Transcript Oxidation-Reduction Processes in Ground-Water Systems Chapelle Groundwater Microbiology and Geochemistry Chapter REDOX • If Dr.

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
– 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:
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
– document
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
Source (donor)
Sink (acceptor)
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
Michaelis-Menton Kinetics
– Steady State Conditions will follow this order
• Methanogens > Sulfate reducers > Fe(III) reducers > Nitrate reducers
• Free Energy from Hydrogen Oxidation Processes
– Dictates the Steady State Concentration
High Hydrogen Concentration
Methanogenic
Sulfate-reducing
Fe(III)-reducing
Mn(IV)-reducing
Nitrate-reducing
Low Hydrogen Concentration
• Hydrogen concentration develop due to
differeing uptake efficiencies
• Ambient TEAPs (terminal electron-accepting process)
• Net Effect- segregate aquifers into discreet zones dominated
by electron accepting processes.
donor poor in pristine aquifer
Dominated by aerobic, then when
O2 used up ->moves to TEAPS
donor rich in contaminated aquifer
Deplete the most efficient electron
donor first-> reversed TEAPS
Lets think about this in terms of hyporheic zone
Can this work in the Hyporheic Zone?
Presence of particular electron acceptor = redox zone
Oxygen decrease at
depth-active O2 reduction
O2 depleted Nitrate fallsnitrate reduction
Completely anoxic Mn2+
accumulatesMn(IV) reduction
Nitrate is completely
consumedFe 2+ accumulates
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
Hyporhiec Zone Redox
USGS, 2013
Riparian Zone Redox
Cygan, 2007; Vadose Zone Journal
Questions