Geochemical Characterisation of Thermal Water from Central Vietnam and Borgarfjordur W-Iceland

Abstract

The geochemistry of low-temperature geothermal water in Central Vietnam and Borgarfjörður (W-Iceland) was studied. Twenty samples of geothermal water were collected in Central Vietnam and twelve samples in Borgarfjörður in the summer of 2018. The water temperature, pH, DIC and H2S concentrations were measured on site and major elemental composition analysed using ICP-OES and IC at the University of Iceland. Stable isotopes of hydrogen and oxygen (δD and δ18O) were also determined at the University of Iceland, by IRMS. The surface temperatures of the geothermal waters in Vietnam were 42- 96°C and in Borgarfjörður they were 40-98°C. The stable water isotope ratios, Cl and B concentrations were used to assess the water origin, mixing and water-rock interaction. Based on the results of this analysis it was concluded that geothermal waters in Vietnam and Borgarfjörður are of meteoric origin affected by rock leaching and mixing with non-thermal water and with possible seawater or salt evaporaties in the case of geothermal water in Vietnam. The SiO2 concentrations of the geothermal fluids in Vietnam and Borgarfjörður were 38-138 ppm and 54-178 ppm, respectively. Using these concentrations and assuming equilibrium with chalcedony the reservoir geothermal temperatures in Vietnam and Borgarfjörður were found to be as high as ~120°C and ~135°, respectively. The geographical projection of the major elemental concentrations and geothermometer temperatures revealed three geothermal anomalies in Vietnam: Le Thuy, Quang Binh; Mo Duc, Quang Ngai; Hoi Van, Quang Binh and the highest temperatures at Hægindi in Borgarfjörður.