Rit (Vísindafélag Íslendinga) - 01.06.1976, Síða 76

as the mean value for the precipitation in the Grimsvötn area. This
is in accordance with the fact that the water discharging in the
burst originates in the central part of Vatnajökull.
The sudden dropping in the discharge on March 24th, which
coincides with the conspicuous ðD-value, is of a special interest,
since it suggests that the tunnels feeding the water from the central
parts of the glacier may have been temporarily hlocked.
The cold springs, as well as the hot springs and drillholes shown
in Fig. 16 and 17, show very small 8D-variations with time. The
rms deviation is even less than in surface streams or similar to the
accuracy of the measurements. Consequently we can say, in accor-
dance with the ahove observation, that a single sample taken from
hot or cold groundwater and from surface streams, except those
streams fed by glacial meltwater, may he used to obtain reliable
information about the mean ðD-value of the respective water source.
This is of a great importance to this work since, as discussed in the
next chapter, 8D-measurements of a single sample of local ground-
water can be used to predict the mean 8D-value of the precipitation
at the place in question.
6. 2 Groundwater and surface streams of local origin.
Table 7 gives the mean 8D-value of the precipitation at the three
meteorological stations, Hveravellir, Vegatunga and Rjúpnahæð,
together with the 8D-values of several cold springs and small rivers
located in the same districts as the meteorological stations. The
cold springs and rivers are all believed to receive their water from
a known recharge area not far from the sampling point, i.e. they
have been evaluated as being of local origin.
Apparently the mean 8D-values of the local groundwater and
surface streams in each district are very similar to the mean 8D-
value of the precipitation in the same area. In case of Rjúpnahæð
and Hveravellir the mean 8D-value of the groundwater and surface
streams is approximately 1%0 higher than that of the precipitation.
At Vegatunga the mean value of the groundwater is 1.8%0 lower
than that of the precipitation.
The rms deviation of the mean for the eight and nine years
measured at Rjúpnahæð and Hveravellir respectively is 1.3%0, and
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