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Tracing The Hydrologic Cycle

Groundwater origin. Mixing. Surface water g/w interaction. Groundwater age. Surface water studies. Tracing The Hydrologic Cycle. With Environmental Isotopes. Precipitation and climate. Environmental Isotopes. Jan 14 Introduction to the environmental isotopes

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Tracing The Hydrologic Cycle

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  1. Groundwater origin Mixing Surface water g/w interaction Groundwater age Surface water studies Tracing The Hydrologic Cycle With Environmental Isotopes Precipitation and climate

  2. Environmental Isotopes Jan 14 Introduction to the environmental isotopes Jan 21 Tracing the water cycle- 18O, 2H Jan 28 Groundwater dating - 3H Feb 4 Carbon cycle – 13C, Radiocarbon Feb 11 Water cycle, carbon and climate – Veizer Feb 25 Nitrogen cycle – 15N Mar 4 Water and carbon cycles on Mars - Fisher Mar 11 Crustal fluids – 18O, D, 87Sr, 129I and 36Cl Mar 18 Noble gases Mar 25 Selected topics - 6Li, 10Be, 11B April 1 Presentations April 8 Presentations

  3. Nucleosynthesis of the elements and isotopes

  4. The Stable Environmental Isotopes Isotope Ratio % natural Reference abundance 2H 2H/1H 0.015 VSMOW 3He 3He/4He 0.000138 Atmospheric He 13C 13C/12C 1.11 VPDB 15N 15N/14N 0.366 AIR N2 18O 18O/16O 0.204 VSMOW, VPDB 34S 34S/32S 4.21 CDT 37Cl 37Cl/35Cl 24.23 SMOC

  5. Delta - permil: d - ‰ ‰ VSMOW

  6. What is the relative enrichment or depletion of 18O in crustal rocks (~0.204%) relative to VSMOW = 17.4‰ VSMOW  crustal rocks are enriched in 18O by 17.4‰ or 1.7% relative to the standard VSMOW

  7. Isotope Ratio Mass Spectrometry

  8. Laser attenuation isotope analyser(Wavelength-Scanned Cavity Ring Down Spectroscopy – WS-CRDS) Laser absorption Reads fraction of heavy isotope bonds Direct reading of BOTH 18O and D ratios Do it in the field! New!

  9. Los Gatos – the original black box Laser attenuation isotope analyser(Wavelength-Scanned Cavity Ring Down Spectroscopy – WS-CRDS) and Picarro – nice small footprint

  10. Laser attenuation isotope analyser(Wavelength-Scanned Cavity Ring Down Spectroscopy – WS-CRDS) Check out the sample requirements – 2 mL. Fill a tray of 100! – lots of good data.

  11. Distribution of isotopes in nature • Isotope fractionation during reaction • Rayleigh distillation during reservoir depletion

  12. Isotope fractionation, a

  13. Physico-chemical fractionation

  14. Isotope partitioning functions  = symmetry value m = mass of isotope E = the energy state summed from the zero-point to the energy of the dissociated molecule (J·mole–1) k = Boltzmann constant (gas constant per molecule) = n · 1.380658 · 10–23 JK–1 T = thermodynamic temperature K

  15. Diffusive fractionation v = molecular velocity (cm · s–1) k = Boltzmann constant (gas constant per molecule) = n · 1.380658 · 10–23 JK–1 m = molecular mass (e.g. 7.3665 · 10–26 kg for 12C16O2) T = absolute temperature K

  16. Diffusive Fractionation e.g. 13C during CO2 diffusion Diffusion in a vacuum Diffusion in air

  17. Units Isotope Enrichment (e) • Isotope difference in permil units between two reacting phases at equilibrium • when a is small, then we can use:

  18. Units Isotope Separation (D) • Isotope difference in permil units between any two phases

  19. For a water – vapor exchange at 25°C what is the d18O of vapor, where: • water d18Ow = 0.0 ‰ VSMOW

  20. For a water – vapor exchange at 25°C what is the d18O of vapor, where: • water d18Ow = 0.0 ‰ VSMOW The fractionation factor (a) is: a18Ow-v = 1.0093 The isotopic enrichment (e): e18Ow-v = (a–1) ·103 = 9.3‰ and e18Ov-w = – 9.3‰

  21. For a water – vapor exchange at 25°C what is the d18O of vapor, where: • water d18Ow = 0.0 ‰ VSMOW e18Ow-v = (a–1) ·103 = 9.3‰ d18Ovapor = d18Owater – e18Owater-vapor = 0.0 – 9.3‰ = – 9.3‰ • vapor d18Ov = –9.30‰ VSMOW

  22. For most reactions in hydrogeology: • d values are typically –50 to +50 ‰ • a values are close to 1 (0.98 to 1.02) • e values are typically –20 to +20 ‰ Except for some extreme reactions and light isotopes . . . e.g. hydrogen gas produced from water is strongly depleted in 2H and has a fractionation factora2HH2O-H2 = 3.76 at 25°C. What will be the d2H value for H2 produced from water with d2HH2O = –75‰ at 25°C?

  23. d2HH2 = –754‰ VSMOW (but using e, d2HH2 = –75 – 2760 = –2835‰)

  24. So, use the e simplification . . . • when a is close to 1 • when the d-values are not too different from the reference (i.e. within a few tens of permil of 0)

  25. Fractionation and Temperature lnaX-Y = aT–2 + bT–1 + c

  26. Fractionation and Temperature

  27. Fractionation - Other Systems

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