Distribution and fate of ¹²⁹I in the seabed sediment off Fukushima

Highlights: • Concentration of ¹²⁹I in seabed sediment off Fukushima is reported for the first time. • Deposition of the FDNPP accident-derived ¹²⁹I to the seafloor was 0.36 ± 0.13 GBq • Until October 2013, ¹²⁹I activity in sediment increased in the shelf-edge region. • Remobilization of ¹²⁹I near the seafloor likely affected the sequential accumulation. • The accident-derived ¹²⁹I is considered to negligibly affect the benthic ecosystem. -- Abstract: In this study, seabed sediment was collected from 26 stations located within 160 km from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) during the 2 years which followed the FDNPP accident of March 2011 and the concentrations of ¹²⁹I and ¹³⁷Cs were measured. By comparing the distribution of these two radionuclides with respect to their different geochemical behaviors in the environment, the transport of accident-derived radionuclides near the seafloor is discussed. The concentration of ¹²⁹I in seabed sediment recovered from offshore Fukushima in 2011 ranged between 0.02 and 0.45 mBq kg⁻¹, with ¹²⁹I/¹³⁷Cs activity ratios of (1.9 ± 0.5) × 10⁻⁶ Bq Bq⁻¹. The initial deposition of ¹²⁹I to the seafloor in the study area was 0.36 ± 0.13 GBq, and the general distribution of sedimentary ¹²⁹I was established within 6 months after the accident. Although iodine is a biophilic element, the accident-derived ¹²⁹I negligibly affects the benthic ecosystem. Until October 2013, a slight increase in activity of ¹²⁹I in the surface sediment along the shelf-edge region (bottom depth: 200–400 m) was observed, despite that such a trend was not observed for ¹³⁷Cs. The preferential increase of the ¹²⁹I concentrations in the shelf-edge sediments was presumed to be affected by the re-deposition in the shelf-edge sediments of ¹²⁹I desorbed from the contaminated coastal sediment. The results obtained from this study indicate that ¹²⁹I/¹³⁷Cs in marine particles is a useful indicator for tracking the secondary transport of accident-derived materials, particularly biophilic radionuclides, from the coast to offshore areas.