Jökull - 01.12.1986, Side 23

Fig. 12. Thickness of the freshwater
layer in the western Reykjanes pen-
insula. The freshwater in the western
part of the Reykjanes peninsula
floats in a thin layer on the seawater
in the bedrock, like oil on water (Sig-
urðsson 1985). Mynd 12. Þykkt
ferskvatnslagsins. Á utanverðum
Reykjanesskaga flýtur ferskvatnið á
sjó í berginu, eins og olía á vatni
(Freysteinn Sigurðsson 1985).
12), in Lágasvæði, in the middle of the western penin-
sula, it is only about 50 m thick. Farther east the
rocks in the freshwater layer include more
hyaloclastites and the thickness of the freshwater layer
•s greater (fig. 13). In the interglacial basalts at the
coast north of Lágasvæði the thickness of the fresh-
water layer remains close to 50 m. The Lágasvæði
area is surrounded by less permeable rocks, where the
freshwater layer is thicker, except in the permeable
'passes” leading to southwest and south. (Sigurðsson
et al. 1978, Ingimarsson & Eliasson 1980, Sigurðsson
1985). The present picture of the thickness of the
freshwater layer is drawn from various sources:
1 • Direct measurements in some few boreholes pene-
trating through the freshwater layer.
2. Thickness calculated from the mean elevation of
the groundwater table in a number of boreholes
and the ratio between this elevation and the thick-
ness measured in the above mentioned boreholes.
3- Approximately 100 geoelectrical soundings, that
are very valuable because of their large number,
despite the indirect nature of the method and the
great range of error for single soundings.
4- Relative elevation of the groundwater table as esti-
mated from groundwater currents indicated in the
distribution of water quality parametres (tempera-
ture, chemistry).
The geoelectrical soundings were partly performed
as Part of a survey for the freshwater and partly in a
geothermal survey (Ágústsson & Þórarinsson 1979,
Georgsson 1979, Georgsson & Tulinius 1983). The
interpretations of the thickness of the freshwater layer
with all four methods gave more or less the same
results.
From the eastem part of the peninsula much less
data is available. The water level of the almost 100 m
deep lake Kleifarvatn should represent approximately
the true groundwater table. Kleifarvatn has no surface
outflow and the water table fluctuates around or
below 140 m asl. An elementary survey of the
Straumsvík area indicates that the groundwater table
in the mountains east of Kleifarvatn hardly rises
much above 200 m asl. (Sigurðsson 1976). West of
the lake geothermal fields cause widespread disturb-
ances in the groundwater level. Alteration and infill-
ings of secondary minerals reduce the permeability of
the rocks, while differences in the density of the water,
due to great differences in temperature, result in
anomalous height of the groundwater level. Farther
west the groundwater level probably falls abruptly,
reaching about 3 m asl. at the western termination of
the highlands (fig. 13). The groundwater level is also
low in the lava covered lowlands along the northern
coast of the peninsula. On the whole the knowledge of
the elevation of the groundwatertable in the peninsula
is still fragmentary and the picture of it, as drawn up
here, must still be largely a guesswork.
Elevation of the groundwater table, location of
spring areas and distribution of water quality para-
meters can be used to reconstruct the groundwater
currents (fig. 10). They are hydrogeologically domi-
nated, the differences in the infiltration playing a
secondary role, because the differences in the perme-
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