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,
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
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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
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||Fabryka-Martin, J.T. (1988). Production of Radionuclides in the Earth
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Iodine-129, University of Arizona, PhD. thesis. 400 pp.
||Fabryka-Martin, J., Davis, S.N., Elmore, D. and Kubik, P.W., (1989).
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||Faure, G. (1986). Principles of Isotope Geology, Second Edition.
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||Nimz, G. J., (1998). "Lithogenic and Cosmogenic Tracers in Catchment
Hydrology", In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers
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