Jökull - 01.12.1986, Blaðsíða 21

range of width of 0.1—2.5 m (fig. 9). This allows a
mathematical treatment of the permeability effect of
the fissures, although it must be based on some as-
sumptions: the permeability in a fissure is assumed to
be equal to the geometrical mean value of the
‘permeabilities” in an open channel and in one filled
with coarse gravel: the effect of the distinct fissures is
thought to be evenly distributed over an area (Sig-
urðsson 1985). The calculated permeability values are
us high as 0.15 m/s for some small areas, in the pre-
valent tectonic direction.
The permeability values for the relative classes of
the lithological types have been estimated from areal
tests (long term pumping tests, tidal effects etc.) and
niodel calculations (Sigurðsson 1976, Ingimarsson &
Kjaran 1978, Ingimarsson & Elíasson 1980, Sigurðs-
son 1985). The approximate values for the areal
Permeability are as follows:
1 • Postglacial lavas
2. Fresh interglacial lavas,
pillow lavas
3. Older interglacial basalts,
unaltered hyaloclastites
4- Altered hyaloclastites,
massive basalts
10‘2 - 10"° m/s
10“3 - 10“' m/s
10“4 - 10“2 m/s
10“5 - 10“3 m/s
Based on the assumed lithology of the reservoir
rocks and on the tectonic pattern a hydrogeological
Permeability map of the western part of the Reykjanes
Peninsula has been prepared (Sigurðsson et al. 1978,
Sigurðsson 1985) and used as a base for a hydrogeo-
logical model (Ingimarsson & Elíasson 1980).
Various methods have been used to approach the
lithological type of the buried rocks in the freshwater
layer:
1 ■ The established systematical distribution of the
reservoir rocks.
2- The structure of the buried reservoir rocks may to
some degree be reflected in the landscape on the
lavacovered surface.
3. Geoelectrical resistivity values can be indicative of
the nature of the reservoir rocks.
4- The groundwater level and the groundwater cur-
rents reflect the permeability pattern of the rocks
m the freshwater layer and thus indirectly indicate
the lithological type.
All four methods have yielded similar results,
strengthening the final synthetical interpretation of
the distribution of the rocks in the freshwater layer.
The areas with the highest permeabilities are situ-
ated at the northern coast of the peninsula and in its

Fig. 9. Number and width of fissures on the Reykja-
nes peninsula. The number of fissures of a certain
width are one-sided normally distributed (Sigurðsson
1985). Mynd 9. Samband víddar og fjölda sprungna.
Fjöldi sprungna af ákveðinni vídd er einhliða mál-
dreifður (Freysteinn Sigurðsson 1985).
western part. Especially favourable are tectonic and/
or geomorphological basins filled with scoriaceous
postglacial lavas and dissected by fissure swarms (fig.
10).
Such is the situation in the freshwater supply area
of Reykjavík and its suburbs, in the Straumsvík area,
probably in the Strandarheiði area, (especially in the
west) and in the Lágasvæði area, where Suðurnes
Regional Heating extracts its freshwater (fig. 2).
Southwest and south of that area are some very prom-
ising areas situated near to the coast, the one on the
south coast being the preferred area for the fishfarm-
ing, which has been recently established on the
Reykjanes peninsula. Areas with strong concentra-
tions of coastal springs on the northern coast seem
also to be interesting for exploitation.
GROUNDWATER HYDROLOGY
The groundwater outflow from the Reykjanes pen-
insula is estimated to be 30—70 m3/s, depending on
the definition of the area , distribution of precipitation
and the rate of evapotranspiration. For the western-
most part of the peninsula an outflow of 10—12 m3/s
seems a reasonable estimate. The coastline of that
part is at least 50—60 km long, so that the mean out-
flow should be about 0.2 1/s for 1 m of the coast. The
observed, visible outflow is usually less, and although
it must be taken into account that locally the outflow
is much greater, it is very probable that some outflow
is taking place below sealevel. The extensive lavas
19