Jökull - 01.12.1985, Blaðsíða 54

Fig. 1. The Upper Borgarfjörður regi-
on and the major thermal fields of the
Reykholt and Baer thermal systems.
1. mynd. Yfirlitskort af ofanverðum
Borgarfirði og helstu hverasvæði í
Reykholtsdal og nágrenni.
The main aquifers appear to be northeasterly trending
faults and dykes. In the lowlands they are intersected by
open northwesterly to northerly trending fractures
which enable the hot water to flow to the surface.
The Vellir thermal field is a part of the Reykholt
thermal system and is adjacent to the Reykholt —
Kópareykir thermal field, about 2.5 km further west. A
detailed study of this field was requested by the Nature
Conservation Council. The Vellir geyser (also called
Árhver) is the only geyser in West Iceland and one of
very few in Iceland still active and unaffected by human
activity. This together with unusual surroundings has
raised an interest in its conservation. This paper pre-
sents the results of the survey.
SURFACE THERMAL MANIFESTATIONS
The Vellir thermal field comprises numerous hot and
warm springs. The majority of the springs are spread
along the 1.5 km long Vellir line, crossing the
Reykholtsdalur valley from SSW to NNE. In addition
the short NW-SE trending Sturlu-Reykir and Hagahús
lines are located on the northern and southern sides of
the valley, respectively. They may be two segments of
the same line that crosses the valley, intersecting the
Vellir line near the Vellir geyser.
Surface geothermal manifestations were mapped in
detail and the natural discharge of individual springs
measured where possible. The total discharge is about
33 1/s and most of the springs are boiling. A map of all
thermal manifestations is shown in Fig. 2. The tempera-
ture range and discharge of the hot springs is summa-
rized in Table 1 but a short description of the thermal
field is presented below.
The chemical composition of thermal water from four
hot springs is listed in Table 2. The chalcedony geo-
thermometer gives values from 126 to 133°C, indicating
a base temperature of about 130°C. The difference
between values for different hot springs is insignificant.
Only a minor part of the hot water from the Vellir
thermal field is utilized for central heating of farm- and
greenhouses and no wells have been drilled so far.
The Vellir Line
The Vellir line has an overall trend close to N15°E and
its total length is about 1.5 km. The total discharge is
about 27 1/s of mainly boiling water. The Vellir line can
be divided into three separate segments, termed here
the northern, central and southern segments.
The northern „segment" includes only one spot of
warm ground (no. 1 in Fig. 2).
The central segment is about 600 m long and stretches
from the Badlaugahver hot spring (no. 2) in the north to
the Vellir geyser (no. 7) in the south and has a trend
close to N3°E. The springs are in two groups, the
Badlaugahver group with Badlaugahver (also named
Badlaugar) as its only major hot spring and the Vellir
52 JÖKULL 35. ÁR