Acidic geothermal waters of pH 3 to 5 are discharged from the high temperature reservoirs in the Kakkonda and Fushime geothermal fields. In order to investigate the possible source of acidity in these waters, numerical modeling based on multicomponent chemical equilibrium calculations was carried out. The results of the calculation indicate that the precipitation of sphalerite from the saline Fushime water could contribute to making the acid water. In contrast, the acidity in the dilute Kakkonda water could not be caused by the precipitation of sphalerite.

However, addition of a small amount of HCI to the Kakkonda water can decrease its pH to the measured value. The deepest well in the Kakkonda field penetrated the Quaternary granite, a probable heat in the geothermal system, but did not indicate existence of HCI. The alteration mineral assemblage deep reservoir also argues against HC1 supply depth. If hydrolysis reactions of NaCl and CaCl2, rate HCI in the Kakkonda deep reservoir during duction, it could explain the low pH.

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