Jökull - 01.12.1962, Blaðsíða 52

is a most important task in the planning of
water-power projects.
The stable isotopes D and O18. Craig, Boato
and White (1956) have studied the abundance
of the stable isotopes in natural waters. Their
results will be summarized below.
Oceanic water contains the following rnain
components:
Com- Relative Vapour pressure at
ponent abundance Mass 30° C 100° C
h2o ... 1 18 31,5 760 mm Hg
HDO . . .. 1/3230 19 29,4 741
H2018 . .. 1/500 20 31,3 756
When natural water evaporates in a system
in which thermodynamic equilibrium is main-
tained, the concentration of the components in
the vapour and liquid phases is controlled by
the respective vapour pressure. The isotopic
relationship can be derived on the basis of a
Rayleigh distillation-model. The vapour is de-
pleted in the heavier isotopes and the con-
densate is enriched.
An experimental study of the isotopic rela-
tionship in meteoric water has shown that the
natural water cycle of evaporation-conclensation
takes place under equilibrium conditions. The
oceanic precipitation is progressively depleted
in the heavy isotopes as a function of the extent
of precipitation from the local atmosperic re-
servoir.
Two main facts stand out. Firstly, there is
roughly a constant ratio AD/AO18 where A
indicates depletion relative to a given standard.
Therefore, water derived from the meterologi-
cal cycle has a natural label and can be dist-
inguished from any water derived from a source
with a different isotopic relationship. This may
provide a method of detecting juvenile com-
ponents.
Secondly, there is a general decrease in the
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