Rit (Vísindafélag Íslendinga) - 01.06.1976, Síða 107

The obtained mean values for the coefficient of transmissivity in
each of the three areas are given in Table 9.
The age of the rock formations in all the areas is believed to be
approximately 1 to 2 million years. The rock formations in the
Mosfellssveit area are, however, probably somewhat younger than
those in the other areas, which might well be the reason for the
appreciably higher value obtained for T in Mosfellssveit. The rocks
in all the areas have undergone appreciable hydrothermal altera-
tions.
Present deuterium measurements indicate that the water of the
Laugames area originates in a recharge area approximately 45 km
north-east of Laugames. The altitude difference between the ther-
mal area and the recharge area is approximately 450 m. In the
same way the Mosfellssveit thermal area is believed to receive water
from a recharge area approximately 15 km towards the east, and
the ElliSaár area receives water from a recharge area approxima-
tely 20 km towards the east. The altitude difference between the
thermal area and the recharge area is approximately 300 m, both
in the case of Mosfellssveit and Elliðaár.
Taking these values as reliable and suggesting some values for
the total thickness of the aquifers and for the effective porosity,
Darcy’s law may be used to calculate the mean flow velocity of
the water in the corresponding thermal area. The deepest drill hole
in Laugarnes, 2200 m deep, has been found to receive water from
aquifers near to the bottom. Here we use D = 2000 m as total
thickness of the aquifers in all the areas.
There exist no direct measurements on the effective porosity in
the deep dense basalt. Einarsson (1942) suggested the value neff =
0.02 and we use the same value here.
Table 9 summarizes the results obtained for mean flow velocity
and average age of the water in the above three areas. The table
shows that, in spite of the fact that these areas are located near to
each other, the mean flow velocity and the average age of the
water differ greatly from one area to another.
It is also interesting to compare the calculated age of the ground-
water in the three thermal areas with results obtained from tri-
tium measurements carried out by Theodórsson (personal commu-
nication). The Mosfellssveit thermal waters show a significant in-
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