The chemical composition of the inflow into Lake Mývatn and the lake itself is stable due to the fact that almost all the inflow is supplied through groundwater by artesian springs. Geothermal and volcanic activity affects the groundwater system and thus the chemistry and biological activity of the lake which is unique especially at this high latitude. The area has been protected by a special law since 1974 and it is listed as an important habitat for birds in the RAMSAR convention on wetlands. This groundwater inflow is abundant and the residence time in the lake is short. The Krafla and Namafjall geothermal areas are close by and their effluent water drains to a large extent into the lavas containing the groundwater. There has been some concern that the effluent might affect the inflow to Lake Mývatn and therefore the groundwater near Lake Mývatn has been thoroughly studied concurrent with the construction of power plants in Krafla and Bjarnarflag (Namafjall), both operated by Landsvirkjun. The results of artificial tracer tests suggest that dilution is great, e.g. that effluent discharged into a fissure at Bjarnarflag has been diluted about 100 million times by the time it reaches the fissure Grjotagja about 2 km to the southwest. The Krafla effluent is discharged into a stream that flows into a fissure about 7 km to the east of Lake Mývatn in Burfellshraun lava. Chemical studies of the area have led to the division of the groundwater into six groups on the basis of stable isotopes and accounting for B/Cl ratios, two of which constitute effluent waters from Krafla and Namafjall and as expected show strong hightemperature geothermal characteristics. The constituent that is most worrisome as regards possible contamination of the lake is arsenic which also happens to be one of the most characteristic constituents of the effluent. Based on that evidence and of great dilution and little effect of previous effluent on Lake Mývatn it has been decided that the proposed enlarged Krafla and Bjarnarflag power plants can carry on discharging effluent into the lavas to the east of Lake Mývatn. The concentrations of certain chemicals characterizing geothermal effluent will be used as natural tracers for monitoring of fluid from five new wells drilled in the area, the old one and several fissures and springs and the effluent.
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