[H-CG27] Nuclear Energy and Geoscience

[HCG27-03] Groundwater dating conducted in and around the Mizunami Underground Research Laboratory

*Takuma HASEGAWA¹, Korato NAKATA¹, Yuichi TOMIOKA¹, Tomoko Ota¹, Katsuhiro HAMA², Teruki IWATSUKI², Toshihiro KATO², Kazuki HAYASHIDA² (1.Central Research Institute of Electric Power Industry, 2.Japan Atomic Energy Agency)

Keywords:groundwater age, carbon 14, Helium 4, noble gas temperature

Groundwater dating was conducted in and around the Mizunami Underground Research Laboratory (MIU). MIU is located at discharge area in local groundwater flow regime, where groundwater age have not been estimated due to low DIC for ¹⁴C and external He flux for ⁴He. The groundwater samples were collected from borehole drilled in the galleries in the MIU. ¹⁴C concentration can be evaluated accurately by changing pre-treatment method from precipitation method to gas stripping method. Helium was accumulated by not only in situ production but also external flux. Helium age can be evaluated by excluding the contribution of external flux using different ratio of external flux contribution data. ¹⁴C age and ⁴He age were estimated about 20,000 years around the MIU. The estimation of the recharge temperature by noble gas is also conducted. The estimated recharge temperature is around 6 ℃. This temperature is about 9 ℃ cooler than present annual average temperature, which agree with estimation of modern analog method conducted around the MIU. Therefore, groundwater around MIU recharged in glacial period. It is consistent with ¹⁴C age and ⁴He age. The groundwater age around MIU can be determined consistently by multiple groundwater dating methods, which are ¹⁴C, ⁴He and noble gas temperature.