Geochemical interpretation of data on thermal fluids from the Laugarvatn low-temperature area, SW-Iceland

Abstract

The Laugarvatn field is one of many low-temperature geothermal fields in the Southern Lowlands of Iceland. The study of the chemistry of the water has been used to suggest movement, processes affecting geothermal water during its up-flow and also possible recharge areas using isotope data ( D and 18O) and Cl/B ratios. Silica and cation geothermometers are applied to predict possible subsurface temperatures using analytical data, and equilibrium calculations with the chemical speciation programme WATCH. The reliability of the geothermometers is checked by studying the equilibrium state of the thermal waters by means of saturation index log Q/K diagrams, Na-K-Mg triangular diagram and by applying both silica-enthalpy and enthalpy-chloride mixing models. In the end, the possible temperatures are predicted after taking into consideration the equilibrium state of the thermal waters. The geochemistry of the thermal water and cold groundwater in the Laugarvatn area shows that the use of geothermometry is dependent on many physico-chemical parameters, suggesting subsurface temperatures in the range of 120-180°C, and probably different sources of water in the eastern and western parts of the area. In fact, physical processes like conductive cooling, mixing with cold water and boiling during up-flow may affect water-rock equilibria and this can result in unreliable temperature predictions. So, it is necessary to apply different methods and geothermometers for the studies of the equilibrium state, to understand which processes occur in the area.