Redox and Eh From electrochemistry:  G R = -nF Eh – E° = -  G R ° / nF – For e - on left side of half-reaction; – If e - on right side: E° = +  G R ° / nF Re-write Nernst Equation: – Oxidized species on side where e - are 1

Measuring Eh The Eh value is usually not very accurate in natural waters because of a lack of redox equilibrium – One half of redox pair often below detection Best to use Eh as a semi-quantitative measurement, giving you a relative idea of the redox potential of the water 2

Eh – pH Diagrams A different type of stability diagram, but using Eh as variable instead of activity – Lines indicate equilibrium – Domains define areas of stability for minerals and aqueous species 3

4 Oxidizing and reducing with respect to SHE O 2 and H 2 are present in entire H 2 O stability range Oxidizing environments may contain only small amounts of O 2

5 We determine what species, minerals are in diagram

Evolution of Water Chemistry 6

Source of dissolved species Primarily from chemical weathering Primary minerals + acid  secondary minerals + dissolved ions – The essential ingredients needed for chemical weathering are water and acid 7

Precipitation Soil water and groundwater start out as precipitation – Very dilute (low TDS), in equilibrium with atmospheric gases (O 2, CO 2, N 2 ) Precipitation passes through the soil zone and unsaturated zone 8

Soils and Weathering In most areas, soils are the first geologic unit to come into contact with precipitation – If soil has organic matter, OM decays, consuming O 2 and producing CO 2 CH 2 O + O 2(g) → CO 2(g) + H 2 O CO 2 + H 2 O  H 2 CO 3  HCO H + Soil P CO2 = – atm – (atmosphere = ) – Due to production of acid (CO 2 ) soils have the highest rate of chemical weathering – TDS increases as minerals dissolve, ions desorbed 9

Unsaturated Zone After passing through the soil zone, water percolates down through the unsaturated zone – Thickness of unsaturated zone is primarily a function of annual precipitation (climate) Also affected by lithology, topography, plant species, nearness to surface water Water can move through the unsaturated zone quickly, or can remain for a long time (years) – Dissolution/precipitation reactions can occur in the unsaturated zone, altering water chemistry 10

Groundwater Chemistry Evolution By the time water reaches the water table, it has acquired the chemical signature of the geologic materials it is flowing through As it moves along a groundwater flow path, the chemistry continues to evolve Evolutionary sequence controlled by mineral availability and solubility – High availability: carbonates and felsic minerals – High solubility: gypsum/anhydrite, evaporites 11

Evolution of Groundwater Chemistry 12

Open vs. closed systems Soil and shallow groundwater (< 10 ft below water table) are open systems with respect to gases (CO 2 and O 2 ) – Gaseous exchange with the atmosphere (or soil gas), which is at or near equilibrium saturation – As CO 2 and O 2 are consumed, replaced by CO 2 from atmosphere – As CO 2 is generated, it will degas Deeper groundwater is a closed system with respect to gases – Water is isolated from the atmosphere – If gases are consumed, their concentrations decrease; if generated, concentrations increase 13

General trends in groundwater with increasing age and/or depth O 2 : rapidly consumed by biological activity (oxidation of organic matter or reduced minerals) pH: usually rises along a flow path as H + is consumed during weathering reactions – A closed system has finite acidity – pH can fall by oxidation of sulfide minerals HCO 3 - : concentration increases because H + in H 2 CO 3 consumed 14

Trends with age/depth As groundwater migrates, concentration of TDS and most major ions increases Anions – Chebotarev took 10,000 groundwater samples from large sedimentary basins in Australia and determined that groundwater evolves towards seawater composition – Determined that relative abundances of anions changed with travel distance/age HCO 3 -  HCO SO 4 2-  SO HCO 3 -  SO Cl -  Cl - + SO 4 2-  Cl - 15

Groundwater Anion Evolution Young Very Old Tri-linear Diagram: Used in Piper Diagrams 16

Trends with age/depth Cations – More difficult to generalize trends – Most common trend: Ca 2+, then Ca-Na, Na-Ca, finally Na + – Driven by cation exchange and CaCO 3 precipitation Redox Species – Sequential reduction of oxidized species 17

Trends with age/depth Groundwater Chemistry Zones – Upper: active groundwater circulation, relatively weathered (leached) rocks, Ca 2+ - HCO 3 - dominate, low TDS Usually not a lot of soluble minerals (like halite and gypsum) HCO 3 - dominant anion, Ca 2+ commonly dominant cation, relatively low TDS (< 500 mg/L) 18

Trends with age/depth Groundwater Chemistry Zones – Intermediate: less active flow, unweathered rocks, SO 4 2- dominant anion, Na + increases but Ca 2+ - usually still important, higher TDS – Lower: slow circulation, unweathered rocks, Na + - Cl - dominant ions, high TDS Highly soluble minerals common 19

