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Redox and Eh

Redox and Eh

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Redox and Eh

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

  2. Measuring Eh • The Eh value is usually not very accurate in natural waters because of a lack of redoxequilibrium • 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

  3. 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

  4. O2 and H2 are present in entire H2O stability range Oxidizing and reducing with respect to SHE Oxidizing environments may contain only small amounts of O2

  5. We determine what species, minerals are in diagram

  6. Evolution of Water Chemistry

  7. 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

  8. Precipitation • Soil water and groundwater start out as precipitation • Very dilute (low TDS), in equilibrium with atmospheric gases (O2, CO2, N2) • Precipitation passes through the soil zone and unsaturated zone

  9. 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 O2 and producing CO2 • CH2O + O2(g) → CO2(g) + H2O • CO2 + H2O  H2CO3 HCO3- + H+ • Soil PCO2 = 10-3 – 10-1atm • (atmosphere = 10-3.5) • Due to production of acid (CO2) soils have the highest rate of chemical weathering • TDS increases as minerals dissolve, ions desorbed

  10. 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

  11. 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

  12. Evolution of Groundwater Chemistry

  13. Open vs. closed systems • Soil and shallow groundwater (< 10 ft below water table) are open systems with respect to gases (CO2 and O2) • Gaseous exchange with the atmosphere (or soil gas), which is at or near equilibrium saturation • As CO2 and O2 are consumed, replaced by CO2 from atmosphere • As CO2 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

  14. General trends in groundwater with increasing age and/or depth • O2: 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 • HCO3-: concentration increases because H+ in H2CO3 consumed

  15. 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 • HCO3- HCO3- + SO42- SO42- + HCO3- SO42- + Cl-Cl- + SO42-Cl-

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

  17. Trends with age/depth • Cations • More difficult to generalize trends • Most common trend: Ca2+, then Ca-Na, Na-Ca, finally Na+ • Driven by cation exchange and CaCO3 precipitation • Redox Species • Sequential reduction of oxidized species

  18. Trends with age/depth • Groundwater Chemistry Zones • Upper: active groundwater circulation, relatively weathered (leached) rocks, Ca2+ - HCO3- dominate, low TDS • Usually not a lot of soluble minerals (like halite and gypsum) • HCO3- dominant anion, Ca2+ commonly dominant cation, relatively low TDS (< 500 mg/L)

  19. Trends with age/depth • Groundwater Chemistry Zones • Intermediate: less active flow, unweathered rocks, SO42- dominant anion, Na+ increases but Ca2+- usually still important, higher TDS • Lower: slow circulation, unweathered rocks, Na+ - Cl- dominant ions, high TDS • Highly soluble minerals common

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

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

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

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

  24. Igneous Rock Type and Water Chemistry • Mafic igneous rocks • High TDS, high Si • Mg2+ and Ca2+ dominant cations • Anions: HCO3- • Felsic igneous and metamorphic rocks • Relatively low (< 500 mg/L) TDS • Anions: HCO3- dominant, F- can be characteristic • Cations: Ca2+ and Na+ dominant • Fine-grained or glassy rocks • High TDS because of high mineral surface area or no mineral structure

  25. 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: HCO3- • Si varies • Dolomite: Ca and Mg equimolar

  26. 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 • SO42- if pyrite is present, and from porewater • Plenty of Si

  27. 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

  28. 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

  29. 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

  30. 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

  31. 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

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

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

  34. 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 SiO2 • CaCO3?

  35. 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

  36. Ephemeral springs in Sierra Nevadas • Began by subtracting snow water chemistry from spring water chemistry to determine how much of each ion/species added

  37. Ephemeral springs in Sierra Nevadas • All SO4 and Cl removed; none added in the subsurface • Remaining species added by reactions

  38. Ephemeral springs in Sierra Nevadas • Hypothesis: plagioclase, biotite and K-feldspar each weathers to kaolinite, amorphous SiO2, and dissolved ions • Allow spring water to back-react with kaolinite to see if could get original minerals • First, react Na, Ca, HCO3, and SiO2 with kaolinite to make plagioclase • All Na and Ca used up • Resulting plagioclase composition close to what is found

  39. Ephemeral springs in Sierra Nevadas • Next, react all Mg along with K, HCO3, and SiO2 to make biotite (KMg3AlSi3O10(OH)2)

  40. Ephemeral springs in Sierra Nevadas • Remaining K, HCO3, and SiO2 used to form K-feldspar • 4% of original SiO2 remains, good enough

  41. Ephemeral springs in Sierra Nevadas • Resulting balance worked remarkably well, explaining the concentration of all ions • Observations • All SiO2 could be accounted for by dissolution of aluminosilicates, no quartz dissolution needed • Waters gain much of their SiO2 over a very short distance; action of high CO2 • Despite abundant K-feldspar, 80% of dissolved ions came from plagioclase weathering

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

  43. Ephemeral vs. Perennial Springs

  44. Ephemeral vs. Perennial Springs • Differences between spring types • Cl assumed to come from NaCl, SO4 from CaSO4 • Weak assumptions, but very low concentrations • SiO2:Na ratio for difference between springs is 1:1 • SiO2:Na ratio in solution for weathering of plagioclase is 2:1 • Some secondary mineral other than kaolinite being produced to remove SiO2

  45. Ephemeral vs. Perennial Springs • Potential candidates for SiO2: clay mineral (smectite); amorphous SiO2 • Hypothesized reactions • Plagioclase and biotitekaolinite • Plagioclase smectite • Ended up with extra Ca and HCO3-, dissolution of CaCO3 • Potential sources of CaCO3 • Summer wet/dry deposition • CaCO3 in fracture fillings

  46. 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”

  47. 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

  48. Factors Controlling Natural Redox Conditions • O2 in recharge • Organic matter content of solids • Occasionally dissolved organics (natural) • Presence of redox buffers, usually minerals • Groundwater residence time

  49. Groundwater Chemistry:Redox Evolution • Water tends to become more reducing as it moves along a flow path • Isolated from atmosphere, so once O2 consumed it is not replenished • Organic matter most commonly oxidized compound • Sulfide minerals can also be important • Most rapidly in the shallow zones

  50. 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)