Fig. 3

δ2H versus δ18O diagrams. a–d Isotopic signatures for the surface waters, High-Cl−, Low-concentration, and High-HCO3− fluids, respectively. Isotopic compositional field for hypothesized magmatic water is after Giggenbach (1992). Local meteoric water line in Kumamoto (ca. 20 km northeast from Uto district) is after Okumura et al. (2018)