Environmental Isotopes in Hydrology Oct 7, 2010 1
What is an Isotope? An element is defined by the number of protons (Z) in the nucleus The number of neutrons (N) defines the isotope(s) of that element The sum of Z and N gives the atomic weight (A) For an element (E): Oct 7, 2010 2
Oxygen Most oxygen has 8 protons and 8 neutrons, giving a nuclide with 16 atomic mass units: About 0.2% of oxygen has 10 N instead of 8, thus the isotope: Usually written just: Oct 7, 2010 3
Stable and Unstable Isotopes? Variation in the number of N in the nucleus can only vary in a limited range Too few or too many will make the nucleus unstable (a radionuclide) A stable isotope does not decay (it won t spontaneously change Z, N or A) Unstable isotopes will decay and are called radioisotopes (they are radioactive) Oct 7, 2010 4
Over 270 stable nuclides and over 1700 radionuclide's have been identified Only a few isotopes are of practical importance in hydrology Oct 7, 2010 5
Mass Spectroscopy Oct 7, 2010 6
Environmental Isotopes Environmental Isotopes are Naturally occurring Found in abundance Principal elements in hydrological, geological and biological systems Relatively light elements (mass ratio) H, C, N, O, S, (advancement and Research) These isotopes serve as tracers for water, carbon, nutrient and solute cycling Oct 7, 2010 7
Advancements and Research Environmental isotopes are new used to trace not only groundwater provenance, but also recharge processes, subsurface processes, geochemical reactions and reaction rates. Their importance in studies of biogeochemical cycles and soil-water-atmosphere processes is increasingly being recognized, and new applications in contaminant hydrogeology are being made. Oct 7, 2010 8
Isotopes in Hydrology Stable isotopic composition of water is modified by meteoric processes Recharge waters in a particular environment will have a characteristic isotopic signature Signatures serve as a natural tracer for the provenance of groundwater Radioisotopes decay provide a measure of circulation time, and thus groundwater renewability Oct 7, 2010 9
Ratios, Deltas (δ) and Permils ( ) Oct 7, 2010 10
Stable Environmental Isotopes Oct 7, 2010 11
Environmental Radioisotopes Oct 7, 2010 12
Mass Ratio Hydrogen (H) 1 H Deuterium (D) 2 H Tritium (T) 3 H Oct 7, 2010 13
What is fractionation? Oct 7, 2010 14
Different Fractionation Oct 7, 2010 15
Temperature Effect on Fractionation Oct 7, 2010 16
Rainout and Rayleigh Distilation Oct 7, 2010 17
Rainout effect on δ 2 H and δ 18 O values (based on Hoefs 1997 and Coplen et al. 2000). Oct 7, 2010 18
Temperature Effect on Precipitation Isotopic Ratio Oct 7, 2010 19
The Global Meteoric Water Line (GMWL) Oct 7, 2010 20
Isotopic Data Oxygen-18 Hydrogen-2 Hydrogen-2 vs Oxygen-18 compared to GMWL Carbon-14 dates corrected with Carbon-13 Oct 7, 2010 21
Oxygen and Hydrogen Isotopes Oxygen - most abundant element in the earth's crust Hydrogen - very common in biosphere. Oxygen and Hydrogen combine to form water DUH! 9 isotopic configurations for water 3 isotopic combinations for most all water molecules Isotopic composition different: sea water polar ice atmospheric water vapor meteoric water Due to Fractionation Oct 7, 2010 22
Oxygen and Hydrogen Isotopes Stable isotope ratios of ( 2 H/ 1 H) and 18 O/ 16 O of water are reported as delta values (δ) expressed as a per mil ( ) deviation from a standard SMOW (Standard Mean Ocean Water) VSMOW (Vienna SMOW). In carbonates PDB, a standard based on the Peedee Formation, carbonate rock found in South Carolina O & H are used for determining precipitation sources and evaporation effects. O isotope ratio of solid phases (e.g. carbonate minerals) can record paleo-climate and paleo-hydrologic information O and H s isotopic composition are used as a hydrosphere tracer Oct 7, 2010 23
Average δd of Meteoric Water in North America (modified from Kharaka and Thordsen 1992, and Taylor and Margaritz 1978) Oct 7, 2010 24
Evaporation and Relative Humidity (h) Effect on Isotopic Ratio Oct 7, 2010 25
Deviation form the GMWL of Different Rivers http://www.sahra.arizona.edu Oct 7, 2010 26
Deviation form the GMWL http://www.sahra.arizona.edu Oct 7, 2010 27
Water Rock Interactions Oct 7, 2010 30
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Carbon Isotopes 12 C & 13 C 12 C & 13 C Help understand food webs and carbon cycling in ecosystems Living matter (i.e., bacteria and plants) takes up carbon through CO 2 in the atmosphere often isotopically selective, generally preferring to break the weaker, light-isotope bonds δ 13 C values can be used to distinguish between C3 and C4 plants C3 - hot, dry (95% species) C4 - tropical, subtropical (higher 13 C content) δ 13 C values are used to understand the biogeochemical reactions controlling alkalinity in watersheds Oct 7, 2010 34
Radioactive 14 C Isotope 14 C Half life of 5715 years Age dating of material containing carbon Tracing hydrologic processes, such as groundwater flow and ocean circulation Radiocarbon dating of groundwater Measured in Percent Modern Carbon (pmc) Oct 7, 2010 35
14 C Dating Corrections In groundwater the initial carbon is diluted with calcium carbonate which is dissolved from the sediments Assume: Closed system Only DIC All dilution is from carbonate t = 8267 ln 14 C pmc q 100 Oct 7, 2010 36
Carbon 13 ( 13 C) Oct 7, 2010 37
13 C vs 18 O Oct 7, 2010 38
34 S Oct 7, 2010 39
15 N Oct 7, 2010 40
15 N vs 18 O Oct 7, 2010 41
15 N vs 18 O Oct 7, 2010 42