Evolution of Groundwater Chemistry Low TDS Intermediate TDS High TDS Aquitard: TDS high relative to aquifers 20

Mineralogy and Water Chemistry Identity of rocks and minerals along groundwater flowpath an important variable affecting water chemistry 21

Mineralogy of Igneous Rocks: Bowen’s Reaction Series 22 Less Stable More Stable At/Near Earth’s Surface: Everything else being equal, Ca > Na > K

Mineralogy of Igneous Rocks: Bowen’s Reaction Series 23 Felsics Mafics

Igneous Rock Type and Water Chemistry Mafic igneous rocks – High TDS, high Si – Mg 2+ and Ca 2+ dominant cations – Anions: HCO 3 - Felsic igneous and metamorphic rocks – Relatively low (< 500 mg/L) TDS – Anions: HCO 3 - dominant, F - can be characteristic – Cations: Ca 2+ and Na + dominant Fine-grained or glassy rocks – High TDS because of high mineral surface area or no mineral structure 24

Sedimentary Rock Type and Water Chemistry Sandstone – Variable, dependent on mineral composition and how “pure” sandstone is – Most often like felsics, but higher TDS Limestone/dolomite – TDS > igneous – Cations: Ca and Mg, little Na – Anions: HCO 3 - – Si varies – Dolomite: Ca and Mg equimolar 25

Sedimentary Rock Type and Water Chemistry Shale – Main minerals quartz and illite are relatively unreactive – Long contact time can lead to high TDS – Most shales form in marine environments, and Na + and Cl - can be elevated from original porewater – SO 4 2- if pyrite is present, and from porewater – Plenty of Si 26

Atmospheric Solids and Water Chemistry Atmospheric input (dust, etc.) – Can provide significant amounts of weatherable material in all climates – In arid regions, this can be a dominant source – Laterites on limestone in Bahamas and Amazon: Al and Fe from dust 27

Chemical Weathering: Climate and Topography Climate – As precipitation increases, mineral dissolution increases, more acid to attack the minerals – For constant precipitation, weathering rate increases with temperature Topography – Some debate about this, but the majority of evidence indicates decreased chemical weathering with increasing elevation – Probably related to thinner soils, cooler temperatures 28

Water Chemistry: Information on Weathering Reactions Knowing starting and ending solution chemistry of a system, we can infer what reactions have taken place to produce the ending solution – Reaction-Path Modeling – In addition to water chemistry, need information on minerals present – As groundwater migrates along a flow path, reactions occur: Dissolution adds ions Mineral precipitation removes ions – The change in water chemistry = the sum of all dissolution/precipitation reactions 29

Water Chemistry: Information on Weathering Reactions Garrels and Mackenzie (1967) first to develop reaction path modeling concept – Applied on watershed scale (Sierra Nevadas) – Initial solution was precipitation (rainfall and snowmelt) – Ending solution was spring chemistry 30

Example: granitic springs in Sierra Nevadas Information that helped characterize the system: Geology: Rocks classified as quartz diorite and quartz microcline gneiss Primary minerals – Feldspars: albite (Na), microcline (K), anorthite (Ca) Average feldspar: andesine (Ca and Na) – Quartz – Biotite/hornblende Climate: high elevation (2-3 km), cool T, high winter snowfall, summer thunderstorms 31

Example: granitic springs in Sierra Nevadas Start building conceptual model: As precipitation recharges the subsurface, which primary minerals would weather most readily? Least readily? 32

Mineralogy of Igneous Rocks: Bowen’s Reaction Series 33 Less Stable More Stable At/Near Earth’s Surface:

Example: granitic springs in Sierra Nevadas G&M predict decreasing weatherability: Ca- plagioclase  Na-plagioclase  Biotite/hornblende  K feldspar  quartz What are expected secondary minerals? – Clays: kaolinite and smectite – Amorphous SiO 2 – CaCO 3 ? 34

Example: granitic springs in Sierra Nevadas Ending solutions: Ephemeral and perennial springs – Ephemeral: short residence time (up to several years), low TDS and pH – Perennial: higher residence time (10-100’s yrs), higher TDS and pH Reaction path model – Starting point: snow chemistry – Ending point: spring chemistry – Difference between the two result of reactions involving dissolution of primary minerals, precipitation of secondary minerals 35

Ephemeral springs in Sierra Nevadas Began by subtracting snow water chemistry from spring water chemistry to determine how much of each ion/species added 36 ephemeral- snow water mM SiO Ca Mg Na K HCO SO Cl0.0140

Ephemeral springs in Sierra Nevadas All SO 4 and Cl removed; none added in the subsurface Remaining species added by reactions 37 ephemeral- snow water mM SiO Ca Mg Na K HCO SO Cl0.0140

