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Distinguishing Iron-Reducing from Sulfate-Reducing Conditions |
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| Authors: | by Francis H Chapelle Paul M Bradley Mary Ann Thomas and Peter B McMahon |
| Institution: | U.S. Geological Survey, South Carolina Water Science Center, Columbia, SC 29210.; U.S. Geological Survey, Ohio Water Science Center, 6480 Doubletree Ave., Columbus, OH 43229.; U.S. Geological Survey, Denver Federal Center, MS 415, Bldg. 53, Rm. E-2313, Lakewood, CO 80225. |
| Abstract: | Ground water systems dominated by iron‐ or sulfate‐reducing conditions may be distinguished by observing concentrations of dissolved iron (Fe2+) and sulfide (sum of H2S, HS?, and S= species and denoted here as “H2S”). This approach is based on the observation that concentrations of Fe2+ and H2S in ground water systems tend to be inversely related according to a hyperbolic function. That is, when Fe2+ concentrations are high, H2S concentrations tend to be low and vice versa. This relation partly reflects the rapid reaction kinetics of Fe2+ with H2S to produce relatively insoluble ferrous sulfides (FeS). This relation also reflects competition for organic substrates between the iron‐ and the sulfate‐reducing microorganisms that catalyze the production of Fe2+ and H2S. These solubility and microbial constraints operate in tandem, resulting in the observed hyperbolic relation between Fe2+ and H2S concentrations. Concentrations of redox indicators, including dissolved hydrogen (H2) measured in a shallow aquifer in Hanahan, South Carolina, suggest that if the Fe2+/H2S mass ratio (units of mg/L) exceeded 10, the screened interval being tapped was consistently iron reducing (H2～0.2 to 0.8 nM). Conversely, if the Fe2+/H2S ratio was less than 0.30, consistent sulfate‐reducing (H2～1 to 5 nM) conditions were observed over time. Concomitantly high Fe2+ and H2S concentrations were associated with H2 concentrations that varied between 0.2 and 5.0 nM over time, suggesting mixing of water from adjacent iron‐ and sulfate‐reducing zones or concomitant iron and sulfate reduction under nonelectron donor–limited conditions. These observations suggest that Fe2+/H2S mass ratios may provide useful information concerning the occurrence and distribution of iron and sulfate reduction in ground water systems. |
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