Jökull - 01.12.1982, Síða 92

STABLE ISOTOPES
IN THE GEOTHERMAL WATER
In Figs. 5a and b are shown the 8 D and 8 18
values measured in the geothennal waters in the
Eyjafjördur region. A few of the 8 D values used are
from Arnason (1976), but the rest is measured in the
same samples as analysed in the present study. A
total of some 40 8 D values is used in this presen-
tation. 8 18 is measured in about 85 samples allow-
ing much more detailed interpretation of the results
than of the 8 D. The 8 D against 8 18 is shown in
Fig. 6. A „best-fit“ line for the points falls near the
meteoric line (Dansgaard 1964) and is displaced as
compared to the best-fit line for precipitation in
Iceland (Amason 1976). Points plotting between
those two lines are considered to have resonable
values not affected by water-rockreaction. The dis-
tinct displacement towards line a (Fig. 6) might be
due to a high age of the water also supported by the
very low values. In view of the sparingly yield of the
springs and the slow recharge in the production
areas this is not unlikely.
The mean 8 D for the regions is -5- 99 ± 4 %o in
springs and drillholes. The only waters which de-
viate significantly from this range are from the
springs in Hólsgerdi and Ytri-Strjúgsá. The water
in the Ytri-Strjúgsá spring can have derived its 8 D
from dilution of the same geothermal water with
local ground-water which has 8 D about -t- 86 %o
(Arnason 1976). The Hólsgerdi water has definitely a
diflerent origin than water in other springs in the
region. 8 18 in the springs in northern part of Eyja-
fjördur is -5- 13.3 ± 0.6 %o and -i- 13.3 ± 0.5 in
samples from springs and drillholes. In southem
part of the region 8 18 is -r 13.3 ± 0.7 %o. Samples
fromEast of the river Eyjafjardará give 8 18 = -r-
13.2 ± 0.5 %o and -h 13.5 ± 0.4 %o West of the
river. Only water from the Hólsgerdi and Ytri-
Strjúgsá fall significantly out of the range of those
means values. The measured 8 18 in two cold
springs from the area was -t- 11.65 %o and -r 12.6
%o in the river Eyjafjardará. The Ytri-Strjúgsá
sample thus could be a mixture of local ground-
water and water of the same 8 18 as the geothermal
water feeding the northern part of the region where-
as the Hólsgerdi water must havean origin different
from the main reservoir. Precipitation with such
isotopic composition falls nowhere in Iceland at
present. Reaction with rocks would not result in
water enriched in the lighter isotopes. The most
likely explanation is thus that the water is older
than ten thousand years (see Arnason 1976).
The much more numerous measurements of8 18
than 8 D permits as mentioned previously a more
detailed interpretation. The mean 8 18 values from
the Ytri-Tjarnir and Laugaland areas Fa.ll within
the range of the mean value for the region. When
separate samples are compared significant differ-
ences in 8 18 values between those two areas app-
ear and by depth in the Laugaland area. Separate
aquifers were sampled in the drillholes both while
they were still free-flowing (in 1977) and after the
waterlevel had been lowered by pumping (in 1979).
Several samples from well heads were analysed
both of free flowing and pumped water sampled
from 1977 to 1981. The results from those analysis
are shown in Fig. 7. In the free flowing wells there is
found a difference of about -s- 1 in the 8 18 in waters
from the main aquifers in Laugaland drillholes on
one side and Ytri-Tjarnir and a drillhole (poorly
productive) at Grísará just north of Botn. Water
from the deepest aq uifer on Laugaland has the same
value ot ð 18 as the two other areas mentioned.
Aquifers between this lowest one and the main
producing zone have values in between. Samples
taken in the deepest drillhole after the water level
had been lowered by pumping have intermediate
8 18 values, indicating circulation within the drill-
hole and little inflow. The samples of free flowing
Fig. 6. 8 D against 8 18 for geothermal water in
Eyjafjördur. Line a represents the meteoric line
(Dansgárd 1964). Line b represents the “best-fit”
line for precipitation in Iceland (Arnason 1976).
Mynd 6. Tvívetnishlutfall (8 D) á móti súrefnisísotópa-
hlutfalli (8 18) í jarðhitavatni í Eyjafirði. Lína a er
tengilína fyrir regnvatn á norðurhveli jarðar og lína b er
tengilína fyrir regnvatn á Islandi.
88 JÖKULL 32. ÁR