Chemical Characteristics of Thermal Fluids from Stykkishólmur, Iceland

The low-temperature geothermal waters that feed the district heating system in Stykkisholmur, W-Iceland, are found in volcanic rocks. The main production aquifer is related to a fracture in a gabbroic intrusion. The high degree of alteration of the reservoir rocks indicates much higher temperatures than presently observed (87°C). Geological investigations reveal that the well is situated within an extinct central volcano. The chemical and isotopic composition of the hot water from the production well in Stykkisholmur differs significantly from that of other lowtemperature areas in Iceland. The geothermal water is brackish, calcium is the dominant cation, and the water type is Cl-Ca-Na. The isotopic composition of the thermal water indicates that it is not strictly a mixture of seawater and present-day freshwater. The results from geothermometry equations indicate that chalcedony controls the silica concentration in the reservoir. Chemical equilibrium calculations for geothermal water in Stykkisholmur were performed with the speciation programs SOLVEQ, WATCH, and PHREEQCI. They show the distribution of aqueous species in the water, as well as saturation indices for minerals occurring in the reservoir. Graphs of log (Q/K) vs. temperature indicate that some alteration minerals found in the well are close to equilibrium with the water at the measured temperature. Chalcedony and anhydrite, in particular, seem to be in full equilibrium.