Jökull - 01.12.1990, Side 134

The low-content tongues stretching out from the
glaciers into permeable regions (Fig. 2 and Fig. 12)
could then be indicative of glacial groundwater, but it
is difficult to distinguish them from low-content areas
having other causes (altitude of mountains, precipi-
tation shadows) on the base of this single parameter
alone. The indications from the carbon dioxide con-
tent have already been mentioned, but three more com-
ponents will be discussed briefly: sulphate, sodium
and magnesium. They have all a fairly good correla-
tion to the chloride content in the precipitation, while
e.g. calcium has only a vague correlation to chloride
(Fig. 8). These components have all been corrected for
the marine component by subtracting the local chlo-
ride content in the groundwater multiplied with the
(linear) ratio (correction coefficient) of the component
against chloride in the precipitation. In the following
the coefficients described by Sigurðsson (1985b) are
used.
For the correcture of the marine component of the
sulphate the value 0.11 is used for the ratio SO^- / Cl-.
This is somewhat lower than the ratio in sea water
(0.13), but it would not change the map of the distri-
bution essentially, if the latter were used instead of the
first (Fig. 13). Two main features are obvious in the
distribution: The high sulphate contents in the Eastern
volcanic zone (>4 ppm) and the difference between
the N- and NW- parts of the country (<1 ppm) as
compared to the S- and SE- parts (>2 ppm).
The first feature shows connections to volcanic and
high-temperature geothermal activity. The sulphate-
rich water of river Skaftá is probably responsible for
the high contents in springs from the Eldhraun lavas
(Skaftáreldar-lava). The second one is probably con-
nected with the origin and or path of the air masses
carrying the precipitation onto the country. Whether
this difference has its origin in a different pollution
from industrial smoke or from a different SO^/Cl-
ratio under different meteorological conditions can not
be decided on with certainty, but the sulphate content
seems not to be very closely related to the chloride
content in this case. A pollutive origin seems there-
fore to be more likely. The sulphate content seems
also to decrease with the altitude of the surface.
These features accounted for, there remain some
abnormal lows, all of which are connected with
glaciers: The areasouth ofLangjökull,perhaps the up-
per reaches of Skjálfandafljót, Breiðbakur southwest
from Vatnajökull and the Mælifellsandur area. South
of Langjökull the relevant springs are connected to
fissure swarms. In the Mælifellssandur area the low
contents are also found in some springs which have
definitely no direct connections to the glaciers.
DISTRIBUTION OF CATIONS
The correction coefficient for the Na+/CF ratio
is 0.55 or almost the same as in sea water. Three
main features are obvious in the distribution of sodium
(Fig. 14): The low content (<2.5 ppm) in the Tertiary
regions, the high content (>4 ppm) in the volcanic
zones, inclusive the now inactive Langjökull-Skagi
zone, and the very high values in fissure swarms in the
volcanic zones (Hofsafrétt, Hólmatungur, Kverkfjöll,
Síða, Þjórsárver, >8 ppm) (Fig. 10).
There remain then highs in Brúaröræfi (low precip-
itation, springs from fissure swarms predominating)
and in the southern highlands (possibly connected with
central volcanoes and or high temperature fields, even
with transitional alkaline affinities (for the petrology
of the area, see Jakobsson, 1979), eventually extinct
or dormant). Some lows are also conspicuous, occur-
ring in areas with other known anomalies: SW and
NE from Langjökull, N and SW from westem Vatna-
jökull and N of Mýrdalsjökull. In all these areas the
groundwater is flowing in relatively shallow aquifers
and could have a glacial origin or at least a strong
glacial component.
The correction coefficient for magnesium is 0.067,
very close to the ratio in sea water. The magnesium
content must also be corrected for an excessive con-
tent of carbon dioxide, the corrective used being-0.2
([CO2]=-30ppm) (Sigurðsson, 1985) (Fig. 15). Two
main features are obvious: A low content in the high-
lands of the Tertiary to Early Quatemary (>1 ppm)
and a high content in the Eastem volcanic zone and
the Skagi-Langjökull zone. Still higher contents (<3
ppm) are found in the fissured geothermal outflow-
zones at Mývatn and Hólmatungur. The fissure zone
of western Hofsafrétt shows similar geochemical char-
acteristics, which possibly indicates the influence of
130 JÖKULL, No. 40, 1990