Groundwater in the Lake Myvatn area, northern Iceland: Chemistry, origin and interaction

Lake Myvatn, northern Iceland is unique in that almost all the inflow is supplied through the groundwater by artesian springs. Hardly any surface water is encountered in the area which is covered by young and porous lava fields and transected by faults. The inflow of water and its chemical composition is, therefore, very stable. Geothermal and volcanic activities affect the groundwater system in the Lake Myvatn area and greatly influence the lake's chemistry and thus the biological conditions especially by providing continuous and ample sources of silica and sulphate. Groundwater studies have been intensified in the area during the last years for further developing the Námafjall geothermal field in the area and the Krafla geothermal field about 10 km from Námafjall. Groundwater chemistry was monitored regularly for 2 years at 22 sampling sites and for determining selected indicator constituents in the samples. Concurrently, several tracer tests were performed in the area, the rate of groundwater flow measured, a reservoir model of the groundwater system established. These studies enabled us to divide cold groundwater and geothermal effluent in the Lake Myvatn area into six distinct groups, based on stable isotope ratios, chemical composition and geographical position. The groundwater has separate origins in the local high ground north of Lake Myvatn and the highlands far to the south, possibly as far south as the glacier Vatnajökull. The waters are to a different extent affected by geothermal activity, and effects of volcanic activity were noted during the Krafla fires in 1975-1984. Although these have diminished, they have not completely disappeared. The effluent from Krafla seems to travel to the east of Lake Myvatn and traces of it have not been found to enter the lake. The Námafjall effluent on the other hand travels along fissures to the lake. Attempts made to simulate the evolution of the geothermal water of Krafla and by theoretically titrating local groundwater with rock at elevated temperatures and adding volcanic gas seem promising and result in a composition close to the natural one.[PUBLICATION ABSTRACT]