Jökull - 01.12.1990, Síða 128

Figure 5. Chemical evolution of glacial groundwater.
Explanations: SeeFig. 3. —Efnafrœðilegþróunjökla-
jarðvatns. Skýringar: sjá nrynd 3.
PROPERTIES OF THE GLACIAL
GROUNDWATER
Three properties of the groundwater can be mea-
sured with a high degree of accuracy: The tempera-
ture, the chemical content and the content of isotopes.
They are all influenced by the hydrological and geo-
logical conditions the water is subjected to on its path
in the hydrological cycle (Fig. 5).
WATER TEMPERATURE
It is common knowledge that the temperature of
the precipitation and of spring water from shallow,
surficial aquifers decreases with the elevation of the
surface and that the temperature of groundwater in-
creases with the depth below surface. In the lowlands
the temperature of springwater is usually 3.5-50 C, but
on Hofsafrétt and Nýjabæjarfjall, north of Hofsjökull,
at elevations above 800 m a.s.l., the temperature is usu-
ally below 3 °C. Melt water from snow and ice has,
in statu nascendi, a temperature of 0 °C. Sunshine and
air temperature (in summer) may raise the temperature
markedly at the surface, especially in turbidated water.
The low temperature of voluminous springs on Mæli-
fellssandur, north of Mýrdalsjökull (below 1.5-20 C),
at elevations close to 600 m a.s.l., may be indicative
of meltwater from the local, thick hibernal snowfields
or even from the glacier.
Water from permafrost areas (e.g. palsa regions)
has also temperature below 2 °C, e.g. on Hofsafrétt.
In fissure swarms in the highlands springs may have
temperature above 4-5 °C, i.e. 1-3 °C higher than
in springs from shallow, surficial aquifers. This prob-
ably indicates that the water has circulated down to
considerable depths. The thermal gradient is high in
many of the highland regions, so that depths of 100
m or less could in some cases be sufficient for this
increase (Sigurðsson, F. and Torfason 1989). In the
volcanic zones the thermal gradient is on the contrary
often low in the uppermost zone of the bedrock, even
down to considerable depths, most probably due to
cooling from the strong groundwater currents, which
in their turn are caused by the high permeability.
Admixture of geothermal water easily leads to
higher temperature. This is clearly seen in Þjórsár-
ver, southeast of Hofsjökull, and in the narrow fissure
swarms of Odáðahraun north of Vatnajökull (Fig. 2),
where voluminous springs with temperature above
6 °C issue in regions characterized by spring tem-
peratures below 4 °C. The fissure swarms of Odáða-
hraun are connected with central volcanoes and or
high-temperature fields over distances of tens of kilo-
metres.
The factors affecting the temperature can be sum-
marized as follows: High elevation, high ratio of fresh
meltwater and the presence of permafrost lead to lower
temperatures. Deep circulation in fissure swarms and
admixture of geothermal water lead to higher tem-
peratures. Of these factors only the meltwater ratio
is indicative of glacial origin, the others representing
different hydrogeological conditions.
SOURCES OF THE CHEMICAL COMPOSITION
The chemical content of the groundwater has var-
ious sources (Fig. 6). The primary one is that of the
precipitation, some changes may occur at the surface
through evapotranspiration or partial melting of snow,
reactions with the rocks take place in the bedrock and
finally an admixture of geothermal water can change
the composition of the water.
124 JÖKULL, No. 40, 1990