[A-HW18] Isotope Hydrology 2016

[AHW18-05] Groundwater age determination by using ⁸⁵Kr in groundwater at the Kakitagawa spring water in Mt. Fuji spring discharge area

*Makoto Kagabu¹, Midori Matsunaga³, Masahiko Ono², Isao Machida², Reo Ikawa², Jun Shimada³, Noriyuki Momoshima⁴ (1.Graduate School of Fisheries and Environmental Sciences, Nagasaki University, 2.National Institute of Advanced Industrial Science and Technology, 3.Graduate School of Science and Technology, Kumamoto University, 4.Radioisotope Center, Kyushu University)

Keywords:Krypton-85, Groundwater age dating, Kakitagawa spring water

Groundwater age dating study around Mt. Fuji area have been conducted by several researches (Ochiai, 1970; Yoshioka et al., 1993; Mahara et al., 1993; Asai and Tsujimura, 2010; Tosaki et al., 2011; Ohta et al., 2012). Various methods were adopted in Mt. Fuji spring discharge area such as ³H, noble gases, CFCs, ³⁶Cl/Cl and ²²⁸Ra/²²⁶Ra activity. All results show relatively young age (0 to several decades) in the Mt. Fuji groundwater, except for CFCs method which affected local anthropogenic source (Asai and Tsujimura, 2010). However, the result in each method contains the “uncertainty” of its age. Unlike these methods, Krypton 85 (⁸⁵Kr) has high resolution for the age estimation, hence we conducted age determination at the Kakitagawa spring water located in the southeastern part of Mt. Fuji. Krypton 85 (⁸⁵Kr) is a man-made trace gas from reprocessing plant origin whose atmospheric concentrations have been increasing over the past few decades. As it is soluble in water, it can be used as groundwater age indicators over timescales ranging from a few years to a few decades. ⁸⁵Kr specific activities in groundwater was 1.022 ± 0.028 Bq/m³, and the estimated age were 5.2 ± 0.4 years. This young age is corresponding to the previous studies, additionally the range of the estimated age by ⁸⁵Kr was much smaller compare to other methods. Furthermore, ⁸⁵Kr method shows the strong advantage against the anthropogenic contamination.