Anthropogenic iodine-129 depositions at the Sea of Japan and Pacific sides of the Japanese archipelago, during 2017-2018
Dr Tetsuya Matsunaka1, Dr. Seiya Nagao1, Dr. Shinya Ochiai1, Mrs. Masumi Matsumura2, Mr. Tsutomu Takahashi2, Dr. Keisuke Sueki2, Dr. Kimikazu Sasa2
1Kanazawa University, Kanazawa, Japan, 2University of Tsukuba, Tsukuba, Japan
The investigation of water dynamics change in the Sea of Japan caused by the recent global warming is essential for forecasting the response of ocean circulation with climate change. Anthropogenic ¹²⁹I (half life = 15.7 million years) produced from the thermal neutron fission, is dominated by release from nuclear fuel reprocessing plants in the Europe and supplied at the Sea of Japan via atmospheric deposition and surface runoff. The long-lived ¹²⁹I is expected as an oceanic tracer of surface and vertical circulations in the Sea of Japan. This study aimed to illuminate the atmospheric deposition flux of ¹²⁹I in the Sea of Japan through the monthly monitoring of ¹²⁹I in precipitation at Nomi located in middle area of the Sea of Japan side of the Japanese archipelago during 2017−2018, by contrasting with the ¹²⁹I depositions in Tsukuba (Pacific side) about 300 km ESE of Nomi and Hirosaki (Sea of Japan side) (Hasegawa et al., 2017) about 600 km NE of Nomi.
Rainwater samples were collected at each station in Nomi and Tsukuba cities using a funnel placed on the buildings of Low Level Radioactivity Laboratory (Kanazawa University) and Tandem Accelerator Complex (University of Tsukuba) on a monthly basis from July 2017 to December 2018. After adding 1 mg of iodine carrier (Woodward old iodine) with an ¹²⁹I/¹²⁷I ratio of 1.5 × 10–¹⁴ to the filtered 200–500 ml rainwater sample, the iodine was isolated by solvent extraction with CCl₄. The purified iodide was precipitated as AgI by adding AgNO₃. The ¹²⁹I/¹²⁷I ratio of the AgI targets was measured using the AMS system at the Tandem Accelerator Complex, University of Tsukuba. A terminal voltage of 5 MV and a charge state of 5+ were chosen for acceleration and detection. The measurement ratios were normalized against the S-Purdue (Z94-0597) reference material, which had an ¹²⁹I/¹²⁷I ratio of 8.378 × 10–¹² and was provided by the Purdue Rare Isotope Measurement Laboratory at Purdue University, USA. Stable iodine (¹²⁷I) in the rainwater was measured by an ICP–MS. The original ¹²⁹I/¹²⁷I ratios and ¹²⁹I concentrations in the rainwater were calculated using ¹²⁹I/¹²⁷I ratio from AMS and ¹²⁷I concentration from ICP-MS.
The overall dissolved ¹²⁹I concentrations in rainwater varied from 18.3 to 326 nBq/L (mean 129 nBq/L) at Nomi and from 32.7 to 223 nBq/L (mean 134 nBq/L) at Tsukuba. Mean monthly deposition fluxes of ¹²⁹I during 2017–2018 were in the range 0.144–1.73 µBq/m²/day at two sites (mean 0.834 µBq/m²/day at Nomi and 0.488 µBq/m²/day at Tsukuba), about one-third to one-half of the mean values observed at Hirosaki (1.28 µBq/m²/day) (Hasegawa et al., 2017). Clear seasonal variations of ¹²⁹I depositions were found at the Sea of Japan side (Nomi and Hirosaki), with the higher levels being recorded in winter (November to February). These latitude and seasonal dependences revealed that the deposited ¹²⁹I at the Sea of Japan side was mainly transported by the prevailing northwesterly winter monsoon from higher latitude area of the North Hemisphere.
2013-2016, Researcher, Tandem Accelerator Complex, University of Tsukuba (Environmental radioactivity)
2017 to present, Assistant Professor, Institute of Nature and Environmental Science, Kanazawa University (Geochemistry)