Jökull - 01.12.1986, Side 25

Fig 14. Origin of the chemical con-
tents of the groundwater on the
Reykjanes peninsula. Storm carried
marine salts, cations and silica from
the bedrock and geothermal influx
are the chief sources of the chemical
contents of the groundwater (Sigurðs-
son 1985). Mynd 14. Uppruni efna i
grunnvatninu. Sœlöður, uppleyst efni
úr berginu og jarðhitapœkill frá há-
hitasvœðunum eru uppistaðan í efna-
innihaldi grunnvatnsins (Freysteinn
Sigurðsson 1985).
what varying ratios, depending on the season and the
climatic conditions. The chloride content in the pre-
cipitation is higher in winter than in summer, and
highest in the wet winter storms. As the storms are
the result of cyclones passing from the southwest and
causing southerly winds, the chloride content is much
higher on the southern side of the Reykjanes penin-
sula than on its northern side. In the spring areas at
the northern border ( Heiðmörk, Straumsvík) the
chloride content is about 10 ppm or less; on the
northern side of the western peninsula it is 20 — 30
Ppm and in the NW-part of the Lágasvæði area it is
40-45 ppm. It increases rapidly towards the Svarts-
engi geothermal field. In the zone of off-flow from
Svartsengi it is 200 ppm CI” or more, due to the in-
fluence of the geothermal water.
Interaction between the rocks and the (cold)
groundwater leads to an overall increase in silica and
the earth-alkaline and alkaline ions. Typical values
for silica are about 15 ppm (table 1). Lower values
seem to be found in water with a “local” character,
perhaps indicating a relatively short stay in the
ground, while notably higher values are interpreted as
indications of a geothermal influence (fig. 15). In
geothermally affected water the variation in the
concentration of the cations is usually large while it
increases gradually and regularly with the chloride
content in the “regional” water.
If the values for the cations are corrected according
to the chloride content, the basic “rock” values are
similiar to those found elsewhere in the Neo-volcanic
zones (Sigurðsson, unpublished preliminary review).
Mixing of freshwater with geothermal fluids leads to
an increase in temperature and total chemical con-
tent. The chloride content in the geothermal fluid of
Svartsengi and Eldvörp corresponds approximately to
a % seawater mixture but in a number of aspects the
chemical composition is different from a diluted mix-
ture of pure seawater (Hauksson 1980, Bjarnason
1984). There is an enormous increase in silica, some
increase in calcium and potassium and a drastic re-
duction in magnesium and sulphate. These differences
make it in many cases possible to distinguish between
heated seawater and true geothermal brine, as a com-
ponent in the freshwater. Both types are found in the
off-flow area from Svartsengi, but heated seawater
seems to be dominant (fig. 15). The freshwater flowing
towards the coast in Grindavík and west of there —
probably all the way west to Reykjanes proper — has
a temperature of 7—10°C and a chloride content of
200 ppm or higher and a composition resembling
diluted seawater. According to various international
standards this water is not potable, but it seems to be
excellent for fishfarming.
At the coast the salinity still increases, due to tidal
effects. The tidal waves can be measured some km in-
land (Ingimarsson & Elíasson 1980, Orkustofnun &
Vatnaskil 1984) and affect the transition layer at the
bottom of the freshwater layer in the western penin-
sula. The amplitude of the tidal waves soon decreases
to some centimetres or even some millimetres with
increasing distance from the shore, but the waves are
still big enough to change the water at a given point at
the bottom of the freshwater layer twice a day. Some
23