Oxidation-Reduction Processes in Ground-Water Systems
ChapelleGroundwater Microbiology and
Geochemistry Chapter
• 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)
Microbial Influence
• Microorganisms– use electron transfer to maintain life functions– respiration, activity, and growth create electro
kinetic conditions
Describing Kinetic Redox Processes• Include three components:
– document source of e that supports microbial metabolism
– document sink for the e that supports microbial metabolism
– document rates of e transfer– Microbes are the catalyst for most reactions
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-
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Why this difference
with microbes
present ?
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)
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
• Free Energy from Hydrogen Oxidation Processes
– Dictates the Steady State Concentration
Methanogenic
Sulfate-reducing
Fe(III)-reducing
Mn(IV)-reducing
Nitrate-reducing
High Hydrogen Concentration
Low Hydrogen Concentration
• 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 diffe
r
• Ambient TEAPs (terminal electron-accepting process)• Net Effect- segregate aquifers into discreet zones dominated
by electron accepting processes.
donor rich in contaminated aquiferdonor poor in pristine aquifer
Lets think about this in terms of hyporheic zone
Dominated by aerobic, then when O2 used up ->moves to TEAPS
Deplete the most efficient electron donor first-> reversed TEAPS
Can this work in the Hyporheic 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
Presence of particular electron acceptor = redox zone
Hyporheic Zone and Riparian Zone more complicated because flowing water
Indicates reduction is occurring somewhere up gradient.
Must be Sherlocky and use simple deducing
Use data in conjunction •Doners present•Acceptors present•H concentrations