Jökull - 01.12.1990, Side 142

Figure 19. Hydrogeology of the Hofsafrétt area.
Explanations: 1) Boundaries of fissure zones.
2) Major spring areas. 3) Plateau escarpment.
4) Inferred groundwater currents. 5) Hyaloclastite
mountain. — Grunnvatnskerfi á Hofsafrétt.
Skýringar: 1) Mörk sprungusvœðis. 2) fíelstu linda-
svæði. 3) Hálendisbrún. 4) Aætlaðir grunnvatns-
straumar. 5) Móbergsfjall.
cases may be explained by local conditions.
The fact is conspicuous, that carbon dioxide is
very high in the fissure zone (>40 ppm), sulphate
is low (<1 ppm) and sodium is high (>8 ppm) as
is magnesium (>3 ppm). All values are corrected
against chloride and carbon dioxide. In accordance
with the previously given interpretation of the country-
wide groundwater chemistry, this composition would
be characteristic for fissure fed springs with a geother-
mal admixture or a magmatic impact, which must be
traced to the caldera in the glacier and thus be con-
nected with the glacial component. The Recent lavas,
which have their eruptive vents beneath the present
glacier, indicate that the activity of the central volcano
is not extinct. Although the glacial origin is not as such
detectable in the chemistry, its composition strength-
ens the conclusion, that approximately one half of the
spring water in the fissure zone has a glacial origin.
These chemical characteristics can be at least traced
as far as Svartárpollar, 30 km away from Hofsjök-
ull. This demonstrates how far such fissure zones can
conduct the groundwater.
NW - VATNAJÖKULL AND THE FISSURE ZONES
OF ÓDÁÐAHRAUN
The fissure zones in the volcanic zone N of Vatna-
jökull are very pronounced and sharply focused. They
would provide excellent and distinct channels for
transport of groundwater over unusually long dis-
tances (Fig. 20). Along Jökulsá á Fjöllum volumi-
nous springs issue, where the fissure zones cut into
the depression of the river bed. On the westem side
of the lava covered area of Odáðahraun great springs
also issue. The connection with fissure zones is not
so obvious as on the eastem side, but nonetheless in
all probability present (Þóroddsson and Sigbjamarson,
1983).
The chloride content of the springwater decreases
almost continuously from the sea and south to the
glacier Vatnajökull. A similar trend is obvious in the
precipitation (Gíslason, 1985). This trend seems to be
pretty regular, and then allows for extrapolations and
interpolations with a fair degree of confidence.
The content in the groundwater seems to be fairly
regularly about three times as high as in the precipita-
138 JÖKULL,No. 40, 1990