Geochemical evaluation of geothermal resources in Ishikawa Prefecture with chemical and isotopic characteristics of hot spring waters

Abstract

Chemical compositions previously obtained from 333 hot springs in Ishikawa Prefecture were firstly examined for the geochemical characteristic. The Cl^- concentration varies from 9 to 17,490 mg/L. Two geothermometers (Na⁺/K⁺ratio and SiO₂ concentration) have been applied to estimate the subsurface temperatures with the chemical data and show that there are several high temperature areas (>150℃).Secondly, 29 hot springs and three river waters were newly collected from selected areas and analyzed for their hydrogen and oxygen isotopic compositions to also examine the source. The chemical and isotopic compositions indicate that the hot spring waters are a mixture of meteoric water, sea water (fossil sea water in a Green Tuff region) and volcanic fluid. The high temperature geothermal resources on the basis of the estimated subsurface temperature (ca. 200℃) may exist in the Hakusan area, the southern part of Ishikawa. The estimated geothermal resources in this area are ca. 60 MW. During geothermal power generation in this area, calcite is expected to precipitate in the reservoir and production wells according to the saturation indices for the hot spring waters and should be prohibited the precipitation.