Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 118

Obviously the lake receives water partly from the mountainous area
around it and partly from groundwater streams originating in the
Langjökull glacier.
The river Brúará (No. 12), located farthest towards the east in
Fig. 28, originates from a number of cold springs at the edge of
an extensive lava field. South-west of Brúará there are also numer-
ous powerful springs, Ljósuár (No. 13) emerging from the side of
a tuff mountain. The deuterium content of Brúará 8D = — 75.5%0
and Ljósuár 8D = — 72.9%0 is much lower than in local ground-
water (8D = — 64%0), but similar to the precipitation deposited on
the south-eastern part of Langjökull. Consequently the 8D-values
of these two sources point to a glacial origin of the water, although
the water has lost all the usual glacial characteristics.
The groimdwater flow patterns presented by arrows in Fig. 28
are based on the deuterium results obtained as well as on the topo-
graphy of the area. The main groundwater divides, shown in the
figure as lines of open circles, are proposed on the basis of un-
published geological studies carried out by G. Sigbjamarson (per-
sonal communication).
The results obtained from deuterium measurements and tlie geo-
logical studies are obviously in excellent agreement. Fig. 28 shows
clearly how the main groundwater stream from Langjökull is split
by the mountains north-east of the lake Þingvallavatn and flows,
partly towards the south-west along the main volcanic rift zone,
until it discharges into the lake Þingvallavatn, and partly towards
the south, where it discharges in the springs Ljósuár and the river
Brúará.
14. 5 The Ódáðahraun — Mývatn — Kelduhverfi drainage area.
The geographical location of this area is shown in Fig. 27.
Fig. 29 shows the location of water sources from which samples
have been collected and the resulting öD-values. The area, which
includes most of the northern part of the neo-volcanic rift zone, is
covered by a postglacial lava, extending nearly unbroken from the
glacier Vatnajökull in the south to the lake Mývatn in the north,
and limited to the west and east by the rivers Skjálfandafljót and
Jökulsá. Except for the caldera lake Öskjuvatn in the mountainous
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