Water geochemistry and soil gas survey at Ungaran geothermal field, central Java, Indonesia

Abstract

A soil gas survey for radon (Rn), thoron (Tn), CO2, and mercury (Hg), and the chemical analysis of hot spring waters, were undertaken in the Ungaran geothermal field, Central Java, Indonesia. The results of soil gas surveys indicate fault systems trending NNE–SSW and WNW–ESE. Particularly high CO2 concentrations (>20%), and high Hg concentrations were detected in vicinity of the fumaroles. Emanometries of Rn, Tn and CO2 also conclusively identified the presence of a fracture zone for the migration of geothermal fluid. The Hg results infer that the up-flow zone of high temperature geothermal fluids maybe located in the north of fumaroles in the Gedongsongo area (near the collapse wall). Chemistry of thermal springs in the up-flow zone are acid (pH=4) and show a Ca–Mg–SO4 composition. The thermal waters are mainly Ca–Mg–HCO3 and Ca–(Na)–SO4–HCO3 types near the fumarolic area and are mixed Na–(Ca)–Cl–(HCO3) waters in the south east of Gedongsongo. The δ18O (between −5.3 and −8.2‰) and δ (between −39 and −52‰) indicate that the waters are essentially meteoric in origin. A conceptual hydro-geochemical model of the Gedongsongo thermal waters based on the soil gas, isotope and chemical analytical results, was constructed.