Isotopes Method
Transcript of Isotopes Method
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Environmental Isotopesin Hydrology
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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 atomicweight (A)
• For an element (E):
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Oxygen
• Most oxygen has 8 protons and 8
neutrons, giving a nuclide with 16atomic mass units:
• About 0.2% of oxygen has 10 Ninstead of 8, thus the isotope:
• Usually written just:
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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 nucleusunstable (a radionuclide)
• A stable isotope does not decay(it won’t spontaneously change Z, N or A)
• Unstable isotopes will decay and are calledradioisotopes (they are radioactive)
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• Over 270 stable nuclides and over 1700 radionuclide's havebeen identified
• Only a few isotopes are of practical importance in hydrology
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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
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Advancements and Research
• Environmental isotopes are new used to
trace not only groundwater provenance, butalso recharge processes, subsurfaceprocesses, geochemical reactions and
reaction rates.• Their importance in studies of biogeo-
chemical cycles and soil-water-atmosphere
processes is increasingly being recognized,and new applications in contaminanthydrogeology are being made.
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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 ameasure of circulation time, and thusgroundwater renewability
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Ratios, Deltas (δ) and Permils (‰)
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Stable Environmental
Isotopes
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Environmental Radioisotopes
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Mass Ratio
• Hydrogen (H) 1H
• Deuterium (D) 2H
• Tritium (T)3
H
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What is fractionation?
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Different Fractionation
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Temperature Effect on
Fractionation
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Rainout and Rayleigh
Distilation
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Rainout effect on δ2H and δ18O values
(based on Hoefs 1997 and Coplen et al. 2000).
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Temperature Effect on
Precipitation Isotopic Ratio
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The Global Meteoric Water
Line (GMWL)
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Isotopic Data
• Oxygen-18
• Hydrogen-2• Hydrogen-2 vs Oxygen-18
compared to GMWL• Carbon-14 dates corrected
with Carbon-13
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Oxygen andHydrogen Isotopes
Oxygen - most abundant element in the earth'scrust
Hydrogen - very common in biosphere.
Oxygen and Hydrogen combine to form water
DUH!
9 isotopic configurations for water
3 isotopic combinations for most all watermolecules
Isotopic composition different:sea water
polar ice
atmospheric water vapor
meteoric water
Due to Fractionation
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Oxygen and Hydrogen Isotopes
• Stable isotope ratios of (2H/1H) and 18O/16O 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) canrecord paleo-climate and paleo-hydrologic information
• O and H’s isotopic composition are used as a hydrosphere
tracer
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A D f M i W i N h
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Average δD of Meteoric Water in NorthAmerica
(modified from Kharaka and Thordsen 1992, and Taylor and Margaritz 1978)
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Evaporation and Relative
Humidity (h) Effect on Isotopic Ratio
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Deviation form the GMWL of Different Rivers
http://www.sahra.arizona.edu
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Deviation form the GMWL
http://www.sahra.arizona.edu
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Water Rock Interactions
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Carbon Isotopes12
C &13
C• 12C & 13C
• Help understand food webs and carbon cycling inecosystems
• Living matter (i.e., bacteria and plants)
• takes up carbon through CO2 in the atmosphere
• often isotopically selective, generally preferring to break the
weaker, light-isotope bonds• δ13C values can be used to distinguish between C3 and C4
plants
• C3 - hot, dry (95% species)
• C4 - tropical, subtropical (higher 13C content)
• δ13C values are used to understand the biogeochemicalreactions controlling alkalinity in watersheds
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Radioactive 14C
Isotope
14C Half life of 5715 yearsAge dating of material containing carbonTracing hydrologic processes, such as groundwater flow and ocean
circulationRadiocarbon dating of groundwaterMeasured in Percent Modern Carbon (pmc)
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14C Dating Corrections
• In groundwater the initialcarbon is diluted withcalcium carbonate which is
dissolved from thesediments
• Assume:• Closed system
• Only DIC
• All dilution is from carbonate
100ln8267
14
⋅
⋅−=
q
C t
pmc
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Carbon 13 (13C)
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13C vs 18O
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34S
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15N
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15N vs 18O
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