Jökull - 01.12.1990, Blaðsíða 147

not more than 10 ppm. The temperature of the springs
in summer is less than 2 °C. In this case hardly any
doubts can be about the glacial origin of the main part
of the spring water. The previously derived interpre-
tation of the glacial effect on the chemistry is here
convincingly demonstrated.
CONCLUSIONS
Very little data have been collected explicitly for
the purpose of examining the glacial component of
the great groundwater systems in Iceland. Although
the hydrology of the highland plateau has been given
much more attention than most other aspects of natural
research in the country, due to the importance of hydro
power in the country’s economy, it is still in many
aspects incomplete and even deficient. There is no
doubt that much more complete, more dense and more
comprehensive data on the glacial groundwater can be
collected with reasonable means in a relatively short
time with a proper scientific approach.
Yet, despite this poor state of the basic informa-
tion, it is clear that the glacial component can in quite
many cases be identified, traced and calculated in
the groundwater systems of the highlands, as demon-
strated on the previous pages. At present, it may not
be done with the uttermost certainty, except in very
few cases, but yet in quite a number of cases with a
masonable probability. It is also clear, that the ways
are known to increase this probability. This is only a
question of the execution of the proper investigation.
So far, it has been shown with a fair degree of prob-
ability, that glacial groundwater is present in shallow
aquifers, bordering on or extending under the glaciers.
It may present a major constituent of groundwater
systems stretching more than 10 km away from the
glacier. It is indeed worth to note, how little the
chemical composition seems to change over such long
stretches, despite the, at least theoretically, expected
reactions between the water and the host rocks of the
aquifers. In restricted fissure zones it can be traced,
probably flowing at greater depths, for some tens of
kilometres. An extrapolation of these observations
would lead to the conclusion, that the glacial ground-
water would be able to flow as a major constituent of
a deep, geothermal system for still longer distances,
perhaps up to some hundreds of kilometres. Such
an extrapolation is of course questionable and no ev-
idence for it has indeed been brought to light in the
present discussion.
On the contrary, strong evidence has been brought
into the field, that the regional infiltration in the
shallow, permeable aquifers will exceed the glacial
component already at relatively short distances away
from the glacier. In precipitation shadows, the glacial
groundwater in restricted fissure zones will be much
more slowly diluted, but a final evidence for this is
still lacking.
On highly permeable ground, with a sufficient
supply of precipitation and glacial melt, the infiltra-
tion through the glaciers can be estimated to reach
1,500-2,000 mm/year (Langjökull, westem Vatnajök-
ull). The permeability of the bedrock is elsewhere
the restrictive factor, forcing the glacial melt water
to flow off as surface water. Estimates can be made
of the total groundwater infiltration under the differ-
ent glaciers, although they must still be regarded with
caution: Langjökull 50-80 m3/s, Hofsjökull perhaps
more than 10 m3/s, northwestern Vatnajökull (ground-
water basins of Tungnaá and Jökulsá á Fjöllum) 50-
100 m3/s, northern Mýrdalsjökull perhaps 20-30 m3/s.
The chemistry of the groundwater, all over the
country, is a key to the classification of the glacial
groundwater as well as other groundwater systems.
The available data are still far from sufficient and a
thorough systematic sampling needs to be carried out
vigorously in the near future. A proper appreciation of
the potential of these investigations has so far not been
published, but from the present discussion it should be
obvious, despite the brief and superficial treatment.
The chemistry of the glacial groundwater seems
to be, as might be expected, characterized by low
chemical contents, especially such mineral compo-
nents as are normally dissolved from the host rock
in the groundwater systems. The temperature of the
glacial groundwater is usually below 2 °C, although
such low temperatures may also have other causes.
The isotopic content is probably less sensitive to the
host rock, so that a revision of the deuterium mapping
would be very promising at the present state.
A comprehensive and scrutineous analysis of
JÖKULL, No. 40, 1990 143