Geochemical interpretation of thermal fluids from low-temperature wells in Stykkishólmur, W-Iceland and Pyrzyce, NW-Polland

Abstract

The low-temperature geothermal waters that feed the district heating systems in Stykkishólmur, W-Iceland, and Pyrzyce, NW-Poland, are found in very different geological environments, namely volcanic and sedimentary rocks, respectively. The geothermal water from the production well in Stykkishólmur differs significantly from other low-temperature waters in Iceland. In particular, its Cl/B ratio is very high, 30,000; while this ratio is 4,350 for seawater and about 100 for typical low-temperature waters in Iceland. The Stykkishólmur water has a very high concentration of calcium, almost 1200 mg/l. The isotopic composition is also unusual for geothermal waters in Iceland. The hot water from Pyrzyce is a brine with mineralization above 120 g/l. This is a typical value for waters occurring in the Polish Lowland. The results from geothermometry equations indicate that chalcedony controls the silica concentration in the reservoir in Stykkishólmur, while quartz controls dissolved silica in Pyrzyce, where the reservoir is a long-lived system. Most of the Na-K equations give a calculated temperature significantly different from the measured temperature. Chemical equilibrium calculations for geothermal water from the production well in Stykkishólmur were performed with the speciation programs SOLVEQ, WATCH, and PHREEQCI. They show the distribution of aqueous species in the water....