isotopes method

7/30/2019 Isotopes Method

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Environmental Isotopesin Hydrology

Oct 7, 2010 1Woocay substituting for Walton



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):




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:




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)




• Over 270 stable nuclides and over 1700 radionuclide's havebeen identified

• Only a few isotopes are of practical importance in hydrology




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 Woocay substituting for Walton



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.




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




Ratios, Deltas (δ) and Permils (‰)




Stable Environmental

Isotopes




Environmental Radioisotopes




Mass Ratio

• Hydrogen (H) 1H

• Deuterium (D) 2H

• Tritium (T)3

H




What is fractionation?




Different Fractionation




Temperature Effect on

Fractionation




Rainout and Rayleigh

Distilation



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Rainout effect on δ2H and δ18O values

(based on Hoefs 1997 and Coplen et al. 2000).



Temperature Effect on

Precipitation Isotopic Ratio




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



Oct 7, 2010Woocay substituting for Walton 22

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



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


A D f M i W i N h




Average δD of Meteoric Water in NorthAmerica

(modified from Kharaka and Thordsen 1992, and Taylor and Margaritz 1978)



Evaporation and Relative

Humidity (h) Effect on Isotopic Ratio




Deviation form the GMWL of Different Rivers

http://www.sahra.arizona.edu


http://www.sahra.arizona.edu/





Deviation form the GMWL

http://www.sahra.arizona.edu





Water Rock Interactions





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




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)




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



Carbon 13 (13C)




13C vs 18O




34S




15N




15N vs 18O


isotopes method

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