Rit (Vísindafélag Íslendinga) - 01.06.1976, Síða 14

Samples have been collected from a large number of local sources, rivers and
streams throughout the country, and their deuterium content measured. The deu-
terium content of the winter layer has also been measured at many places on the
country’s main glaciers. These results are used in drawing a detailed map showing
the amount of deuterium in precipitation over the whole country.
The map reveals that the deuterium content of the precipitation varies con-
siderahly from one place to another. Precipitation on the coast is richest in deu-
terium, which reduces uniformly and rapidly towards the centre of the countryr.
Topography can, however, cause some departure from this, since precipitation on
high mountains generally contains little deuterium.
From investigations of the deuterium content of an ice core taken from the
glacier Vatnajökull, together with measurements of the oxygen-18 content in an
ice core from Greenland, it is possible to conclude that the deuterium content of
the precipitation at any particular localitý has remained more or less unchanged
during the last 8000 years.
According to this, the deuterium map shows the proportion of hydrogen isotopes
in precipitation in Iceland during the last 8000 years.
Before this time, i.e. dating back 60 000 years from the time of the last glacia-
tion, the deuterium content of precipitation in Iceland was much smaller than it
is today. If groundwater contains very little deuterium, there is, therefore, reason
to believe that it is more than 10 000 years old. Such water is fotmd in one region
in Iceland, i.e. in boreholes at Húsavík and in the northem part of Aðaldalur in
S-Þingeyjarsýsla.
Measurements of both the deuterium and the oxygen isotope 018 in the ground-
water have confirmed that all groundwater in Iceland, both hot and cold, is origi-
nally meteoric. Furthermore these measurements show that the deuterium con-
tent of the water does not change on its way through the bed-rock.
The deuterium content of cold and hot springs of unknown origin is often
very different from the deuterium content of precipitation at the same place. On
the other hand, by comparing obtained results with the deuterium map, it is often
possible to find where this water has fallen as rain and to determine its flow path
underground.
In this way it has been possible to make a detailed map of the cold ground-
water flows in three regions in the country. These regions are: 1) The drainage
area of tlie Lake Þingvallavatn, 2) the drainage area of the Lake Þórisvatn and
the area south-west of it down into Landsveit, and 3) the area north of Vatna-
jökull up to Axarfjörður.
Deuterium measurements have been made on water from nearly all the geo-
thermal areas of the country. These measurements have definitely confirmed the
following ideas concerning the origin of the hydrothermal systems.
The thermal water is of meteoric origin. In most cases it is precipitation which
has fallen on the highlands. There it manages to percolate deep down into the
bed-rock, where the ordinary heat-flow of the roclc heats the water up to a tem-
perature which is to a considerable extent determined by the depth, since the
geothermal heat increases with depth. The water can then flow a distance of as
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