Jökull - 01.12.1990, Page 136

Figure 15. Regional distribu-
tion of magnesium in
groundwater. Values (in
ppm) corrected for the
marine component.
— Svœðisdreifing magnes-
íums í grunnvatni. Gildi
(í ppm) leiðrétt fyrir haf-
rcenan þátt í úrkomu.
a geothermal activity connected with the central vol-
cano (caldera) in Hofsjökull (Bjömsson, 1988). A
third high seems to be in the Veiðivötn area, also con-
spicuous iri sodium and carbon dioxide, and possibly
also indicating some geothermal or magmatic activity,
hidden below an aquifer of perhaps 100 m thickness
and with a flow of 15 m3/s or more of cold freshwater.
Low contents of magnesium (< 1 ppm) are found in
' the fissure swarms of Síða and Þjórsárver. Geothermal
water is usually very low in magnesium, uniformly
from low temperature fields but also from many high
temperature fields. In some high temperature fields,
and in groundwater in the vicinity of active central vol-
canoes, the magnesium contents may be high, prob-
ably connected with a heavy presence of magmatic
or geothermal gases, increasing the acidity of the wa-
ter (Arnórsson et al., 1983). Relative lows may per-
haps be detected in the Langjökull basins, the upper
reaches of Skjálfandafljót and in the Mælifellssandur
area. They may be indications of glacial groundwater.
All in all, the chemical components regarded indi-
cate perhaps some shallow aquifers in the vicinity of
the glaciers as conduits of glacial groundwater. They
do not reach farther away from the glaciers than 10-30
km, with the exception of the Langjökull - Þingvalla-
vatn basin. In the Eastem volcanic zone the chemical
indications could be masked by the effects of the vol-
canism in the zone. The chloride content could point
to a glacial component in the water in Krákárbotnar
or Suðurárbotnar south of lake Mývatn. That would
not be in disagreement with the geothermal signs in
the chemistry when the position of the central volcano
Dyngjufjöll is taken in account.
Glacial groundwater, originating from Hofsjökull,
marked by geothermal and or volcanic activity, is pos-
sibly present in westem Hofsafrétt. Farther to the
NW the concentration of most components decreases,
probably due to infiltration of local water into the fis-
sure swarms and or aquifers.
These indications are hardly decisive but the co-
incidence of indications from different components
enhances the probability for the presence of glacial
groundwater considerably in the above mentioned
cases. The distinction between a glacial origin and
an altitudinal effect is not always clear. That is indeed
not to be expected from a single group of parameters.
132 JÖKULL, No. 40, 1990