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

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.