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

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 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, H 2 S mixes with O 2 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, CO 2 – BUT Carbon is the most abundant Microbial Ecology and Competition – H 2 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 differ

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 Lets think about this in terms of hyporheic zone Dominated by aerobic, then when O 2 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 O 2 reduction O 2 depleted Nitrate falls- nitrate reduction Completely anoxic Mn 2+ 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

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

Questions