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).