Jökull - 01.12.1990, Page 139

and extending to west of Eiríksjökull. The area with
lacking or restricted surface runoff reaches west to the
rivers Norðlingafljót and Geitá. The drainage area of
the above mentioned springs covers most probably the
whole ”dry“ area.
The chemistry of the springwater shows relation-
ships to that of the volcanic zones, but hardly any
signs of direct volcanic or geothermal influence, nor
°f a glacial or altitudinal origin. The temperature in-
creases slowly but consistently from E to W, probably
due to infiltrationon the local lava fields. The ground-
water seems to be concentrated in a strong current and
ls probably overflowing from a shallow valley, hidden
beneath the lava. The icefree drainage basin cov-
ers <300 km2 and the infiltration in it must be 3,000
mm/year, if it should alone supply the springs. This
value is most likely much too high, as great parts of
the area are considered to be rather ”dry“, i.e. with a
low precipitation.
The surface divide on the glacier points to an addi-
tional drainage basin on the glacier of near to 150 km2.
Even if a great part of the precipitation flows off on the
surface, this addition is certainly sufficient to account
for the volume of the springwater. On the other hand
11 does obviously not constitute so great a part of the
total water, that its glacial chemistry becomes domi-
nant. If this interpretation is near to reality, despite its
slender documentary base, it shows that the infiltra-
tion rate is probably lower on the glacier than in the
tcefree surroundings, and that the glacial component
must be dominant in the quantity to be recognizable
m the quality of the water, at least at the present state
of knowledge.
THE SEYÐISÁ BASIN
This is probably the case in the drainage basin of
Seyðisá, N of Langjökull (Fig. 18). The tempera-
íure of the springwater is low (2.5-3.5 °C) as is the
chemical content. Many of the springs are connected
t0 fissures, on some of which occur warmer springs,
richer in chemistry, and most likely connected to the
Hveravellir thermal field.
The icefree drainage basin is only 100-150 km2
and the total infiltration not more than 2-3 m3/s, if
the precipitation at Hveravellir is taken as represen-
tative for the area. The mean discharge is probably
near to 6 m3/s (Orkustofnun, Vatnamælingar, 1989),
so that the greater part of the water must have another
origin, which then must be glacial. This would be
in agreement with the properties of the water. The
shallow, regional aquifers permit the water to flow
into the spring area, where they thin out and the wa-
ter is forced to the surface on S-N running fissures,
cutting the stratified and anisotropic aquifers. This in-
terpretation is at present the most plausible, although
not beyond doubts, and demonstrates a case, where
a major, glacial component of a shallow groundwater
current can plausibly be recognized.
THE SOUTHERN LANGJÖKULL BASIN
The mountains bordering the Southern lowlands
(Fig. 16) constitute a system of mountain chains
and tablemountains, covering the eastem part of the
groundwater basins south of Langjökull, and reaching
heights of 800-1,000 m a.s.l. Voluminous springs is-
sue from their basement and the immediate forelands,
many of which are associated to fissures but other to
lava fields. The total discharge is 50-70 m3/s (20-25
m3/s to the river Tungufljót, 25-30 m3/s to upper Brú-
ará and at least 10-15 m3/s to Hagaós (Hjartarson and
Sigurðsson, 1988), distributed along a front of nearly
35 km. The mountain zone has an areal extension
of near to 500 km2. With a probable infiltration of
3,000^4,000 mm/year the above mentioned total dis-
charge would accurately be supplied. There would
then hardly be a need nor a space for a substantial
glacial groundwater component.
The chloride content in the groundwater is much
lower in the mountains than in the lowlands but this
can equally well be accounted for by the altitudinal
effect (if compared to e.g. Tröllaskagi or Austfirðir)
and needs not to be due to a glacial origin. The water
of some of the fissurefed springs shows signs of a deep
penetration in fissures (a lack in magnesium and sul-
phate but not in sodium). The most plausible origin of
the springwater is then from the mountain zone itself,
the glacial factor, if present, not being identifiable.
Ámason (1976)presumed a separate drainage sys-
tem of similar extension for the same spring area in
the ice free zone, but included also a considerable area
JÖKULL, No. 40, 1990 135