Rit (Vísindafélag Íslendinga) - 01.06.1976, Page 101

In some cases, corrections should be made on samples from high-
temperature areas because a part of the water has evaporated dur-
ing adiabatic cooling as the water flows to the surface, but only the
liquid phase has been collected. Some of the samples are collected
as the water discharges at one atmosphere pressure and thus the
liquid phase is at a temperature of 100°C. Other samples are col-
lected from drill holes, by using steam-water separators as shown
in Fig. 4, at pressures higher than one atmosphere and thus the
liquid is at a correspondingly higher temperature.
The method used for the correction is described in section 3. 3.
The corrections made are shown in Fig. 26 by arrows. When
the arrows are drawn by solid lines the base temperatures are ob-
tained by measurements in drill holes. Arrows drawn by broken
lines mean that there are no drill holes available, in which the
base temperature can be measured. In those cases the temperature
is estimated by indirect methods, such as by using measurements of
the silica content of the water discharging (Arnórsson 1970).
The values of cold groundwater samples show about the same
scatter around the meteoric line as the precipitation values. Thus
it is not possible to verify from the 8D — 8o18 relationship that cold
groundwater has changed its isotopic composition in the ground,
either the hydrogen or the oxygen isotopes. The same holds for the
values obtained for samples from low-temperature areas. The val-
ues of samples from high-temperature areas, on the other hand, are
obviously more scattered. The most reasonable explanation for the
greater scattering of the high-temperature samples is, according to
Craig (1956) and Banwell (1963), that in some cases the water
has undergone variable oxygen isotope exchange with silicate rocks.
Since Craig and Banwell always found the 8D-value of the geo-
thermal water to be the same as in associated surface water they
concluded that there has been no change in the 8D-value of the
groundwater. 8D-measurements from two high-temperature areas
in Iceland also support this conclusion.
These areas, which will be discussed in detail later in this work,
are the Krísuvík high-temperature area on the Reykjanes peninsula
in Southwest Iceland and the Hveravellir high-temperature area in
the western part of central Iceland, between the glaciers Hofsjökull
and Langjökull. Both of these areas are among those with ÖD — 8018
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