Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 149

It is interesting to compare the Cl~ and SD data obtained for the
two wells Akranes and Leirá and the hot spring Brautartunga. These
sources, which will be discussed in detail later, are located in SW-
Iceland (see section 17.12). They lie close to a line which can be
drawn south-west from the south-westem part of the Langjökull
glacier. All these sources discharge water with about the same
SD —— — 72.0%0, which is the same as the present-day precipitation
in the southern part of Langjökull and the area west of it. Since
the sources all discharge water with the same SD-value, they are
expected to belong to the same deep groundwater system. The CL
content, however, increases from 40 mg/1 in the hot spring Brautar-
tunga to 3000 mg/1 in the Akranes well, indicating that at Akranes
the groundwater has already entered marine sediments and has
acquired a large amount of salt. Akranes is located on the coast at
a distance of approximately 45 km south-west of Brautartunga. If
we accept the assumption that all three above sources derive water
from the same groundwater system, a depletion in ÖD-value of the
water due to exchange reactions in the ground seems to be of little
importance. The same conclusion can also be drawn from consider-
ation of the results obtained for the previously discussed areas, the
southern-lowland and the Hengill area.
Because of lack of fundamental data on hydrogen exchange
among water and hydrated minerals, it is not possible entirely to
rule out exchange as a factor in determining the ðD-value in water
in contact with such minerals. The available experimental data,
however, suggest that these effects are small.
From the above discussion it seems unlikely that the water in
the Reykjanes thermal brine has changed its ðD-value by reaction
with rocks.
A possible explanation for the low 8D-value in the Reykjanes
brine is that the thermal water is sea water, which to some ex-
tent has mixed with juvenile water. Measurements carried out on
volcanic gases collected during the eruption in Surtsey, in the
years 1964—1967, showed that the vapour released from the erupted
magma has ðD = — 55%0 (Ámason et al. 1968). The oxygen isotope
composition of the magmatic water was not measured but accord-
ing to Craig (1963), juvenile water should have 8018 + 7%0.
The sea water south of Reykjanes has been found to have 8D =
145
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