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Hydrogeochemistry

Hydrogeochemistry. “Geochemistry of Natural Waters” No wastewater, water resources Class not related directly to social aspects of water Study natural controls of chemistry of rivers, lakes, ground water, oceans etc. Questions considered:.

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Hydrogeochemistry

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  1. Hydrogeochemistry • “Geochemistry of Natural Waters” • No wastewater, water resources • Class not related directly to social aspects of water • Study natural controls of chemistry of rivers, lakes, ground water, oceans etc.

  2. Questions considered: • Why do different waters have different chemical compositions? • What controls the compositions? • This will lead to discussion of chemical reactions between water and rocks • How do compositions vary with setting? • How do they vary with time?

  3. Why consider these questions? • Diagenesis ≡ All chemical (and physical) alteration of solid material (low T and P) • Material: rocks, sediments, minerals, plants, animals, bacteria, archea • Typically occurs in water with dissolution/precipitation reactions • Often involves gas phases

  4. Why consider these questions? • Hydrology and Hydrogeology • Variations in chemical composition can be used to understand (map) flow paths • Flow can alter chemistry – through mixing of different composition waters • Diagenesis and hydrology are linked • This link critical for all earth surface processes • Also – may be used to understand paleoenvironments

  5. Concentration terminology • We will need a way to discuss what is in water • Dissolved components (ions, gasses, complexes etc.) • Solid components

  6. Total dissolve solids • Total dissolved solids (TDS) – mass of solid remaining after evaporation of water • Bicarbonate converted to carbonate • Units of mass/volume (e.g. g/L, kg/L, etc.)

  7. Salinity • Salinity – similar to TDS except includes only charged species • Operational definition • Salinity reported as ratio of electrical conductivity to standard • Originally “Copenhagen seawater” • Now KCl standard • Ratio so dimensionless (commonly ppt, ‰, Practical Salinity Units - PSU, nothing)

  8. Chlorinity • Determined by titration with AgNO3 • Definition • Mass (g) of Ag necessary to precipitate Cl, Br, and I in 328.5233 g of seawater • Total number of grams of major components in seawater: • gT = 1.81578*Cl(‰) • S(‰) = 1.80655*Cl(‰)

  9. Dissolved concentrations • Fresh water • Potable, generally < 1000 mg/L TDS • Brackish • Non-potable, but < seawater • Seawater, salinity 34 to 37 • Saline water/brine > seawater salinity • We will see later that seawater salinity important cutoff for chemical models

  10. Other measures of TDS • Refractive index • Amount of refraction of light passing through water • Linearly related to concentrations of dissolved salts • Conductivity/resistivity • Current carried by solution is proportional to dissolved ions

  11. Conductivity • Inverse of resistance • Units of Siemens/cm • Siemen = unit of electrical conductance • 1 Siemen = 1 Amp/volt = 1/Ohm = 1 Mho • Conductance is T dependent • Typically normalized to 25º C • Called Specific Conductivity

  12. Reporting units • Need to report how much dissolved material (solute) in water, two ways: • Moles • Mass • Need to report how much water (solvent) • Volume of water, typically solution amount – analytically easy • Mass of water, typically solvent amount – analytically difficult

  13. Molar units • Number of molecules (atoms, ions etc) in one liter of solution • Most common – easy to measure solution volumes • Units are M, mM, µM (big M) • Example Na2SO4 = 2Na+ + SO42- 1 mole sodium sulfate makes 2 moles Na+ and 1 mole SO42-

  14. Molal Units • Number of molecules (atoms, ions etc) in one kg of solvent • Abbreviation: m or mm or mm (little m) • Difficult to determine mass of solvent in natural waters with dissolved components • not used so often in natural waters • Useful for physical chemistry because doesn’t depend on T or X (composition of dissolved constituents).

  15. Why use molar units? • Reaction stoichiometry is written in terms of moles, not mass CaCO3 = Ca2+ + CO32- 100 g = 1 mole 40 g = 1 mole 60 g = 1 mole Simple to convert between mass (easily measured) and moles

  16. Example • Nitrate a pollution of concern • Commonly measured as mass • Reported as mass of N in NO3 • E.g., g N in NO3 • NO3 is measured • Its concentrations is about 4 X bigger than N concentration when reported by weight • Moles of NO3 and N are identical

  17. Alternative – Weight units • Mass per unit volume • For example: g/L or mg/L • If very dilute solution • Mass per unit volume about the same as mass per mass • 1L water ~ 1000 g, BUT variable with T, P and X as density changes

  18. Mass – Mole conversion • Conversion from weight units to molar units • Divide by gram formula weight • Molar units to weight units • Multiply by gram formula weight

  19. Alternative – charge units • Equivalents – molar number of charges per volume • eq/L or meq/L • Used to plot piper diagrams • Used to calculate electrical neutrality of solutions

  20. Calculation: • Moles (or millimoles) of ion times its charge Na2SO4 = 2Na+ + SO42- 1 mole of Na = 1 eq Na solution 1 mole of SO4 = 2 eq SO4 solution Although different number of moles, solution is still electrically neutral

  21. Example

  22. Charge Balance • Electrical neutrality provides good check on analytical error • Charge Balance Error – CBE SmcZc - SmaZa CBE = SmcZc + SmaZa Where: m = molar concentration of major solutes z =charge of cation (c) or anion (a)

  23. Possible causes of errors • Significant component not measured • Commonly alkalinity – can be estimated by charge balance • Analytical error • +5% difference OK – acceptable • + 3% good • 0% probably impossible

  24. Graphical data presentation • Stiff diagram – geographic distribution • Piper diagram – comparison of large numbers of samples

  25. Stiff Diagrams Cations on left Anions on right Top K+Na & Cl = seawater Middle Ca & HCO3 (alkalinity) most fresh water Bottom Mg & SO4 – other major component 4th optional – redox couples

  26. Free software for stiff diagrams • http://www.twdb.state.tx.us/publications/reports/GroundWaterReports/Open-File/Open-File_01-001.htm

  27. Piper Diagrams • Two triangular diagrams • Projected on quadralinear diagram • Very useful figure for comparing concentrations of water

  28. Santa Fe water chemistry

  29. Construction • Convert concentrations to meq/L • Use major cations and anions concentrations • Cations = Ca, Mg, Na + K • Anions = SO4, Cl, HCO3 + CO3 (or alkalinity)

  30. Calculate %’s of each element on ternary diagram • For example Ca is: • Plot %’s on ternary diagrams • Project each % onto diamond diagrams [Ca] *100 [Ca] + [Mg] + [Na + K]

  31. Composition: Ca = 22.3% Mg = 13.7% Na+K = 64% Alkalinity = 31.3% Sulfate = 54.5% Chloride = 14.2%

  32. Free software to plot piper diagrams: • http//water.usgs.gov/nrp/gwsoftware/GW_Chart/GW_Chart.html

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