Geochemical Interpretation on Origins and Underground Behavior of Geothermal Water from Yamagawa Geothermal Power Plant, Kyushu, Japan

Abstract

Yamagawa geothermal power plant, located in the southern Kyushu, commenced its operation in 1995. Since then, geothermal fluids from production wells have been continuously collected mostly at an interval of half a year, and analysed for their physical and chemical properties. The relationships between specific enthalpy and Cl content and also hydrogen and oxygen isotopic ratios of the geothermal fluid revealed the followings. The geothermal fluid is a mixture of a high-enthalpy parent water from deep and a low-enthalpy hot water at shallow depth. The parent water is of seawater origin, of which temperature and Cl content are about 290∼310°C and 18∼19 g/kg. The shallow hot water is also a mixture of seawater and local meteoric water, though its temperature and Cl content are lower than the parent water, and about 150°C and 14 g/kg. The mixing ratios of these two waters are various among individual production wells. In some production wells, underground boiling has taken place under adiabatic condition, while in the other wells under isothermal condition caused by heat supply from rocks. Admixing of re-injection hot water from the power plant is observed in several production wells.