Geochemistry of Hot Spring Waters at Bajawa Area, Central Flores, Nusa Tenggara Timur, Indonesia

Abstract

Thermal and cold spring water, and stream water were analyzed to understand geochemical characteristics of geothermal resources in Bajawa area, central Flores, eastern part of Nusa Tenggara Timur, Indonesia. In this area, three active volcanoes, Inerie, Inerika and Ebulobo, form an isosceles triangle with sides 26-27 kilometers (Eburobo-Inerie and Eburobo-Inerika) and 18 kilometers (Inerie-Inerika). More than 30 monogenetic volcanoes are distributed in the triangle. Only one fumarolic area exists at western outside of a crater rim of Wolo Bobo monogenetic volcanoes.

Seven prospective manifestation areas are situated near the volcanoes. Mataloko, Nage, Keli and Soka areas are in south to southeast of Bajawa city, and Mengeruda, Gou and Boba- Soa areas are in northeast of the city. Temperatures of thermal springs are > 90?C at Mataloko, >75?C at Nage, >70?C at Keli and 40-45?C at other areas. Geochemical characteristics of most thermal waters are acid sulfate type, except for Nage thermal waters, acid sulfate-chloride type.

Acid thermal springs are formed by mixing relatively low temperature volcanic gases containing hydrogen sulfide gases, interstitial waters in acid alternation zone and meteoric/subsurface/stream waters, whereas Nage thermal springs may originate from high temperature volcanic gases, which contain hydrogen chloride and sulfur dioxide.

Chemical and isotopic characteristics of dissolved sulfate show that Mataloko, Nage and Keli thermal waters are similar to those in Rank A volcanic geothermal resources areas in Japan, whereas Mengeruda, Gou and Soka thermal waters are related to those in Rank B volcanic geothermal resources areas in Japan. Thus it is deduced that geothermal resources of Mataloko, Nage and Keli areas are more prospective compared with those of Mengeruda, Gou and Soka areas.