Chemical Variety of Water in Icelandic Heating Systems

Abstract

Icelandic heating systems are traditionally of very simple design; water is pumped up from the wells, distributed to the users who use the water directly for heating and as tap water without using heat exchangers. The spent water is mostly disposed of into the sewer or possibly first used for melting snow in the driveway or for the domestic hot pool. This is based on the reality that commonly the geothermal water is slightly mineralized, typically with TDS 200-400 mg/l and with a low corrosion and scaling potential. The pH is typically 9-10, devoid of oxygen and contains enough hydrogen sulfide to act as an inbuilt corrosion inhibitor by reaction with oxygen taken up from the environment. The waters are in equilibrium with silica and silica concentration rises in direct relation with temperature, but in the low and medium enthalpy systems silica scaling does not pose any problems. The waters are in equilibrium with calcium carbonate at reservoir temperatures and are highly depleted in magnesium, even at moderate temperatures. The permeability of the fields is normally high and there is ample recharge to the systems. The waters utilized in the first big heating systems in the country were typically of such a character. However there are also found geothermal waters with relatively high salinity, waters with some concentration of oxygen, waters devoid of hydrogen sulfide and even carbonated water. Waters with extreme high pH, 10-11, are also encountered. By time and increased utilization of geothermal resources for heating there have been utilized geothermal waters with a very varied geochemical properties in the Icelandic heating systems.