Jökull - 01.12.1990, Page 146

Figure 23. Hydrogeology of
Mælifellssandur. Explana-
tions: 1) Major spring areas.
2) Inferred groundwater cur-
rents. — Grunnvatnskerfi á
Mœlifellssandi. Skýringar:
1) Helstu lindasvœði.
2) Aœtlaðir grunnvatns-
straumar.
almost certainly be traced in the model for a distance of
70 km or more, although increasingly diluted by local
infiltration on the way. The conditions for a flow over
such long distances are extremely favourable in the
Recent lavas and fissure zones of the basin (transmis-
sivities in the model commonly 0.1-1 m2/s). Despite
the probable errors in the model, due to the insuffi-
ciency of the basic data, it presents the most accu-
rate, quantitative informations on a single groundwa-
ter basin with a strong glacial component available at
present. A hydrological and hydrogeological analysis
of the groundwater conditions in the basin is therefore
very promising, especially as other types of informa-
tion have also reached a high level in the area: Hy-
drogeological maps in the scale 1:50,000 are available
or in preparation, a considerable chemical sampling
has been carried out in the area and various other in-
formations on the springs in the basin have also been
collected.
MÝRDALSJÖKULL AND MÆLIFELLSSANDUR
On the eastern margin of the plateau of Mæli-
fellssandur, N of Mýrdalsjökull, voluminous springs
issue at an elevation of near to 500 m a.s.l., with a
total discharge of almost 20 m3/s in summer but pos-
sibly as low as 10-15 m3/s in winter (Fig. 23). The
drainage area in front of the glacier does hardly exceed
40 km2 in extent, although its boundaries are not ex-
actly known. The discharge rate is thought to be near to
100 1/s/km2 (Sigurðsson and Einarsson 1982) most of
it being infiltrated intothe ground. This corresponds to
maximal 5 m3/s input to the groundwater, but it might
be as low as 1-2 m3/s. The rest must be of a glacial ori-
gin. The plateau is covered by lavas, protruding from
below the glacier, but of a much greater importance
is most likely the volcanic and tectonic fissure cutting
NNE - SSW through Háalda and disappearing under
the glacier. It is a part of the Eldgjá volcanic system
which reaches probably from Vatnajökull to Mýrdals-
jökull (Jakobsson, 1979; Larsen, 1979; Sigurðsson,
1988).
The fissure zone could drain a subglacial area of
100-150 km2, which could yield a mean outflow of
10-20 m3/s. A small part of this is released and in-
filtrated in front of the glacier during the glacial melt
in summer. The glacial component in the springwater
seems to be 70-90%. The chloride content is around 4
ppm, but else the chemical content is very low, SÍO2 is
142 JÖKULL, No. 40, 1990