Environmental Isotopes in 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 atomic

weight (A)• For an element (E):

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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:

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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 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)

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

• Only a few isotopes are of practical importance in hydrology

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Mass Spectroscopy

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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, but also recharge processes, subsurface processes, 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 contaminant hydrogeology 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 a measure of circulation time, and thus groundwater 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) 3H

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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).

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 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

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) can record paleo-climate and paleo-hydrologic information

• O and H’s isotopic composition are used as a hydrosphere tracer

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Average δD of Meteoric Water in North America

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

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

Water Rock Interactions

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Carbon Isotopes12C & 13C

• 12C & 13C• Help understand food webs and carbon cycling in

ecosystems• 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 biogeochemical reactions 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 initial

carbon is diluted with calcium carbonate which is dissolved from the sediments

• Assume:• Closed system• Only DIC• All dilution is from carbonate

100ln8267

14

⋅⋅−=

qC

t pmc

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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15N vs 18O

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