Oxidation-Reduction Processes in Ground-Water Systems

ChapelleGroundwater 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:

– 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, 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

Hyporhiec Zone Redox

Riparian Zone Redox

Cygan, 2007; Vadose Zone Journal

USGS, 2013

Questions