Ephemeral springs in Sierra Nevadas Hypothesis: plagioclase, biotite and K-feldspar each weathers to kaolinite, amorphous SiO 2, and dissolved ions – Allow spring water to back-react with kaolinite to see if could get original minerals – First, react Na, Ca, HCO 3, and SiO 2 with kaolinite to make plagioclase All Na and Ca used up Resulting plagioclase composition close to what is found 38

Ephemeral springs in Sierra Nevadas Next, react all Mg along with K, HCO 3, and SiO 2 to make biotite (KMg 3 AlSi 3 O 10 (OH) 2 ) 39 ephemeral- snow water-plagioclase mM SiO Ca Mg Na K HCO SO Cl

Ephemeral springs in Sierra Nevadas Remaining K, HCO 3, and SiO 2 used to form K-feldspar 4% of original SiO 2 remains, good enough 40 ephemeral- snow water-plagioclase-biotite mM SiO Ca Mg Na K HCO SO Cl

Ephemeral springs in Sierra Nevadas Resulting balance worked remarkably well, explaining the concentration of all ions Observations – All SiO 2 could be accounted for by dissolution of aluminosilicates, no quartz dissolution needed – Waters gain much of their SiO 2 over a very short distance; action of high CO 2 – Despite abundant K-feldspar, 80% of dissolved ions came from plagioclase weathering 41

Perennial springs Can same reactions be assumed to be occurring in perennial springs? – Not necessarily – Look at ratio of ions in solution 42

Ephemeral vs. Perennial Springs ephemeralperennialdifference mM SiO Ca Mg Na K HCO SO Cl TDS (ppm)3675 pH

Ephemeral vs. Perennial Springs Differences between spring types – Cl assumed to come from NaCl, SO 4 from CaSO 4 Weak assumptions, but very low concentrations – SiO 2 :Na ratio for difference between springs is 1:1 SiO 2 :Na ratio in solution for weathering of plagioclase is 2:1 – Some secondary mineral other than kaolinite being produced to remove SiO 2 44

Ephemeral vs. Perennial Springs Potential candidates for SiO 2 : clay mineral (smectite); amorphous SiO 2 – Hypothesized reactions Plagioclase and biotite  kaolinite Plagioclase  smectite – Ended up with extra Ca and HCO 3 -, dissolution of CaCO 3 Potential sources of CaCO 3 – Summer wet/dry deposition – CaCO 3 in fracture fillings 45

Reaction Path Models Good for simple systems where flowpaths are well defined – The larger and more complex the systems, the harder it is to constrain potential reactions Can consider redox reactions, gas exchange, isotopic reactions, mixing of waters, etc. N.B.: there is no unique solution – Modeler determines which phases to consider – Based on available data and “intuition” 46

Redox reactions in Groundwater Redox reactions are extremely important in groundwater and soil water Many key elements are redox sensitive: – C, N, S, Fe, Mn, As, heavy metals Very important in terms of water quality/chemistry 47

Factors Controlling Natural Redox Conditions O 2 in recharge Organic matter content of solids – Occasionally dissolved organics (natural) Presence of redox buffers, usually minerals Groundwater residence time 48

Groundwater Chemistry: Redox Evolution Water tends to become more reducing as it moves along a flow path – Isolated from atmosphere, so once O 2 consumed it is not replenished – Organic matter most commonly oxidized compound Sulfide minerals can also be important – Most rapidly in the shallow zones 49

Microbes and Redox Reactions in Groundwater Almost all redox reactions in groundwater are biogeochemically mediated – Microorganisms catalyze almost all redox reactions and use the energy released – Microbes also need a carbon source (as well as other nutrients) 50

Role of Microrganisms Microorganisms produce enzymes that bring reactants into close proximity Enzymes specific to substrate: carbon source and terminal electron acceptor (TEAP) (i.e., O 2, NO 3 -, Fe(OH) 3, …) – Enzyme induction: ability to create new enzymes to adapt to new carbon source (i.e., organic contaminants) In any soil, there exists a huge variety of microorganisms but there is usually a dominant species or set of species – DNA/RNA techniques used to identify dominant species – Non-dominant species exist in isolated microenvironments Biofilms (“slime”): “engineered” microenvironments 51

Groundwater Chemistry: Redox Evolution Dissolved oxygen (DO) – In clayey/silty soils, DO commonly below detection in shallow groundwater – DO is generally detectable in recharge areas and in sandy soils and karstic limestones – If there is little or no soil over permeable fractured rock, detectable DO can persist far into the flow system Occasionally an entire flow system is oxygenated 52

