[AHW35-P10] Groundwater flow system in Fukushima Prefecture traced by tritium-3He and 129I dating methods
In this study, we investigated the groundwater flow system in Fukushima Prefecture and the anthropogenic contamination of groundwater caused by the nuclear accident using 3H-3He dating method, which is possible to acquire not only the age of groundwater but also the information on the water source based on the initial 3H concentration, and 129I, one of the radionuclides released by the nuclear accident.
Sampling was conducted in September 2016 and January 2017 mainly on groundwater and spring water at the Hama-dori region, Fukushima Prefecture. The water samples for 3H-3He dating method were collected and sealed in copper tubes with special care to atmospheric contamination and analyzed using a noble gas mass spectrometer at Department of Basic Science, University of Tokyo. After the first extraction of originally dissolved helium, the sample water was sealed in the copper tube again and stored for about a month, and then the amount of 3He produced by the decay of 3H was extracted by the second degassing and analyzed to obtain the initial 3H concentration and the residence time. Water samples for iodine analysis were sampled in 1-L plastic bottles. Iodine in the sample water was extracted and back-extracted to yield silver iodide pellet, and then 129I/127I ratios and concentration of stable isotope 127I was measured using an accelerator mass spectrometer and ICP-MS at Micro Analysis Laboratory, Tandem accelerator, University of Tokyo.
For example, in Minami Soma Sports Park located about 30 km north of the nuclear plant, the concentration of tritium-derived 3He was approximately 2.2x10-18 mol/g. The current 3H concentration has been determined by liquid scintillation counter to be 9.3 TU for a sample collected from the same locality (Yabusaki et al., 2015). These values give the initial 3H concentration of 10.1 TU and the residence time of about 1.5 years. This suggests the groundwater system in this region has already been almost completely refreshed since March 2011 due to the short residence time.
For 129I/127I ratios, most of the samples measured so far showed values below 1.0x10-11, which is the lower limit of the isotope ratio including radioactive iodine of anthropogenic origin (Fehn, 2012). The low 129I/127I ratio of water from Minami Soma Sports Park is in the range of natural background, which is consistent with the short residence time obtained with the 3H-3He method.
Up to the present, it is not possible to confirm the apparent contamination due to the nuclear accident, but depending on the residence time of the groundwater, the possibility that polluted water will discharge in the future cannot be ruled out. We are planning to carry out further sampling to estimate the residence time and water source by the 3H-3He method and combine it with the 129I results to investigate the environmental impact.
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S. Yabusaki, N. Shibasaki, Y. Takagi (2015) J. Center Regional Affairs, Fukushima Univ., 27, 37-46.
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