[A-HW23] Isotope Hydrology 2021

[AHW23-07] Supply processes of dissolved inorganic carbon and flow system of groundwater in Fukui-Ohno Basin

★Invited Papers

*Yamanaka Masaru¹, Seongwon Lee², ZHAO MINGZHE¹, Takanori Nakano³ (1.Department of Earth and Environmental Sciences, College of Humanities and Sciences, Nihon University, 2.Rissho University, 3.The Research Institute for Humanity and Nature)

Keywords:groundwater, carbon isotope

A combination of carbon stable isotopic compositions, d¹³C, and chemical compositions of groundwater in the lowland of the Fukui-Ohno Basin, FOB, was employed to elucidate supply processes of dissolved inorganic carbon, DIC, and additionally to give some constraints on groundwater flow systems in the FOB. Ca-HCO₃ type groundwater is dominated with relatively high concentrations of Ca²⁺ and HCO₃^-, which can be attributed to weathering processes of anorthite and partially of peridot in the aquifer with partial CO₂ pressure as typically observed in soil. Moreover, d¹³C values for the CO₂ gas initially equilibrated with DIC in the FOB groundwater are calculated to be -28.6~-22.8‰, indicating that the FOB groundwater has two carbon sources; one is C3 organic material mainly distributed in mountainous area, ≈-28‰, and another is organic materials in lacustrine deposits in the lowland, ≈-15‰. Considering a model calculation that 20~50% of DIC is lacustrine deposit origin, the FOB groundwater is likely composed of two components; 20~50% of penetrated precipitation in the lowland and 50~80% of groundwater which has been recharged mainly in mountainous area and horizontally transported in the aquifer.