Evaluation of chemical characteristics and reservoir temperature of geothermal water around the Shinyu Fault, North Hakkoda Volcanoes

Abstract

The southwestern foot of the Hakkoda-Odake volcano has many geothermal manifestations where geothermal development has been limited by the Natural Parks Law. The current target area, which is classified as Class II special zone has been deregulated for individual geothermal development. We conducted an investigation to determine the chemical and isotopic characteristics of the hot waters around the Shinyu Fault and to estimate the temperature of the geothermal reservoir. According to our δD-δ18O diagram, the hot waters in the study area are meteoric in origin. In addition, the triangle diagram for Cl-, SO42-, and HCO3- concentrations indicates that these hot waters can be classified as steam-heated waters and volcanic waters. The B/Cl and Br/Cl ratios show that the volcanic waters are diluted with shallow groundwater as they flow through Jigokunuma downstream to Shinyu. We estimated the geothermal reservoir temperature to be between approximately 150 and 160 °C, based on anhydrite geothermometry and the chloride-enthalpy mixing diagram.