[A-HW23] Tracer Hydrology: Advances in Measurement and Modelling

[AHW23-P04] ³H, ³⁶Cl and ¹²⁹I in the underground of Koriyama, Fukushima

*Tomoko Ohta¹, Keith L. Fifield², László Palcus³, Stephen G. Tims², Stefan Pavetich², Hiroyuki Matsuzaki⁴, Daisuke Tsumune⁵, Yasunori Mahara⁶ (1.Nagaoka University of Technology, 2.Australian National University, 3.HUN-REN Institute for Nuclear Research, 4.The University of Tokyo, 5.Tsukuba University, 6.Kyoto University)

Keywords:tritium, 36Cl, 129I

Several studies in the past fifty years have exploited the bomb-pulse tritium and ³⁶Cl released in nuclear weapon tests in unsaturated soil water and shallow groundwater across the globe as a hydro-tracer. A significant pulse of tritium was also released from the damaged reactors in the Fukushima nuclear accident, which was caused by a large earthquake and the resulting tsunami on March 11th, 2011. Rather than use this as a hydrological marker, however, the study reported here aims to reconstruct the amount of tritium deposited around the epicenter of the accident. To this end, a continuous depositional record of tritium and ³⁶Cl was obtained from the soil water in a 6 m long drill-core through the unsaturated soil zone. It was collected in September 2014, 3.6 years after the accident, at the site of Koriyama, Fukushima. The contributions of ³H and ³⁶Cl from the accident were determined to be 1.4 × 10¹³ and 2.0 × 10¹² atoms m⁻², respectively, at this site. Unlike approaches based on radionuclide migration analysis, the ³H and ³⁶Cl concentrations in precipitation during the approximately six weeks following the accident were accurately recovered (607 Bq/L and 4.74 × 10¹⁰ atoms/L, respectively) by analyzing the depositional flux in the unsaturated soil column from depths of 0 m to 4.25 m. Both the ³H and ³⁶Cl concentration profiles were reassessed at the site in 2016. In contrast, ¹²⁹I was primarily found in the litter layer and the soil near the ground surface (¹²⁹I: 10^-4 ~3 mBq/kg, ¹²⁹I/¹²⁷I: 5×10^-12 ~ 2×10^-8).