Jökull - 01.12.1990, Page 133

Figure 12. Regional distribu-
tion of chloride in groundwa-
ter. Values in ppm.
— Svœðisdreifing klóríðs í
grunnvatni. Gildi íppm.
on the whole the concentrations show a reasonable dis-
tribution and relations to hydrogeological conditions.
The regional distributiondemonstrates the above men-
boned effects pretty well: Low contents in the high-
lands and in non-thermal fissure zones, high contents
m moorlands and in the runoff from active central vol-
canoes. Low contents are found in the highly perme-
able surroundings of Langjökull, northem Mýrdals-
jökull and westem Vatnajökull. There they could be
mdicative of a glacial origin of the groundwater. At
the base of the glaciers and in the subglacial aquifers
the groundwater is at least semiconfined. Vegetation,
as a source of carbon, is of course absent. Low carbon
dioxide contents should therefore indeed be expected
in the glacial groundwater (Fig. 12).
The chloride content is thought to be indicative
°f the marine component in the groundwater. In the
Precipitation shadows north of the great glaciers the
chloride content is less than 2 ppm. This could be
the result of differentiation by the glacial melt, but
similar values are also found on Tröllaskagi and Nýja-
hæjarfjall (Fig. 10). The unaltered bedrock on top of
Nýjabæjarfjall is well permeable, so that an excessive
surficial runoff is probably not the case. The concen-
Iration in the groundwater should therefore represent
the concentration in the precipitation pretty well.
The chloride content decreases gradually and con-
stantly towards these areas which low content, and it
is not probable that the precipitation has completely
lost all its chloride when reaching them. In the precip-
itation shadows the potential evapotranspiration may
exceed the precipitation, so that a considerable con-
centration could indeed be expected in the groundwa-
ter. There is then hardly any space left for more than
0.5-1 ppm chloride dissolved from the bedrock and it
could easily be less.
In general the chloride content diminishes with
the distance from the sea and with higher altitudes.
The inflow of geothermal water, increases the chloride
content locally, and even regionally. This could be the
case in the Kverkfjöll-Dyngjufjöll area (Fig. 10.), but
there an extremely high ratio of evapotranspiration
could indeed be another potential explanation. The
chloride contents increase with increasing snow/rain
ratio (Matthess, 1973), which again increases withthe
elevation. This could to a certain degree be offset
by the reductive altitude effect. Outside the glaciers
the latter is obviously stronger. It is therefore to be
expected, that the glacial groundwater is relatively low
in chloride content.
JÖKULL, No. 40, 1990 129