Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 175

by the hydrostatic pressure in the ground. The general flow pattern
of the deep thermal water is therefore likely to be rather simple.
As the water approaches the surface it is likely, however, that
the flow pattern is more or less directed by the tectonics. The up-
wards flow from the base to the surface is possibly sometimes quite
complicated. Such complicated upwards flow has in fact been ob-
served in Reykjavík and at S-Reykir in Mosfellssveit at the boun-
daries between the two thermal water systems feeding Laugarnes
and Elliðaár.
If the deep thermal water flow were directed mainly by hydro-
static pressure in the ground it should in principle be possible to
construct a flow pattem for thermal water, when the hydrostatic
pressure is known. As far as the author knows there is, however,
no information available from which a picture of the hydrostatic
pressure of the deep groundwater can be obtained.
The following may be considered as an attempt to obtain qualita-
tive information about how the hydrostatic pressure of the deep
thermal water changes, in general, within the country.
The average heights of rectangular areas, approximately 520 km2,
are used to construct an average topographic map of Iceland. The
data for the average height of each rectangle have been made avail-
able through the courtesy of Mr. Gunnar Þorbergsson, the head of
the Survey Department of the National Energy Authority. The
contours of this average topographic map are shown in Fig. 41.
Now assume that the hydrostatic pressure of the deep thermal
water changes in a similar way as the average heights on the map.
The general flow pattern of the water should then always be per-
pendicular to the altitude contours. As appears from Fig. 41 the
agreement between the general flow pattem, obtained from deu-
terium measurements only, and the flow pattern which could be
predicted from the average topographic map, is in fact surprisingly
good.
The deuterium map, the average topographic map and the 8D-
values obtained for the thermal water samples discussed in chapter
15, are now used to construct the main underground flow pattern
for most of the thermal water systems in Iceland.
The results obtained are shown in Fig. 41, where the thermal
area and the respective recharge area are joined by arrows, in such
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