岐阜県焼岳西麓奥飛騨温泉郷の温泉水の地球化学的特徴と地熱資源解析

抄録

Chemical and isotopic (δD and δ¹⁸O) compositions of 30 hot spring waters and 5 river waters in Okuhida Hot Springs, Gifu, were investigated to examine the geochemical characteristics and geothermal resources. Most hot springs were of meteoric origin and of Na⁺–Cl^-・HCO₃^- and Na⁺–HCO₃^- types with low salinity (less than 30 meq/L). In the study area, meteoric water infiltrates into ground and is heated by magmatic source and enriched in Ca²⁺and HCO₃^- during interaction with underlying carbonate rocks. A part of the hot spring is enriched in CO₂ due to contribution of CO₂-rich volcanic fluid. The estimated underground temperatures by several geothermometers are 150 to 200℃ at the depth of ca. 1 km. The total geothermal resources in the study area are estimated to be ca. 400 MWe for producing the electricity. In the study area, most hot springs have been provided for bathing in hotels and a part of them (60℃, 3,000 L/min) has not been used. We provide this unused geothermal energy to apply for snow melting on road and room heating. For the safety use of geothermal resources in the study area, scaling problem was considered. On the basis of the saturation index of several minerals from the observed chemical compositions of hot spring waters, calcite scale is expected to be precipitated in the reservoir and in the production wells and should be considered to prohibit the precipitation.