Organic Matter Oxidation O 2 has low solubility – 9 mg/L at 25°C (2.8 x moles/L) – 11 mg/L at 5°C Half reactions – OM oxidation: CH 2 O + H 2 O  CO 2 + 4H + + 4e - – O 2 reduction: O 2 + 4H + + 4e -  2H 2 O – CH 2 O + O 2  CO 2 + H 2 O For every mole of OM oxidized, one mole O 2 of reduced Therefore, DO typically consumed in the soil zone and shallow groundwater, resulting in anoxic groundwater 53

DO Consumption Flooded soil 54

Groundwater Chemistry: Redox Evolution After DO is consumed, other TEAPs are used by microbes based on thermodynamics – NO 3 - reduction (denitrification) – MnO 2 [Mn(IV)] reduction – Ferric [Fe(III)] mineral reduction – SO 4 2- reduction – Fermentation and methanogenesis (CO 2 reduction) – “Redox ladder” The order of the reactions based on obtainable energy for the microbes Kinetics: the less the energy, the slower the reaction 55

Role of Microrganisms Microorganisms are subject to the laws of thermodynamics (as are we) – They catalyze reactions until equilibrium is reached (ΔG = 0) or until TEAP is consumed (reaction goes to completion) – For example, when O 2 is TEAP CH 2 O (aq) + O 2  CO 2 + H 2 O – When O 2 is consumed and NO 3 - is present, denitrifying organisms have competitive advantage because they get more energy from reaction than Fe, Mn, or SO 4 2- reducers 56

Redox Ladder: electron acceptors and donors 57

Post-DO redox reactions involving OM Unbalanced reactions: CH 2 O + NO 3 -  CO 2 + N 2 (denitrification) – 5 CH 2 O + 4 NO H +  5 CO N H 2 O – This reaction causes pH to increase, which is indirect evidence that denitrification has occurred CH 2 O + NO 3 -  CO 2 + NH 3 (ammonification; toxic to fish) 58

Post-DO redox reactions involving OM CH 2 O + Fe(OH) 3  CO 2 + Fe 2+ (iron reduction; dissolves Fe(III) minerals) – CH 2 O + 4Fe(OH) H +  CO Fe H 2 O CH 2 O + SO 4 2-  CO 2 + H 2 S (sulfate reduction) – 2CH 2 O + SO H +  2 CO 2 + HS H 2 O – Water from Normal well field has a rotten egg (H 2 S) smell—why? High organics and sulfate-reducing bacteria active 59

Fermentation and Methanogenesis Reactions that occur when all external electron acceptors have been used; methane (CH 4 ) is produced, CO 2 both produced and consumed Transformation of complex organics into simpler compounds Fermentation: CH 3 COOH  CH 4 + CO 2 – CH 3 COOH = Acetic acid – Also produces H 2 2 H e -  H 2 – Fermentation byproducts are used by methanogenic microbes 60

Fermentation and Methanogenesis Methanogenesis: CO H 2  CH H 2 O – Methane production characterized by increasing H 2 – Methanogens need fermenters H 2 is a reactive intermediate product, produced and consumed by metabolic processes – Low at high Eh, higher at lower Eh – H 2 is best indicator of dominant TEAP, but difficult to measure (field GC) 61

General order of microbially-mediated redox reactions Conceptual change in concentrations with time/distance 62

TEAPs in Groundwater Uncontaminated Contaminated FLOW 63

TEAPs While thermodynamics predicts an orderly progression of the dominance of individual TEAPs, it’s not so simple in nature – Often have 2 (or more) TEAPs active in same part of aquifer e.g., often have Fe 3+ -reduction and SO reduction occurring together, even though Fe 3+ reduction more thermodynamically favorable – Due to: micro-environments, different microorganisms responsible, solid vs. aqueous environments – Where there’s energy to be gained, microbes are working 64

TEAPs and Eh Ranges 65

Determining predominant TEAP 66

Redox Buffering The Eh of groundwater does not linearly decline as oxidizers are consumed along a flow path Instead, the Eh remains relatively constant as a particular oxidizer is consumed, then the Eh drops and stabilizes again 67

Redox Buffering 68

Redox Buffering System is buffered if oxidizable or reducible compounds are present that prevent a significant change in Eh when strong oxidizing/reducing agents added – Expect Eh of natural waters to generally be in buffered ranges – Values in unbuffered ranges unstable 69

Computed vs. Measured Field Eh 70 - Vertical bands indicate buffered ranges; reflect the standard E°

Redox Buffering Example: recharging water has dissolved O 2, Eh will remain high until O 2 is consumed; after O 2 gone, Eh drops rapidly and stabilizes at the value determined by next oxidizer Buffers can be dissolved species or solid matter – Dissolved species: usually limited in concentration and consumed rapidly (if right conditions exist) – Solid matter: can provide large buffering capacity – E.g., Fe(OH) 3 can provide buffering until equilibrium is reached with dissolved Fe concentration 71