Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 71

6. DEUTERIUM CONTENT OF GROUNDWATERS
AND STREAMS
6. 1 Time variation§ of 8D in groundwater and streams.
As discussed previously the stable isotope content of the precipi-
tation varies considerably from one month to another. In constrast
to the precipitation, regular samples collected from hot and cold
springs and from surface streams show much smaller variations
in isotope content with time. This can be seen by considering the
data in Figs. 15, 16 and 17. The figures show the 8D-values of
samples collected from 6 streams, 5 cold springs and 1 hot spring
and 4 hot drill-holes. Some of the samples have been collected
rogularly while others have been collected occasionally. The 8D-
values are plotted as a function of time. The figures also give the
niean 8D-values and the rms deviation (n) together with informa-
tion about the characteristics and the mean discharge of the rivers.
Information about characteristics given by capital letters and mean
discharge are obtained from Rist (1956 and personal communi-
cation).
Consider first the rivers Hólmsá, Grímsá, Brúará and Sog, which
can be taken as representative of all the different river types ex-
cept those containing significant amounts of glacial meltwater.
All these rivers show small time variations (the rms deviation be-
tween 1%0 and 3%0) compared to the monthly variations in the
Precipitation (rms deviation approx. 13%0, see 4.2). Therefore a
single sample taken from such a river is believed to give reliable
information about the mean 8D-value of the river in question.
The river Þjórsá, which is partly fed by glacial meltwater, has
a similar behaviour to the above-mentioned rivers. The rms devia-
tion is 2.2%0.
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