[H-CG25] Nuclear Energy and Geoscience

[HCG25-P07] Estimation of hydraulic conditions of groundwater using carbon isotope

*toshihiro kato¹, Teruki Iwatuki¹, Kotaro Nakata², Takuma Hasegawa² (1.Japan Atomic Energy Agancy, 2.Central Research Institute of Electric Power Industry)

Keywords:carbon isotope, groundwater, hydraulic condition

Groundwater flow is an indispensable information to estimate the material cycle in underground environment. Hydraulic simulation referring chemical index of groundwater enables to realize the evaluation of groundwater flow. Japan Atomic Energy Agency (JAEA) has been carried out the hydrochemical and hydraulic monitoring in sedimentary rocks and granite at Mizunami underground research laboratory (MIU) to evaluate an environmental influence around the large-scale underground facility. We studied the relationships between isotopic / chemical composition of groundwater and hydrogeological structure to infer the hydraulic condition.
The δ¹³C value of groundwater increase with decreasing 14C activity at depths between 200 - 400 m. The saturation index of minerals indicates that calcite may not dissolve in this domain. The δ¹³C value of groundwater is probably changed by mixing of isotopically distinct groundwaters. 14C activity ranges 4 to 31 pMC in relatively highly fractured domain at depths between 200 - 400 m, and ranges 2 to 16 pMC in sparsely fractured domain at the depth of 500 m of granite. It depends on difference of permeability of rock formation.