Estimation of microbial respiration rates in groundwater by geochemical modeling constrained with stable isotopes
- Pacific Northwest National Lab., Richland, WA (United States). Environmental and Health Sciences Div.
- Shepherd Miller, Inc., Fort Collins, CO (United States)
Changes in geochemistry and stable isotopes along a well-established groundwater flow path were used to estimate in situ microbial respiration rates in the Middendorf aquifer in the southeastern United States. Respiration rates were determined for individual terminal electron acceptors including O{sub 2}, MnO{sub 2}, Fe{sup 3+}, and SO{sub 4}{sup 2{minus}}. The extent of biotic reactions were constrained by the fractionation of stable isotopes of carbon and sulfur. Sulfur isotopes and the presence of sulfur-oxidizing microorganisms indicated that sulfate is produced through the oxidation of reduced sulfur species in the aquifer and not by the dissolution of gypsum, as previously reported. The respiration rates varied along the flow path as the groundwater transitioned between primarily oxic to anoxic conditions. Iron-reducing microorganisms were the largest contributors to the oxidation of organic matter along the portion of the groundwater flow path investigated in this study. The transition zone between oxic and anoxic groundwater contained a wide range of terminal electron acceptors and showed the greatest diversity and numbers of culturable microorganisms and the highest respiration rates. A comparison of respiration rates measured from core samples and pumped groundwater suggests that variability in respiration rates may often reflect the measurement scales, both in the sample volume and the time-frame over which the respiration measurement is averaged. Chemical heterogeneity may create a wide range of respiration rates when the scale of the observation is below the scale of the heterogeneity.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC06-76RL01830
- OSTI ID:
- 338616
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 62, Issue 21-22; Other Information: PBD: Nov 1998
- Country of Publication:
- United States
- Language:
- English
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