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Periodic Table--Iodine

Iodine has only one stable isotope, 127I. However, radioactive isotopes of iodine have been used extensively. 129I (half-life 17 Myr) is a product of 129Xe spallation in the atmosphere, but is also the result of 238U decay. As 238U is produced during a number of nuclear power- related activities, its presence (as an 129I/I ratio) can indicate the type of activity going on at any one site. For this reason, 129I was used in rainwater studies following the Chernobyl accident (Paul et al., 1987). It also has been used as a ground-water tracer (Brauer and Rieck, 1973) and as an indicator of waste dispersion into the natural environment (Brauer and Ballou, 1975). Other applications may be hampered by the production of 129I in the lithosphere through a number of decay mechanisms (see Fabryka-Martin, 1989).

In many ways, 129I is similar to 36Cl. It is a soluble halogen, fairly non-reactive, exists mainly as a non-sorbing anion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies, 129I concentrations are usually reported as the ratio of 129I to total I (which is virtually all 127I). As is the case with 36Cl/Cl, 129I/I ratios in nature are quite small, 10-14 to 10-10 (peak thermonuclear 129I/I during the 1960's and 1970's reached about 10-7; Fabryka- Martin et al., 1989). 129I differs from 36Cl in that its half- life is longer (1.6 vs 0.3 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I- and iodate) which have different chemical behaviors.

Ground-water age dating with 129I faces most of the same obstacles faced by the 36Cl method. However, input values are probably easier to estimate because the 129I/I ratio in the oceans and atmosphere is quite homogeneous due to the long half-life of 129I (Fabryka-Martin et al., 1985). The production of thermonuclear 129I can be a problem near nuclear power plants and production facilities, but for older subsurface processes, the input ratio will be approximately 10-12 (Fabryka-Martin et al., 1989). In-situ 129I production can be significant in some geologic environments where production values can exceed precipitation input values. In addition, subsurface addition of stable iodine (127I) to ground water is less of a problem than the possible addition of stable Cl to 36Cl dating because iodine is uncommon in most geologic settings. The longer half-life of 129I relative to 36Cl means that 129I can only be used for dating older ground-waters; the longer residence times allow more time for geochemical processes to adversely affect iodine isotope ratios. Finally, more substantial problem is that the long 129I half-life makes it appropriate for dating old systems only. Old ground waters have experienced a wider variety of hydrogeologic environments and phenomenon than have younger waters. Each of these can add complications to the interpretation of 129I/I ratios. Finally, 129I is more analytically challenging than 36Cl (Roman and Fabryka-Martin, 1988), and this is a mjor factor in its use.

131I and 133I are both short-lived (8 days and 21 hours, respectively) isotopes. Both have seen some use in studies where waters will have short residence times (see, for example, Robertson and Perkins, 1975). Because both are radiogenic, they are usefully applied as indicators of radioactive pollution.

Source of text: This review was assembled by Dan Snyder and Carol Kendall, primarily from Fabryka-Martin (1988) and Nimz (1998).

Brauer, F.P., and Ballou, N.E. (1975). "Isotopic ratios of iodine and other radionuclides as nuclear power pollution indicators", in Isotope Ratios as Pollutant Source and Behaviour Indicators, IAEA, Vienna. pp. 215-230.
Brauer, F. P, and Rieck, H.G. Jr. (1973). 129I, 60Co, and 106Ru Measurements on Water Samples from the Hanford Project Environs. USAEC Rep. BNWL-SA-4478.
Fabryka-Martin, J.T. (1988). Production of Radionuclides in the Earth and their Hydrogeologic Significance, with Emphasis on Chlorine-36 and Iodine-129, University of Arizona, PhD. thesis. 400 pp.
Fabryka-Martin, J., Davis, S.N., Elmore, D. and Kubik, P.W., (1989). "In-situ production and migration of 129I in the Stripa granite, Sweden". Geochim. et Cosmochim. Acta, 53: 1817.
Fabryka-Martin, J., Whittemore, D.O., Davis, S.N., Kubik, P.W. and Sharma, P., (1991). "Geochemistry of halogens in the Milk River aquifer, Alberta, Canada". Appl. Geochem., 6, 447.
Fabryka-Martin, J., Bentley, H., Elmore, D. and Airey, P.L., (1985). "Natural iodine-129 as an environmental tracer". Geochim. et Cosmochim. Acta, 49: 337.
Faure, G. (1986). Principles of Isotope Geology, Second Edition. John Wiley and Sons, New York. pp. 589.
Nimz, G. J., (1998). "Lithogenic and Cosmogenic Tracers in Catchment Hydrology", In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology. Elsevier, Amsterdam, pp. 247-290.
Paul, M., Fink, D., Hollos, G., Kaufman, A., Kutschera, W., and Magaritz (1987). "Measurement of 129I in the environment after the Chernobyl reactor accident". Nucl. Inst. Meth. Phys. Res., B29: 341-345.
Robertson, D. E., and Perkins, R. W. (1975). "Radioisotope ratios in characterizing the movement of different physical and chemical species through natural soils", in Isotope Ratios as Pollutant Source and Behaviour Indicators, IAEA, Vienna. pp. 123-134.
Roman, D. and Fabryka-Martin, J., (1988). "Iodine-129 and chlorine-36 in uranium ores 1. Preparation of samples for analysis by AMS". Chem. Geol. Isotope Geosciences Section, 72: 1.
Related Links
Periodic Table
Fundamentals of Stable Isotope Geochemistry
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