Rit (Vísindafélag Íslendinga) - 01.06.1976, Page 142

There are about 50 groups of hot springs distributed over an
area of approximately 50 km2. The elevation of the springs ranges
from 40 to 600 m. Above 100 m elevation, the hot springs mainly
occur as acid sulphurous pools and fumaroles with negligible flow
of water, but a few springs in depressions discharge several liters
of water per second. At lower elevations, both in north-eastem and
south-western parts of the area, many powerful drill holes discharg-
ing large amounts of water and steam have been drilled. The deep
circulating temperature of the water has been found to be 260°C
in the north-eastem part but only 230°C in the south-western part.
The geographical location of the Hengill area is shown in Fig. 31,
while the places from which samples have been collected for deu-
terium measurements, together with the mean ðD-value obtained
for each place are shown in Fig. 35. The detailed results are given
in Table IV in the Appendix.
Water samples collected from several drill holes at Nesjavellir in
the north-eastem part of the area (samples No. 52-55) gave a mean
value of ðD = — 71.7%0, when corrected for evaporation due to the
high temperature of the deep circulating water.
Water samples from the drill holes and hot springs below 100 m
altitude at Hveragerði in the south-western part of the area (sam-
ples No. 56-61), all gave similar ðD-values, which when corrected
for evaporation gave a mean value of — 65.4%0.
The hot springs at elevations higher than 100 m (samples No.
63-75) all have higher ðD-values, with mean ðD = — 56.5%0, which
is approximately the same as in local precipitation in this area. The
30°C spring on the lowland south-west of Hengill, with ðD —
— 56.6%0, (sample No. 76), obviously is also mainly of local origin.
On the basis of the above data, a hydrological model of the Hengill
hydrothermal system is proposed. A similar model was initially
suggested by Einarsson (1950), before the present data became
available.
The Hengill area is fed by two deep circulating groundwater
systems which originate farther inland. According to data obtained
for the drill holes at Nesjavellir (Tómasson et al. 1974), the deep
circulating water system at Nesjavellir is at a depth of more than
800 m, but on the lowland in Hveragerði the deep water system
reaches the surface. In the upper part of the Hengill area there is
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