Natural iodine-129 as a ground-water tracer

Abstract

Iodine-129 (t ½, 16 My) is a naturally-occurring tracer which can be used to study hydrologic and geologic processes on time scales up to 100 My. Global modelling suggests that the pre-bomb atmospheric ratio ¹²⁹I/I should have been constant in time and space. This ratio is the starting value in ground-water recharge, and subsequent ratio changes are determined by isotope contributions from three sources: recharge water, iodine leached from the formation, and in situ uranium fission. This expected behavior is compared to field study results. Ground-water samples from the Great Artesian Basin, Australia, provide an estimate of the atmospheric equilibrium ratio, 6 x 10⁻¹³. Down-gradient changes in water up to 1 My old suggest that subsurface production can be significant. The usefulness of ¹²⁹I as an indicator of brine source and age is verified in brines collected in and around Louisiana salt domes. The method leads to ages of 7 and 9 My for two brine pockets trapped within Jurassic salt, and 32 to > 40 My for oil-field brines in Miocene sands adjacent to the domes.