Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 103

12. THE AGE OF THE GROUNDWATEKS
As discussed previously the stable isotope content of the precipi-
tation has not changed greatly during the past 8000 years. The
ðD — value of groundwaters younger than 8000 years should there-
fore be comparable to the ðD-values of the deuterium map. Before
this time, however, the isotopic content of the precipitation differs
greatly from that of the present-day precipitation. Comparison of
the deuterium content of the Vatnajökull deep ice core with the
oxygen-18 content of the Camp Century ice core, indicates that the
öD-value of the precipitation deposited in the period 10 000 to 60 000
years ago was — 50%o to — 100%o lower than to-day.
Consequently, if the age of the groundwater is somewhere be-
tween 10 000 and 60 000 years, this should he reflected in an excep-
tionally low ðD-value. This is, however, not the fact, since in all
cases except for one geothermal system, the 8D-value of the ground-
water is within the same limits as that of the precipitation found
somewhere in the country at the present. The geothermal system
with a öD-value lower than any present-day precipitation is the
Húsavík and Hafralækur system with 8D = — 130%0 and 8D =
— 142%0 respectively, located in the northeastern part of the coun-
try near the coast (see section 15. 18.3). A possible explanation of
the unusually low 8D-value of this water might be that this water is
derived from precipitation deposited some time during the ice-age.
In general, however, the deuterium measurements suggest that
groundwaters in Iceland are younger than 8000 years.
The flow velocity of groundwater is likely to vary greatly, rang-
ing from very high velocity in the uppermost postglacial lavas to
a very low velocity in the deep dense Tertiary basalt. Consequently
the groundwater of the various groundwater systems can be quite
different in age. The youngest groundwater will be found in the
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