¹²⁹I and ¹³⁷Cs in groundwater in the vicinity of Fukushima Dai-ichi nuclear power plant

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This paper reports iodine (¹²⁷I and ¹²⁹I) and cesium (¹³⁷Cs) isotope concentrations in groundwater of confined and unconfined aquifers in the vicinity of the Fukushima Dai-ichi nuclear power plant (FDNPP). ¹²⁷I and ¹²⁹I concentrations range from 2–13 μg/L and 5 × 10⁷–8 × 10¹⁰ atom/L respectively, resulting in ¹²⁹I/¹²⁷I atomic ratios from 5 × 10^–9 to 2 × 10^–6. In all samples, ¹³⁷Cs concentrations were below detection limit. The deep-sealed groundwater from the confined aquifer did not contain significant quantities of Fukushima-derived ¹²⁹I compared to the groundwater in the unconfined aquifer. The minimal ¹²⁹I/¹³⁷Cs activity ratios in the groundwater are more than 2–500 times higher than the FDNPP source ratio. These data can be explained by rainwater infiltrating through the surface soils, with the more water-soluble ¹²⁹I preferentially extracted into the aqueous phase and the ¹³⁷Cs preferentially retained in the soil.