Jökull - 01.12.1986, Blaðsíða 26

Fig. 15. Chemistry of groundwater in the western
Reykjanes peninsula. The different sources of the
chemical components can be distinguished in the
groundwater chemistry (Sigurðsson 1985). Mynd 15.
Efnainnihald í grunnvatni á vestanverðum Reykja-
nesskaga. Greina má uppruna efna í grunnvatninu
(Freysteinn Sigurðsson 1985).
water is always left behind in the pores and fissures of
the rocks, and will then be mixed with the water at
the other end of the tidal amplitude. In course of time
saline water is “pumped” step by step upwards into
the freshwater, and freshwater the opposite way into
the seawater. It can be seen that the greater the tidal
amplitude , the higher is the salinity of the freshwater
(fig. 16). The salinity increases along the flowpath of
the groundwater as more and more seawater is added.
Other causes for a transition layer at the bottom of the
freshwater layer are the long term changes in the
thickness of the layer, heterogenous flow-paths due to
the geological structure and chemical diffusion. In the
Lágasvæði area the transition layer occupies approxi-
mately 15 m of the more than 50 m thickness of the
freshwater layer. The temperature increases rapidly
downwards in the transition layer compared with the
low temperature gradient usually found in the fresh-
water layer and the uppermost part of the seawater
below it (fig. 11). The seawater is probably almost
stagnant, at least at some distance away from the sea,
while the freshwater is flowing horizontally. The
transition layer has probably an intermediate position
with regard to the flow, the velocity decreasing down-
wards.
Some layering has ben observed in the freshwater in
the western part of the peninsula. The uppermost
5—6 m are commonly influenced by annual or epi-
Fig. 16. Salination of freshwater through tidal effects.
The salinity of the freshwater increases as the flowing
groundwater approaches the coast (Orkustofnun &
Vatnaskil 1986). Mynd 16. Aukning seltu í grunnvatni
vegna sjávarfalla. Seltan eykst, þegar nœr dregur
ströndinni (Orkustofnun & Vatnaskil 1986).
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