Regional hydrological investigations of the subsurface drainage in the neovolcanic area around lake Thorisvatn on the central Icelandic plateau have been carried out as well as geological exploration. Environmental isotopes, deuterium and tritium, proved decisive in finding the groundwater flow pattern and in separating the different groundwater systems and explaining local deviations as barriers and perched aquifers. The regional groundwater flow is only slightly dependent on the. topography but highly on the geological conditions, as it virtually flows under mountain ranges as well as under the river Tungnaa.