Japan Geoscience Union Meeting 2018

Presentation information

[EJ] Oral

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

[A-HW23] Residence time of groundwater / surface water and water / mass cycle processes in watershed

Thu. May 24, 2018 9:00 AM - 10:30 AM 106 (1F International Conference Hall, Makuhari Messe)

convener:Maki Tsujimura(Faculty of Life and Environmental Sciences, University of Tsukuba), Shigeru Mizugaki(PWRI Public Works Research Institute), Masanori Katsuyama(京都大学農学研究科, 共同), Maksym Gusyev(International Centre for Water Hazard Risk Management, Public Works Research Institute), Chairperson:Gusyev Maksym, Katsuyama Masanori

9:35 AM - 9:50 AM

[AHW23-03] Groundwater flow system in Fukushima Prefecture, Japan, traced by tritium-3He, 4He and 14C dating methods and 129I/127I ratio

Maiko Sakuraba1, *Hirochika Sumino1, Hiroyuki Matsuzaki2, Haruka Kusuno2, Yusuke Yokoyama3, Yosuke Miyairi3, Takeshi Ohba4, Kana Nishino4 (1.Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, 2.Micro Analysis Laboratory, Tandem accelerator, The University of Tokyo, 3.Analytical Center for Environmental Study, Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Department of Chemistry, School of Science, Tokai University)

Keywords:tritium-3He age, 14C age, 4He age, 129I/127I ratio, oxygen and hydrogen stable isotope ratio, Fukushima Daiichi Nuclear Power Plant

For effective utilization of groundwater resources, it is necessary to understand the large-scale circulation of water [1]. In particular, studying the residence time and source of the water is important for evaluating the groundwater flow system. For Fukushima Prefecture, northeastern Japan, there are few reports on the behavior and contamination status of groundwater affected by 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. We investigated the groundwater flow system in the Fukushima Hama-dori region and the anthropogenic contamination of groundwater caused by the nuclear accident using the 3H–3He dating method, with which it is possible to acquire the age of groundwater and information on the water source based on the initial 3H concentration and 129I, one of the radionuclides released by the nuclear accident. The results were compared with 14C and 4He ages to identify mixing between recent (post-1950’s) and old water (up to 1 million years).

A high 3H concentration of 9 T.U. associated with the age consistent with the date of the nuclear accident within analytical error was observed at only one of the 27 sampling sites. Some samples with which 3H-3He ages were determinable showed significantly older 14C and/or 4He ages than the upper limit of 3H-3He method, indicating mixing between recent and old water. For some samples, measured 14C ages were older than those estimated from mixing ratios of old and recent water based on their 3H concentrations, which suggests additional “dead” carbon supplies from their aquifer. The 129I/127I ratios of almost all samples were lower than the recent precipitations [2] but significantly higher than the lower limit of the value of anthropogenic iodine (1.5 x 10-12) [3], indicating mixing of older water than 1950’s to the recent water. Until the present, the apparent groundwater contamination due to the nuclear accident has not been confirmed except at the single site with high 3H and less than 10 years residence time, though the possibility that polluted water might discharge in the future cannot be ruled out.

[1] Mahara (1996), RADIOISOTOPES, 45, 435-445. [2] Toyama et al. (2012) J. Environ. Radioact., 113, 116-122. [3] Fehn (2012) Annu. Rev. Earth Planet. Sci., 40, 45-67.