An anomalous thermal water from Hofsstadir western Iceland: Evidence for past CO2 flushing

The geothermal field at Hofsstadir northern Snæfellsnes peninsula, Iceland produces low-temperature geothermal water with about 5.4‰ salinity. The fluid temperature is 87 °C, near the reservoir temperature of 90 °C as assessed from mineral solution/equilibrium conditions. The stable isotopic ratios δ²H and δ¹⁸O show that the water is significantly lighter than present day precipitation anywhere on the Snæfellsnes peninsula. It is offset from the meteoric water line towards isotopically depleted ¹⁸O values, most likely due to CO_2(g) – H₂O exchange at earlier times during evolution of the system. Such a concentration of stable isotopes is unique for Icelandic groundwaters and has not been encountered anywhere else in Iceland. The water may either have its origin far north of the Bay of Breidafjörður in the highland of the western fjordlands or dating back to a Pre-Holocene age when local precipitation was considerably lighter due to the cold climate at that time. The water is highly concentrated in Ca in comparison with seawater and also compared to that of geothermal saline water elsewhere, which indicates intensive and prolonged water–rock interaction. The ¹⁴C concentration is low, about 7.4 pMC (percent modern C), compared to the cold local groundwater of about 74.6 pMC. δ¹³C for the thermal and cold waters is −4.9‰ and −2.3‰, respectively. The geothermal water is used for heating the small town of Stykkishólmur through a central heat exchanger plant due to the high salinity of the water. The outbuilding of a health resort has been planned and the water has been used successfully for the treatment of psoriasis and is claimed to have beneficial effects in bathing therapy for rheumatism as well as for drinking cures.