Jökull - 01.12.1990, Blaðsíða 145

Figure 22. The groundwater
basin of Tungnaá. Explana-
tions: 1) Major spring areas.
2) Fissure zones. 3) Inferred
groundwater currents.
4) Boundaries of highly
permeable formations. 5)
Mountain area of Torfajökull
central volcano. —Grunn-
vatnskerfi Tungnaár. Skýringar
1) Helstu lindasvæði. 2)
Sprungureinar. 3) Aœtlaðir
og útreiknaðir grunnvatns-
straumar. 4) Mörk vel lekra
jarðmyndana 5) Fjalllendi
umhverfis Torfajökul.
proximately 2/3 of the total groundwater flow. This
ratio is perhaps reflected in the chloride content. At
the glacier margin it is probably near to 2-2.5 ppm,
but in the most voluminous springs it is 3-4 ppm. On
the highland slope, below the basin, the chloride con-
tent seems to be 5-8 ppm. The content in the great
springs is then probably too low for the local precipi-
tation alone, so that an inflow of low-chloride, glacial
groundwater is indeed very likely.
The chemistry indicates strongly a glacial compo-
nent in the groundwater east of river Tungnaá and on
its upper reaches. In the Veiðivötn - Útkvísl area the
chemical indications may be obscured by the effects
of a hidden geothermal field, as mentioned above. On
the whole the calculated glacial component seems to
be quite reasonable. A comparison of the estimated
infiltration rate with that on Langjökull (see above)
is interesting, in both cases it seems to be near to
1,500-2,000 mm/a. The permeability of the bedrock
ts probably not a decisive, limiting factor, nor is the
available precipitation. These values could then be in-
dicative of maximal values for the possible infiltration
through the ice mass of the glaciers.
The process of meltwater supply and infiltration
is not a simple one. The ice mass is transported to-
wards the glacier margins with the movements of the
glacier ice. The ice melt is strongest at the margins and
the infiltration may be strongest there too, at least dur-
ing the summer time. The restricted channels in which
the meltwater flows, intraglacially and or subglacially,
limit on one hand the time of stay for the water, and on
the other hand the areal access to the bedrock. Both
factors limit the infiltration considerably. The per-
meability of the bedrock may be sufficient for infiltra-
tion of more than 3,000-4,000 mm/year (Recent lavas,
fresh hyaloclastites and pillow lavas), as observed on
the Reykjanes peninsula (Sigurðsson, 1986), but the
water supply from the glacier is not. A variously
great part of the glacial meltwater in summer is in-
filtrated in the immediate, highly permeable foreland
of the glaciers, e.g. Langjökull S and NW, from the
river Sylgja from Tungnaárjökull and from the braided
Jökulsá á Fjöllum.
In the Tungnaá basin the glacial groundwater can
JÖKULL, No. 40, 1990 141