This paper summarizes the result obtained for hydrothermal systems in Iceland, by using deuterium and oxygen-18 as natural tracers.

The deuterium content of a single sample of a local cold spring or river, except those rivers fed by considerable amount of glacier melt water, can be used to estimate the mean deuterium concentration of precipitation in the corresponding locality. Furthermore the deuterium content of the last winter layer, collected on Icelandic glaciers in spring before the melting season, is practically identical to the mean value of the corresponding annual precipitation. These facts have been used to draw a detailed map showing the amount of deuterium in precipitation over the whole country. Deep ice core studies show that the deuterium content of precipitation in each place has remained more or less unchanged during the last 8000 years.

Measurements of both deuterium and oxygen-18 in the groundwater have confirmed that all groundwater in Iceland is originally meteoric and that the deuterium content of the thermal water does not change on its way through the bed-rock.

The deuterium content of water discharging from hot springs or drill holes is often very different from the deuterium content of local precipitation. On the other hand, by comparing results obtained with the deuterium map, it is often possible to find where this water has fallen as rain and to trace its underground flow path. Deuterium measurements have been made on water from nearly all geothermal areas in the country. The results are drawn up together in one picture, which shows the origin and flow paths of most of the hot groundwater systems in Iceland.

Finally it has been possible to give som idea of the age of the thermal groundwater, i.e. the time past since precipitation. The thermal water is obviously of varying ages. The »youngest« water seems only to be a few decades old, whereas the »oldest« appears to be from the last glaciations.

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