Jökull - 01.12.1986, Blaðsíða 22

Fig. 10. Permeable areas on the
Reykjanes peninsula. The highest
permeabilities are found in Postgla-
cial lavas and fissure zones. Mynd 10.
Vatnsgœf svœði. Lekt er mest í
hraunum frá nútíma, einkum ef þau
eru á sprungusvœðum.
Explanations:
1. Permeable areas/vel lek svœði.
2. Fissurezones/sprungusvœði.
3. Strong groundwaters outflow.
4. Surface limits of interglacial ba-
salts/mðrk grágrýtis á yfirborði.
5. Hycloclastite mountains/wó-
bergsfjöll.
HE5Í
0 5 10 15
Fig. 11. Schematic profile of coastal
groundwater at Staður (location see
fig. 10). Tidal waves have a strong
effect on the groundwater in the
highly permeable recent lavas at the
coast. The result is a transition from
freshwater to seawater (Orkustofnun
& Vatnaskil 1986). Mynd 11. Fersk-
vatnslag við strönd. Sjávarfallasveifl-
ur í grunnvatni valda blöndun á
ferskvatni og sjóvatni í hinum vel
leku hraunum við ströndina (Orku-
stofnun & Vatnaskil 1986).
from the lava shields reach down to considerable
depths below sea level at the coast, even 20 m or
more. They may act as confined or semiconfined
aquifers, conducting some overpressured groundwater
out into the sea. Near to the coast some mixing of
groundwater and seawater takes place due to tidal
effects. The increase in salinity is discernible some
hundred metres or more inland from the shore at some
places (Tómasson & Tómasson 1966. Orkustofnun &
Vatnaskil 1984). Under these circumstances the salin-
ity of the groundwater gradually approaches that of the
seawater until there is no measurable difference, where
the water flows out from the rocks into the sea (fig-
11). There is then no difference in the density of the
fluids so that a considerable part of the groundwater
could flow out below sea level in hydrostatic equi-
librium. In the coastal spring areas pure freshwater is
flowing out into the sea, where it must be in floating
equilibrium with the heavier seawater and a large part
of the outflow must take place at or above sealevel, at
least in unconfined aquifers (Sigurðsson 1976).
The freshwater in the western part of the Reykjanes
peninsula is floating on sea water in the rocks like oil
on water.The freshwater layer is relatively thin ( fig-
20