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Iodine has one stable isotope and one comsogenic/anthropogenic isotope. 129I, the cosmogenic isotope, can be used to date groundwaters from 3 million to 80 milllion years old and trace groundwater flow.


Cost of Analysis
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Accelerator Mass Spectrometry (AMS):
$200 for batch processing plus $175 to $350 for analysis (depending on precision required).

(See, for example, the Purdue Rare Isotope Measurement Laboratory (PRIME Lab) for more information)

(See our AMS page for more information about the technique)

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Natural sources of 129I
The natural sources of 129I are cosmic spallation of xenon and fission of uranium found in the subsurface. The fission of uranium generates a release of 129I into the groundwater and into the atmosphere from volcanic emissions. The residence time for 129I in the atmosphere is 2 weeks, whereas in the oceans it is 40,000 years. Therefore, oceanic stable iodine buffers the 129I /I ratio to an average constant value of 1.1 x 10-12. Young groundwaters, marine sediments, and current precipitation have a 129I/I ratio on the order of 10-12.

Anthropogenic sources
Anthropogenic sources are responsible for the majority of 129I in the atmosphere, particularly the chemical reprocessing of irradiated fuel from nuclear power reactors. Anthropogenic 129I is mainly a fission product of 235U and 239Pu. The isotopic ratio increased in some parts of the world in the 1960's due to above ground nuclear testing. The 129I/I ratio in the atmosphere increased to ~10-7, and has been as high as 10-4.

Release of 129I also has occurred by accident. In April 1986, Chernobyl Reactor 4 in the Ukraine experienced a series of explosions that destroyed the reactor core. Radioactive xenon, iodine, cesium, krypton, and tellurium were released into the environment for nine days.

Measurement Techniques (return to top)

Initially, 129I/I ratios were measured by ß- decay counting, neutron activation to 130I, and negative-ion mass spectrometry (Delmore 1982). Now they are typically measured with accelerated mass spectrometry, which provides higher precision with a smaller sample size. Nevertheless, because of the normally low concentration of 129I, 5 to 10 liters of water, and 300 to 500 grams of ice are normally required for this analysis.

(See our AMS page for more information)

Hydrological Applications
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129I is used for estimating groundwater residence times, tracing brine migration, identification of hydrocarbon source rocks, and as a tracer of radioactive contaminant plumes. Its long half-life extends the opportunity to date groundwater to 80 million years, but some limitations must be considered:

1. The estimate of the initial atmospheric ratio is uncertain due to shifts in production rates over time. A possible solution that has been suggested is to measure the ratio in marine sediments, coral, peat and organic soils.

2. Sources of 129I and stable iodine are different. The transformation and equilibration rates from these sources to waters need to be estimated.

3. Old groundwater in contact with uranium-bearing minerals may result in 129I from spontaneous fission of 238U. The rate of release is dependent on the contact area and porosity.

4. The release of stable iodine from sediments or fluid inclusions must be estimated.

5. The amount of 129I released by volcanic emissions is not known and cannot be separated from the overall atmospheric production of 129I.

6. 129I is a potentially useful tracer of groundwater contamination from nuclear facilities. However, studies indicate that it may bind with organic compounds, limiting its use. For more information see I. Clark's "Partitioning of 129I in the Environment"

References and Further Reading (return to top)

  • Clark, I., and P. Fritz, Environmental Isotopes in Hydrogeology, Lewis Publishers, Boca Raton, 1997.

  • Delmore, J.E., Isotopic analysis of iodine using negative surface ionization, Int. J. Mass Spectrometry and Ion Phys. 43: 273-281, 1982.

  • Fabryka-Martin, J., Iodine-129 as a groundwater tracer, in Radioactive tracers in Hydrogeology, edited by P.G. Cook and A.L. Herczeg, 504-510, Kluwer Academic Publishers, Boston, 2000.

  • Fontes, J.C., and J.N. Andrews, Accelerator mass spectrometry in hydrology, Nucl. Instrum. Phys. Res. B92, 367-375, 1994.

  • Kocher, D.C., A dynamic model of the global iodine cycle and estimation of dose to the world population from releases of iodine-129 to the environment, Environ. Int. 5, 15-31, 1981.

  • Martin, J.K., S.N. Fabryka, and D. Davis, Applications of 129I and 36Cl in hydrology, Nucl. Instrum. Meth. Phys. Res. B29, 361-371, 1987.

  • Paul, M., D. Fink, G. Hollos, A. Kaufman, W. Kutschera, and M. Magaritz, Measurement of 129I in the environment after the Chernobyl reactor accident, Nucl. Instrum. Meth. Phys. Res. B29, 341-345, 1987.

Internet Resources (return to top)

USGS Periodic Table - Iodine


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