Feasibility of 129I groundwater dating calibrated by both 81Kr and 4He for the assessment of deep geological repositories in Japan

Iodine-129, which is a promising tracer for dating old groundwater, has been used as a tracer for deep upwelling groundwater. The nuclide is expected to be one of the key factors for site selection for high-level radioactive waste disposal, which is a global societal issue. The pre-anthropogenic 129 I/ 127 I ratio for marine iodine is (1.50 ± 0.15) × 10 −12 , which could be considered the initial value for 129 I dating. This study identifies the challenges in groundwater age dating using 129 I/ 127 I. We measured the ratios of 129 I/ 127 I and 81 Kr/Kr and concentration of 4 He in groundwater from boreholes on the northern coast of Japan. The 129 I dating results were not coincident with the other groundwater dating results. The iodine in the groundwater was inferred to be released in situ from marine organisms in sediments of various ages. We estimated that the primordial iodine ratio originating from seawater was ~ 1 × 10 –13 (8 × 10 –14 ~ 2 × 10 –13 ). The groundwater age deduced from the 129 I/ 127 I ratio using this value agrees with other groundwater dating results.