Aquifer Chemistry of Thermal Waters in Obama Geothermal Field, Japan- Between 1984 and 2001

Abstract

Thermal waters hosted by volcanic rocks emerge along the faults lineaments in the Obama geothermal area (Western part of Kyushu Island, Japan). All thermal waters belong to Na-Cl facies. Those waters are slightly alkaline and have high dissolved contents with total dissolved solids around 8404 mg/kg in 1984 and 8196 mg/kg in 2001. The chemical-isotope geothermometers, silica-carbonate and enthalpy-chloride mixing models suggest a reservoir temperature is around 190 oC in 1984 and 210 oC in 2001. The chloride composition of the parent fluid is 6200 mg/kg in 1984 and 3800 mg/kg in 2001. The silica composition of the parent fluid is 310 mg/kg in 1984 and 400 mg/kg in 2001. The Obama hot waters are products of mixing of three phases: sea water, meteoric water and magmatic water. The Obama geothermal field is in a recovery state with rising in the water temperature and decreasing in the salinity.