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Oxidation-Reduction Processes in Ground-Water Systems

Oxidation-Reduction Processes in Ground-Water Systems. Chapelle Groundwater Microbiology and Geochemistry Chapter. REDOX. If Dr. Alpers has taught me anything, its. REDOX. If Dr. Alpers has taught me anything, its. 3+. 2+. Non Equilibrium of Groundwater Various Redox Couples

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Oxidation-Reduction Processes in Ground-Water Systems

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  1. Oxidation-Reduction Processes in Ground-Water Systems Chapelle Groundwater Microbiology and Geochemistry Chapter

  2. REDOX • If Dr. Alpers has taught me anything, its...

  3. REDOX • If Dr. Alpers has taught me anything, its... 3+ 2+

  4. Non Equilibrium of Groundwater • Various Redox Couples • 2 ways to figure out Redox • Nerst Equation • Thermodynamic Problem: including a known free ENERGY • Kinetic Approach • Measure electrical potential: another form of ENERGY transfer e creates energy -> elec. current generated =amperage - Fig 10.4 The lack of internal consistency between Ehs calculated with different redox couples as observed by Lindberg and Runnels (1984)

  5. Microbial Influence • Microorganisms • use electron transfer to maintain life functions • respiration, activity, and growth create electro kinetic conditions

  6. Describing Kinetic Redox Processes • Include three components: • document source of e that supports microbial metabolism • documentsink for the e that supports microbial metabolism • document rates of e transfer • Microbes are the catalyst for most reactions - - - Why this difference with microbes present ?

  7. Identifying Electron Donors • An Example: • Long, Long ago scientist were confused. • Ocean had low magnesium and nodules with high concentrations of other metals • Submarine Alvin found gushing hot springs at spreading centers • Water rich in Fe, Mn, H2S mixes with O2 rich Sea Water • Ground Water Carbon is abundant • but it is important to figure out the species • DCE-common contaminant can be Electron Donor Sink (acceptor) Source (donor)

  8. Identifying Electron Acceptors • Acceptors in Microbial Metabolism • Oxygen, Nitrate, Mn(IV), Fe(III), Sulfate, CO2 • BUT Carbon is the most abundant • Microbial Ecology and Competition • H2 is most widely used for anaerobic respiration • Steady State Conditions will follow this order • Methanogens > Sulfate reducers > Fe(III) reducers > Nitrate reducers Michaelis-Menton Kinetics

  9. Free Energy from Hydrogen Oxidation Processes • Dictates the Steady State Concentration • High Hydrogen Concentration • Low Hydrogen Concentration • Methanogenic • Sulfate-reducing • Fe(III)-reducing • Mn(IV)-reducing • Nitrate-reducing

  10. Hydrogen concentration develop due to differeing uptake efficiencies Organic Matter initially consumed by fermenting bacteria producing H Depending on which donors are available, H concentrations will differ

  11. Ambient TEAPs (terminal electron-accepting process) • Net Effect- segregate aquifers into discreet zones dominated by electron accepting processes. donor rich in contaminated aquifer donor poor in pristine aquifer Deplete the most efficient electron donor first-> reversed TEAPS Dominated by aerobic, then when O2 used up ->moves to TEAPS Lets think about this in terms of hyporheic zone

  12. Can this work in the Hyporheic Zone? Presence of particular electron acceptor = redox zone Oxygen decrease at depth-active O2 reduction O2 depleted Nitrate falls- nitrate reduction Completely anoxic Mn2+ accumulates- Mn(IV) reduction Nitrate is completely consumed- Fe 2+ accumulates Hyporheic Zone and Riparian Zone more complicated because flowing water Indicates reduction is occurring somewhere up gradient.

  13. Must be Sherlocky and use simple deducing • Use data in conjunction • Doners present • Acceptors present • H concentrations

  14. Hyporhiec Zone Redox USGS, 2013 Riparian Zone Redox Cygan, 2007; Vadose Zone Journal

  15. Questions

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