Long-Term Variations in ¹²⁹ I Seawater Concentrations off Fukushima and Potential of ¹²⁹ I as an Oceanographic Tracer for ALPS-Treated Water

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.