Rit (Vísindafélag Íslendinga) - 01.06.1976, Page 150

+ 1.5%0 and 8018 = +0.2%0 (Ólafsson 1974). Thus a mixture of
60% juvenile water and 40% sea water would give the same 8D-
value as the thermal brine, but the 8018 of this mixture would be
+3.5%0 or possibly higher when an additional oxygen isotope ex-
change has occurred between water and rocks in contact. The actu-
ally measured value in the brine is 8018 = — 0.52%0. We therefore
rule out the possibility of an appreciable amount of juvenile water
in the brine.
If, in accordance with our working hypothesis, we assiune that
the brine water has not changed the 8D-value by isotopic exchange
with rocks, this leads to a very simple hydrological model. It tums
out that the Reykjanes water is a mixture of sea water and fresh
water.
The deep fresh groundwater flow found in the wells in Krísuvík
continues towards the west along the peninsula and possibly farther
westwards under the bed of the ocean, where it more and more
mixes with sea water derived from the ocean. It is likely that a part
of the deep groundwater that flows from the inner and upper parts
of the country can continue underground to some distance from
the coast, where it finally emerges into the ocean. 8D-measurements
of the water collected in the 1565 m deep well in Vestmannaeyjar
(see section 15.4) and of thermal water that emerges in some islands
in Breiðafjörður (see section 15.12) support this.
The Reykjanes area is situated on the North-Atlantic rift zone.
Earthquakes are frequently observed in the zone, indicating that
the rocks may be strongly fractured and thus allow mixing of fresh
water with sea water in the ground.
Our final model will then be as follows: The fresh deep ground-
water flow found in the wells in Krísuvík apparently continues
farther towards the west, along the peninsula. On its passage, the
groundwater mixes more and more with sea water, which descends
into the grormd through faults and reaches depths where the fresh
aquifers are found. In the well H-6 at Krísuvík, the thermal water
contains only 6% sea water but at Svartsengi and Reykjanes the
sea water is 50% and 53% respectively. Thus wells drilled into the
bottom of the ocean somewhere south-west of the Reykjanes penin-
sula might be expected to deliver thermal water of pure oceanic
origin.
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