THE GLOBAL DISTRIBUTION OF IODINE-129 AND ITS POTENTIAL APPLICATION AS AN AGE TRACER IN THE TUCSON BASIN

Abstract

Radiogenic tracers help determine groundwater recharge rates and flow paths. Decreasing environmental tritium concentrations necessitate the use of a new radiogenic tracer. Iodine-129 (129I), a long-lived radioisotope generated by nuclear weapons and fuel reprocessing, offers a potential alternative. This study compared the isotopic ratio of 129I to stable 127I with ages previously calculated with tritium. A strong logarithmic correlation was found between the tritium ages and the isotopic ratio. Values from Sabino Canyon and Marshall Gulch precipitation and surface water were compared to global values from published research and in both cases isotopic ratios in precipitation were higher than for surface water, which were higher than ocean or groundwater. The Arizona samples show a seasonal variation in isotopic ratio between summer monsoon and winter frontal precipitation, with values significantly higher in the summer. Mixing model analysis performed on Marshall Gulch surface water and using deep groundwater, precipitation, and soil water as end members, showed a shift in sulfate concentration between these seasons that mimics the variation in isotopic ratio. However, the difference in precipitation source between the summer monsoon and winter frontal precipitation may be a factor in the variations.