Jökull - 01.12.1986, Blaðsíða 13

Hydrogeology and Groundwater
on the Reykjanes Peninsula
FREYSTEINN SIGURÐSSSON
National Energy Authority
Grensásvegi 9, Reykjavík, Iceland
abstract.
There is hardly any surface waterflow on the
Reykjanes peninsula, while from its groundwater 60 %
°f the population of Iceland is supplied, as well as a
number of fishfarms with high water consumption.
The young volcanic rocks of the peninsula, all from
the present Brunhes magnetic epoch, are highly
permeable, so the greater part of the precipitation is
infiltrated and jlows off as groundwater, although the
Precipitation is very high, probably exceeding 3000
mm/y in the highest parts of the peninsula.
The peninsula is the product of the Reykjanes vol-
canic zone, connecting the Mid Atlantic Ridge —
Reykjanes Ridge with the volcanic zones across Ice-
land. This zone stretches WSW-ENE and is in turn
crossed obliquely by volcanic systems, orientated
SW—NE. Each system consists of one or more lava
shields and a number of eruptive fissures, grouped
iogether in narrow volcanic strips. All the rocks are
basaltic. During the glacial phases steepsided moun-
tains of various hyaloclastite facies and tuffs were
formed in subglacial eruptions. The rocks consist of
tuffs, breccias, and pillow lavas and are occasionally
covered by lava caps. During icefree periods, as at pre-
sent, extensive lavafields were been formed, covering
the lowlands on both sides of the mountainous vol-
canic zone, as well as the westernmost part of the pen-
insula. The volcanic systems are accompanied by tec-
tonic fissure swarms of similar extent and orientation.
The permeability is highest in the postglacial,
scoriaceous lavas but is also high in the interglacial
basalts and the pillow lavas, while the finer grained
hyaloclastites are less permeable, especially near to
the high temperature geothermal fields where they are
subject to alteration and their cavities filled with
secondary minerals. The fissure swarms enhance the
Permeability strongly and cause a strong anisotropy in
the directions of the swarm.
In the eastern part of the peninsula the ground-
watertable reaches perhaps 200 m asl., while it lies
only l—2m asl. in the extreme west. There the fresh-
water is floating as a thin layer (50 m or less) on sea
water in the bedrock.The hydrogeological structures
direct the groundwater flow towards spring areas at
the northern coast and the northern border area of the
older rock formations, where the discharge exceeds
1 m3/s in the areas with the highest yield. Smaller
concentrations have also been observed in the western-
most part of the peninsula.
The temperature of the groundwater is usually
3.5—5°C, but it is higher, where influenced by the
geothermal fields. This also affects the chemical com-
position. The salinity of the geothermal water varies
from some few promilles at Krýsuvík and up to a
hypersaline brine in the Reykjanes field. Salt particles
of marine origin carried onto the land by the frequent
and strong southerly winds are another source of
salinity. In the spring areas on the northern border of
the peninsula the chloride content is near to 10 mg/l,
increasing to 20—30 mg/l on the northern coast of the
western peninsula and increasing higher towards the
south coast.
INTRODUCTION
There is no perennial surface runoff on the Reykja-
nes peninsula west of a line Hafnarfjörður — Þorláks-
höfn, except two small brooks in Krýsuvík, and even
these may dry up occasionally (Locations see fig. 4).
This is due to the high permeability of the lava cover
and the bedrock as the precipitation is rather high in
this area, probably exceeding 3000 mm/year in the
mountains in the east. These permeable rocks are all
volcanic and dating from the present Brunhes magnet-
ic epoch, which began 700.000 years ago. The penin-
sula is part of the Neovolcanic zones of Iceland (fig.
1).
The groundwater resources of the Reykjanes-penin-
sula are very important from a national point of view.
The areal extension of the peninsula is 1000—1500
km2, but over half the population of Iceland lives in
the peninsula and its northern border. The city of
Reykjavík and its suburbs have about 130,000 inhabi-
11