Jökull - 01.12.1967, Page 75

The i'ollowing geological conditions favour
sea-water origin of the cliloride in the water
of the aquifers mentioned above: In the Vest-
mann Islands and at Húsavík the water issues
from thick sedimentary formations that are
partly marine. The wells at Seltjarnarnes and
Akranes are situated on the enter part of
headlands, so it seems quite conceivable that
sea-water has percolated into the ground water
system. Selfoss and Sölvholt are both situated
some 12 kms from the present sea-level. The
wells cut massive non-marine sedimentary
formations, but the area subsided below sea-
level during finiglacial time so the strata have
possibly been impregnated with sea-water and
the non-marine sediments acted as filters re-
taining the salt as in the case of the marine
sediments. Comparable explanation could fit
with Leirá, but the geological conditions are
much less known.
This hypothesis of seawater origin of the
chloride content of some hot aqifers does not
apply to the hot springs in general, where the
temperature is beneath 150° C and the chloride
content is of the same order of magnitude as
that of the precipitation.
The magnesium content lias decreasecl in all
the samples comparecl with sea-water and in
most of them almost disappeared, so most of
the points plot very close to the Na-Ga line
(Fig. 10). The magnesium has most likely been
absorbed into clay minerals (montmorillonite)
and chlorite minerals, which are abundant in
both basalts and sediments. Magnesium has
been lost from the Vestmann Islands water to
the least extent, the primary cause being higher
concentration of clissolved material, the com-
paratively low age of the sediment ancl the
relatively low temperature of the water.
In most of the samples calcium has increas-
ed to the same extent as magnesium has de-
creased, but in some of them sodium has also
decreased when calcium has increased. Sodium
rich zeolites found in the sediments possibly
account for the loss of sodium from solution
relative to calcium. When calcium increases
ancl sodium decreases the sodium/chloride ra-
tio decreases relative to sea-water (Table II).
TABLE II
Analyses of samples of tlie discharge of the welf in Vestmann Islands during pumping tests.
Efnagreining sýna úr Vestmannaeyjaholu, meðan dcelt var úr holunni.
Date Sea-water (llankama and
PH Sahama 1950)
SiOo ppm
S04 „ 2649
C1 „ 18980
hco3 .... 140
F „
Ca 400
Mg 1272
Na „ 10556
K „ 380
Total dissolved solids . .. . 34816
SO4/CI 0.139
Ca/CI 0.022
Mg/Cl 0.067
Ca/Mg 0.315
Na/Cl 0.56
N/K 28
Samples from the well in Vestmann Islands
29.4.1964 18.10.1965 1.11.1965 2.11.1965
6.70 7.0 6.45
98 114 104
594 870 1040 855
5770 9220 9540 9220
. 477 551 346
0.3 0.55 0.55
389 928 950 • 512
306 467 349 509
2889 121 4550 4700 4740
11284 18800 19264
0.095 0.095 0.109 0.093
0.069 0.100 0.100 0.055
0.053 0.051 0.037 0.051
1.25 1.98 2.77 1.00
0.50 24 0.49 0.49 0.51
JÖKULL 17. ÁR 309