Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 123

The groundwater flow in the central areas contains water signifi-
cantly lower in deuterium than the local precipitation. The south-
ernmost branch of the system (area b on Fig. 30) shows a fairly
constant ðD-value, 8D = — 81%0 to §D = — 82%0, in the whole area,
while the tritium concentration, in general relatively low, decreases
downstream from 80 tritium units to 14 tritium units. Hence there
must be a large amount of groundwater flowing from the south-
westem part of Vatnajökull in a direction north-west through the
entire area.
The northem part of the central groundwater flow (area c in
Fig. 30) originates in the westem part of Vatnajökull, and drains
mostly via Þórisvatn. The deuterium content of the water does not
change significantly on this course. As the groundwater drains
further south-west it mixes with local groundwater or possibly with
some leakage from lake Þórisvatn, as a result of which it becomes
enriched in deuterium. The groundwater stream can, however, still
be easily distinguished from perched aquifers of local origin, which
contain much more deuterium and tritium. In certain cases, espe-
cially in the area south and west of lake I>órisvatn, springs are
found with relatively high tritium content (more than 100 tritium
units), indicating relatively young groundwater. The deuterium
content (§D = — 77%0) also indicates a local origin of this water.
By comparison with the piezometri surface it turns out that these
springs issue from perched aquifers supported by tillite layers. For
location of these perched aquifers the reader is referred to the paper
of Sigbjamarson et al. (1975).
The third groundwater system appears in the north-westem part
of the area (area d on Fig. 30), along the river Kaldakvísl. This
system has a deuterium concentration ðD = — 77%0 to §D = — 79%0,
indicating a local origin of this water.
The above discussion only concerns the groundwater flow within
the area bounded by the rivers Kaldakvísl, Tungnaá and the glacier
Vatnajökull, the Þórisvatn area. It is clear, however, that such stu-
dies when extended further towards the south and south-west are
of great interest, since they could possibly allow us to trace the
groundwater flow farther towards the lowland. Therefore further
deuterium measurements were carried out on several springs located
farther to the south-west. The location of these springs and the
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