Geochemical evaluation of geothermal resources in Toyama Prefecture, Japan, based on the chemical and isotopic characteristics of hot spring waters

The chemical compositions, including hydrogen and oxygen isotopes, of 35 hot spring waters in Toyama Prefecture, central Japan, were analyzed to examine their geochemical characteristics and geothermal resources. The hot spring water samples were classified into two groups: low and high chloride (Cl⁻) concentrations of less than and greater than 2000 mg/L, respectively. The samples in the low-salinity group were determined to be of meteoric origin and Ca²⁺–HCO₃⁻ type. The samples in the high-salinity group were determined to be of Na⁺–Ca²⁺–Cl⁻ type and assumed to contain a mixture of fossil seawater and meteoric water. In the eastern area of the study region, the Na⁺/K⁺ and SiO₂ concentrations were lower and higher, respectively, than those in other areas, which corresponded to high-temperature geothermal resources. Thus, the subsurface information obtained in this study enabled effective use of the low to medium geothermal resources in Toyama Prefecture.