Long-lived radioactive iodine-129 (129I) was released into the environment during the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011. 129I has also been released into the ocean through the discharge of advanced liquid processing system (ALPS)-treated water since 2023. To clarify the long-term variations in 129I seawater concentrations off Fukushima and examine the potential use of 129I as an oceanographic tracer for seawater circulation, 129I concentrations were measured in seawater around the FDNPP between 2012 and 2024. The maximum 129I concentration (445 × 10-8 Bq/L) was observed at station NP0 in 2013, located closest to the FDNPP, shortly after the evacuation order was lifted. Results indicated that 129I seawater concentrations around the FDNPP decreased over time and approached the preaccident level of 1.76 × 10-8 Bq/L. Spatiotemporal variations in the ratio of 129I to cesium-137 (137Cs) suggested that these radionuclides are temporarily trapped in water masses with several kilometers in size and migrated to the interior of the ocean. Moreover, 129I-rich seawater with an elevated 129I/137Cs ratio was detected following the discharge of ALPS-treated water. Results suggested that the 129I/137Cs ratio exhibits strong potential as an oceanographic tracer in regions with complex circulation patterns, and 129I may serve as a useful tracer for studying local seawater movement.
Suzuki et al. (Sun,) studied this question.