Jökull - 01.12.1985, Page 53

The Vellir thermal Field in Borgarfjörður,
West Iceland
LÚDVÍK S. GEORGSSON
GUDMUNDUR INGI HARALDSSON
National Energy Authority,
Grensásvegur 9,
108 Reykjavík, Iceland
HAUKUR JÓHANNESSON
Icelandic Museum of Natural History,
P.O. Box 5320,
105 Reykjavík, Iceland
EINAR GUNNLAUGSSON
Reykjavík Municipal District Heating Service,
Grensásvegur 1,
108 Reykjavík, lceland
ABSTRACT
The Vellir thermal field in the Reykholtsdalur valley
belongs to the Reykholt thermal system. The hot springs
are distributed along two lines. The main thermal activity
follows the 1.5 km long Vellir line, which has a general
trend near N15°E. The other line is divided into two short
northwesterly trending segments on each side of the
valley, the Sturlu-Reykir and Hagahús lines. The Vellir
geyser is the largest hot spring. All significant hot springs
are close to boiling and the accumulated discharge is 33l/s.
The base temperature of the field is about 130°C.
Geological and geophysical evidence suggests that the
regional flow towards the field is controlled by at least
two northeasterly trending faults. They are intersected by
open northerly trending fractures. The major hot springs
are located at or close to the intersections.
INTRODUCTION
The Borgarfjördur thermal area in the valleys of
Upper Borgarfjördur is the largest low temperature
thermal area in Iceland. The total natural discharge is
equivalent to about 450 1/s of boiling water (Georgsson
et al. 1981a). The Borgarfjördur thermal area has been
divided into five separate thermal systems mainly on the
basis of the chemistry of the thermal fluid and the
results of a resistivity survey (Gunnlaugsson 1980,
Jóhannesson et al. 1980), which have been named after
their centres: Reykholt, Baer, Brautartunga, England
and Húsafell. The Reykholt thermal system is by far the
largest system and includes numerous thermal fields
along with numerous minor hot and warm springs. The
hot springs are usually distributed along lines.
In recent years the Geothermal Division of the Natio-
nal Energy Authority (NEA) has conducted a syste-
matic mapping of the hot springs in Borgarfjördur along
with geological, geophysical and geochemical studies of
the thermal systems. This includes studies on a regional
scale and detailed studies on individual thermal fields
and systems. An overview of the results on the
Reykholt thermal system was presented by Georgsson et
al. (1984). The central part is the Reykholt —
Kópareykir thermal field in the Reykholtsdalur valley
(Fig. 1) where the base temperature exceeds 140°C. The
temperature decreases in all directions from the centre.
The thermal water originates as a precipitation on the
Arnarvatnsheidi highlands. The water percolates down
to 1-3 km depth, and flows laterally for about 50 km
towards southwest driven by the hydrostatic gradient.
JÖKULL 35. ÁR 51