Jökull - 01.12.1986, Side 24

Fig 13. A profile of the groundwater
along the Reykjanes peninsula. The
groundwater table is much higher in
the less permeable hyaloclastite
mountains of the eastern peninsula
than in the highly permeable lava
series in its western part (Sigurðson
1985). Mynd 13. Grunnvatnssnið eft-
ir Reykjanesskaga. Grunnvatnið
stendur miklu hœrra í móbergsfjöll-
unum á austanverðum skaganum,
heldur en í hraununum á honum
vestanverðum (Freysteinn Sigurðs-
son 1985).
ability are orders of magnitude greater than the dif-
ferences in the infiltration/precipitation. The hydro-
geological structures concentrate strong groundwater
currents toward the spring areas on the northern
border of the Reykjanes peninsula (Heiðmörk >3
m3/s; Straumsvík area >5 m3/s; Vatnsleysuvík and
Vogavík, each probably >1 m3/s) (fig. 18). Investi-
gations under progress show also that there is a strong
outflow at the coast around Þorlákshöfn on the south-
east border of the peninsula. Elsewhere along the
coast the outflow seems to be more evenly distributed.
GROUNDWATER QUALITY.
The temperature of the groundwater is dependent
on atmospheric as well as terrestrial factors. The
temperature of the precipitation is variable: snowmelt
water has a temperature close to 0°C, air and pre-
cipitation temperature decreases with altitude, while
the snow/rain ratio increases. A stay in ponds or at
the surface can lead to heating from the sun or adjust-
ment to the air and/or surface temperature. The tem-
perature in the great springs usually remains relatively
constant with time, as it represents mean values
over long periods, (a number of years). Annual
changes are often within 0.5°C. Great variations in
temperature may indicate a local origin of the water.
In the ground the water acquires a higher temperature
due to the local or regional heat flow. The strongest in-
fluence is from the numerous geothermal fields in the
peninsula. The typical temperature of spring water is
3.5—4.5°C (Selvogur, Kaldársel, Straumsvík, Lága-
svæði NW-part, etc.). Lower temperatures are met in
the northernmost parts of the western peninsula,
where snowmelt water may have a disproportionate
effect because of excessive evapotranspiration during
the summer, due to the soil conditions. In the western
part of the peninsula the temperature increases to-
wards the geothermal field of Svartsengi. The same
probably applies to the other geothermal fields. South
and southwest of Svartsengi the groundwater has a
temperature of 6 — 15°C. West of Straumsvík tempera-
tures of 6—8°C have been measured, probably indicat-
ing an off-flow from the geothermal field of Trölla-
dyngja. Temperatures of 5 —6°C have been measured
in Þorlákshöfn and adjacent areas north of there
Similar, or even higher, temperatures are found at the
coast in Hafnir. On the south coast of the western
peninsula freshwater temperatures are 7°C or higher.
The relatively high temperatures there are favourable
for the newly installed fishfarming industry. All tem-
peratures higher than 5°C probably indicate geother-
mal influences, except close to the coast where the
mean annual temperature of the ocean can raise the
temperature of the groundwater up to perhaps 7°C.
The primary source of the chemical content of the
freshwater is the chemistry of the precipitation, which
has both marine and atmospheric factors (fig. 14).
Most of the chloride in the freshwater is thought to be
of marine origin, where no geothermal influx is pre-
sent. A small part of the chloride may be traced to
industrial smoke from both sides of the northern
Atlantic, like some considerable part of the sulphate
in the precipitation. Other elements and chemical
compounds accompany the marine chloride in some-
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