Jökull - 01.12.1986, Side 8

in the interior highlands to the east (Árnason 1976).
The interpretation of the deuterium data are not dis-
puted as far as the source areas are concerned. How-
ever, the inferred deep circulation of the water from
the recharge areas and to the discharge areas is dis-
putable in the light of available geological data.
Drillhole data show that permeability decreases in
general with age of the flood basalt piles in Iceland
(.Scemundsson and Fridleifsson 1980). By inference to
studies of Walker (1960) in eastern Iceland it is ex-
pected that the decreasing permeability with age is
due to compaction and increasing abundance of
secondary minerals filling vugs and cracks.
In the 4 —5 million years old bedrock in Reykholts-
dalur the primary permeability is in all likelihood
very low as judged from permeability data for drill-
holes in the Tertiary basalt formations in Iceland
(Sœmundsson and Fridleifsson) 1980). Therefore,
ground water flow would be expected to be concen-
trated in fractures of secondary permeability. Re-
charge from the northeast along the NE-striking frac-
tures is, therefore, likely as suggested by Georgsson et
al. (1984). There may also be contribution to the re-
charge along pores of primary permeability at rela-
tively shallow depths.
Unconsolidated post-glacial sediments cover the
Tertiary bedrock in the westernmost part of the Reyk-
holtsdalur field and the area to the west. Rivers deriv-
ing most of their water from the highlands meander
over these sediments. The rivers are likely to contri-
bute water to the ground water body soaking the sedi-
ments. Infiltration of such water into the bedrock
could contribute to the recharge of the geothermal
system, especially if permeable fractures underly the
sediments as the ground water body in the latter will
act as an infinite reservoir to the fractures.
Cold ground water flowing along fissures into the
northern end of Lake Thingvallavatn represents pre-
cipitation which has fallen as far north as Langjökull
some 40 km away (Árnason 1976). Water in cold
springs emerging on the lower slopes of the mountains
immediately north of the Upper-Árnessýsla geother-
mal system originates in southern Langjökull and the
deuterium content of the geothermal water is similar
to that of the cold springs or higher (Árnason 1976).
There is, therefore, no need to explain the deuterium
content of the hot springs by deep circulation from the
recharge area.
Permeability data and temperature distribution in
some drilled low-temperature fields in Iceland indi-
cate that convection is sustained by the pressure dif-
ference exerted by the hot water column of the upflow
and the denser and colder water column in the down-
flow zone (Björnsson 1980a). High flow rates from hot
springs in Upper-Árnessýsla are indicative of good
permeability. The rocks in the area are Upper-
Quaternary and would, therefore, be expected to have
some primary permeability. Together with informa-
tion on subsurface temperatures, this favours that the
Upper-Árnessýsla geothermal system is of the convec-
tive type just described. The convection is envisaged
to be concentrated in tectonic fractures but downflow
may occur additionally over a larger area through
pores of primary permeability, still within the geo-
thermal field. The distantly derived ground water
mixes in some instances with local ground water as
indicated by the deuterium data of Árnason (1976).
Such mixing can take place either in the downflow
zones or in the upflow.
EVOLUTION OF GEOTHERMAL SYSTEMS
The low-temperature geothermal activity in Iceland
has most likely not a common origin as assumed by
the model of Einarsson (1942). Some low-temperature
systems seem to have evolved from high-temperature
ones. Others may have developed through convection
in tectonic fractures in otherwise impermeable bed-
rock. Still others may be due to deep circulation of
ground water from the interior highlands towards the
coastal areas.
When classifying geothermal fields in Iceland as
high- and low-temperature Bödvarsson (1961) was
emphasizing economic aspects of exploitation. How-
ever, as pointed out by him, the ciassification found
strong support in the geological settiríg of the two
types of fields. The high-temperature systems had a
volcanic heat source whereas the low-temperature
ones were nonvolcanic.
Alteration mineralogy and measured temperatures
in wells in several geothermal fields in SW-Iceland
show that present-day low-temperature systems are
located within fossil high-temperature systems. It may
be that these low-temperature systems developed from
the high-temperature ones conjuncture with the drift-
ing of the latter out of the volcanic zone. After cess-
ation of emplacement of magma into the roots of a
high-temperature system, the cooling of the hot rock
will depend on time and on the rate at which ground
water circulates through the system. The geothermal
activity around Hveragerði is considered to represent
a high-temperature system which is in an early stage
of cooling down. Just north of Hveragerði maximum
drillhole temperature is 230°C. Temperatures de-
crease towards south and in the southernmost well in
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