JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

A (Atmospheric and Hydrospheric Sciences ) » A-HW Hydrology & Water Environment

[A-HW34] Isotope Hydrology 2020

convener:Masaya Yasuhara(Rissho Univ.), Kohei Kazahaya(Geological Survey of Japan, AIST), Shinji Ohsawa(Institute for Geothermal Sciences, Graduate School of Science, Kyoto University), Kazuyoshi Asai(Geo Science Laboratory)

[AHW34-01] Development of 3H-3He groundwater dating method and its application to costal region of Fukushima Prefecture, northeast Japan

★Invited Papers

*Hirochika Sumino1, Naoki Nomata1, Maiko Sakuraba1 (1.Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo)

Keywords:3H-3He dating, Residence time , Groundwater

Residence time of groundwater is an important parameter to elucidate groundwater flow system. The 3H-3He method is one of methods to determine groundwater residence time ranging from several months to 120 years. The best advantage of this method is that it is applicable to accurately determine an age of young water compared to other methods (CFC & SF6 method and 36Cl method, etc.), because initial 3H concentration is directly determined as the sum of 3He and 3H in the water at present, if the groundwater flow system has been a closed system for 3He and 3H until its discharge [1]. We have been developing the method to investigate the groundwater flow system in the Fukushima Prefecture, Japan, where there is concern about contamination by anthropogenic radionuclides released by the accident at the Fukushima Daiichi Nuclear Power Plant triggered by the earthquake of 11 March 2011 off the Pacific coast of Japan and the subsequent tsunami [2]. However, the reliability of the determined residence time is still insufficient to discuss the groundwater residence time because of difficulty with tritium concentration determination by the in-growth 3He method.

We tested accuracy of our tritium determination by analyzing standard water samples provided by IAEA with known tritium concentrations. The 3He/4He and 4He/20Ne ratios of samples which had been degassed and stored for a month suggested insufficient degassing and contamination from the ambient air. Therefore, we improved the equipment for degassing and method for storage, and succeeded in preventing atmospheric helium contamination. This presentation will discuss how this method is applied to analyze groundwater samples from Fukushima Prefecture in Japan.

[1] Y.Mahara et al., J. Groundwater Hydrology., 35, 201-215 (1993). [2] M.Sakuraba et al., Goldschmidt 2017 abstract (2017).