Geothermal Influence on Groundwater in the Lake Mývatn Area, North Iceland

Cold groundwater and geothermal effluent in the Lake Mývatn area has been divided into six distinct groups according to origin and geothermal influence. This division is based on stable isotope ratios, chemical composition and geographical positions. The groundwater has apparently two basically separate origins, i.e. the local high ground north of Lake Mývatn and the highlands far to the south, possibly as far south as the glacier Vatnajökull. No traces of seawater are observed and the concentrations of conservative constituents suggest extensive water-rock interaction. The waters are to a different extent affected by geothermal activity and effects of volcanic activity were noted during the Krafla fires 19751984. These have diminished but not disappeared completely. The effluent from the Krafla geothermal power plant seems to travel to the east of Lake Mývatn and traces of it have not been found to enter the lake. The Námafjall diatomite plant effluent on the other hand travels along fissures to the lake. An attempt at simulating the composition of Krafla and Námafjall geothermal water by titrating local groundwater with rock at 205°C and adding volcanic gas seems promising.