Jökull - 01.12.1984, Side 107

Geothermal Exploration of the Reykholt Thermal
System in Borgarfjördur, West Iceland.
LÚDVÍK S. GEORGSSON (1) HAUKUR JÓHANNESSON (2)
EINAR GUNNLAUGSSON (3)
GUDMUNDUR INGI HARALDSSON (1)
(1) National Energy Authority,
Grensásvegur 9
108 Reykjavík, Iceland.
(2) Icelandic Museum of Natural History,
P.O. Box 5320,
105 Reykjavík, Iceland.
(3) Reykjavík Municipal District Heating Service
Drápuhlíd 14,
105 Reykjavík, lceland.
ABSTRACT
The Reykholt thermal system covers an area of
250-300 km2 in the valleys of the Upper Borgar-
fjördur region. The natural discharge is equivalent
to about 400 l/s of boiling water. It is the largest
low-temperature thermal system in Iceland. It
comprises all the largest thermal fields in Borg-
arfjördur, incl. the Deildartunga-Klepp-
járnsreykir, Hurdarbak-Sídumúli and Vellir
fields. The highest base temperature is at the
Reykholt-Kópareykir thermal field where it
exceeds 140PC. The temperature decreases in all
directions from the centre.
The thermal water is of meteoric origin and has
fallen as precipitation on the Arnarvatnsheidi
highlands. Itpercolates down to a depth of 1-3 km
and is heated by regional heat flow. It flows
laterally for about 50 km to southwest, driven by
the hydrostatic gradient. The main aquifers appear
to be permeable northeasterly faults and occa-
sionally dykes. They are intersected in the low-
lands by open northwesterly to northerly trending
fractures which enable the hot water to flow to the
surface.
INTRODUCTION
The Borgarfjördur thermal region in the val-
leys of Upper Borgarfjördur (Fig. 1) is the largest
low temperature thermal region in Iceland; the
second being the South Iceland thermal region
(Saemundsson and Fridleifsson 1980). These two
regions are adjacent to the Reykjanes-Langjökull
axial rift zone bordering its western and eastern
margin, respectively. The natural discharge of
the hot springs in the Borgarfjördur region is
estimated to be equivalent to about 450 1/s of
boiling water.
The Borgarfjördur thermal region has been
divided into five separate thermal systems mainly
on the basis of the results of a resistivity survey
and the chemistry of the thermal water (Jóhan-
nesson et al. 1980, Gunnlaugsson 1980). These
systems have been named after their centres:
Reykholt, Baer, Brautartunga, England and
Húsafell. Each system comprises a few to numer-
ous thermal springs or spring clusters. The hot
springs are often distributed along lines. One
thermal field may include more than one line of
hot springs. The Reykholt thermal system is by
far the largest system in Borgarfjördur. It com-
prises many thermal fields along with numerous
minor hot and warm springs.
The thermal activity in the Borgarfjördur
region has in the past attracted the attention of
many scientists, who have tried to explain its
origin and relate the distribution of hot springs to
tectonic structures. Thoroddsen (1891) was the
first to map the hot springs in the Borgarfjördur
region. He suggested that they were aligned
along semicircular faults demarcating the Faxa-
flói bay depression.
Einarsson (1937) mapped the main geological
features around the hot springs in Reykholtsdalur
and suggested that the hot water ascended along
JÖKULL 34. ÁR 105