Rit (Vísindafélag Íslendinga) - 01.06.1976, Side 108

crease in tritium content during the years 1963—1970. Since 1970
no tritium measurements have been carried out on the water. The
increase in the tritium content of these aquifers can only be ex-
plained on the assumption that the thermal water contains some
amount of precipitation deposited after the hydrogen-bomb tests
which began in the year 1952.
The thermal water in the Laugarnes and the Elliðaár area, on
the other hand, contained practically no tritium when last mea-
sured in 1970.
The tritium measurements thus support the results obtained in
Table 9, that the Mosfellssveit thermal water is likely to he only
some decades old on the average, but the Laugames and the Ell-
iðaár thermal water is much older.
It is obvious from Table 9, that the permeability of the rocks
can vary greatly from one thermal area to another, even when the
formations are of similar age. Thus it is by no means possible to
consider the above data as representative for the entire country. It is
possible that the permeability of the Tertiary formations in North
Iceland (approximately 2 to 10 million years old) is even less than
that found in the Elliðaár area. By contrast the neo-volcanic rock
formations, which are less than 700 000 years old, are likely to be
much more permeable. According to Pálmason (1971), the thick-
ness of the neo-volcanic formations, which have been classified as
seismic layer 0, ranges between 0—1.0 km. Thus it is reasonable
to assume that the deep thermal aquifers are mainly associated with
the seismic layers 1 and 2, which are found to consist of Tertiary
flood basalt.
Since there exist no other measurements of the permeability of
the deep rock formations in Iceland, we will assrnne the transmis-
sivity coefficient of the Tertiary formations to be generally within
the limits 2 • 10“3 to 20 • 10'3 m2/sec and from that derive the pos-
sible age of various groundwater systems in Iceland.
Assume that the thermal groundwater usually flows underground
from the central parts of the country towards the coastal areas,
where it discharges from the hot springs in the lowland. The alti-
tude difference would be approximately 700 m on the average. In
the case of the thermal areas in Húnavatnssýsla, for instance, (see
section 15.13) the average distance between the thermal areas and
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