thus one manifestation of microbial competition for scarce substrates. The wide occurrence of similar water‐chemistry patterns implies that microbial competition mechanisms are important to the ground‐water geochemistry of many hydrologie systems.