Hydrogen isotopes in hydrological studies in Iceland

The deuterium content in monthly samples of precipitation at Reykjavik, Iceland, has been measured continuously for the past 8 yr. The results show considerable fluctuations for individual months and a slight annual variation. At another station in south-west Iceland deuterium measurements have been carried out for two years. Numerous water samples from cold springs in south-west Iceland show no time variation in deuterium content, whereas samples taken from streams in the same area show a small variation. Cold springs and rivers have similar deuterium content as the total yearly precipitation in their recharge area as judged from monthly samples. Because the time variations are small, these sources are very useful indicators of the mean deuterium content in the precipitation and its variation with geographical position. A map showing the mean deuterium content of precipitation in south-west Iceland is presented. This map is based on cold-surface and near-surface water samples. It shows clearly how the deuterium content varies with the distance from the sea and how the distribution is modified by the landscape. A number of deuterium analyses of thermal subsurface waters is presented with a discussion of the sampling technique for thermal water. Samples of steam and liquid water from some boiling boreholes show that equilibrium is achieved between deuterium concentrations in the vapour and the water phase. This makes it possible to calculate the deuterium content of the recharge water for a hot spring from deuterium measurements of the water phase alone, if the base temperature of the recharge water is known. Our results give no reason to believe that the thermal water changes the deuterium content during its passage underground. In Iceland hydrological conditions are such that deuterium analysis can be used to trace the recharge area of thermal springs. In many cases they differ markedly in deuterium content from the precipitation at their location. The conclusion drawn is that the hot water emerging from some of the hot springs and boreholes in south-west Iceland has flowed under ground for a distance of 50 km or more. (author)