Jökull - 01.12.1984, Blaðsíða 115

Figure 6. The Borgarfjördur thermal systems.
6. mynd. Jarðhitakerfi í Borgarfirði, myndin sýnir líklegar rennslisleiðir jarðhitavatnsins og skilin milli
jarðhitakerfa.
Not only do the northeasterly faults serve as the
main flow channels for the system and the frac-
tures open the way for the water to the surface
but the associated seismicity of the area keeps
them open, as it counteracts low temperature
zeolitization or sealing which otherwise would
gradually fill the channels.
The flow pattern of the thermal waters of the
five thermal systems of the Upper Borgarfjördur
region and their recharge areas (Fig. 6) can be
inferred from the combined results of the geologi-
cal studies and the regional resistivity survey, in
addition to the deuterium measurements by
Árnason (1976). The Arnarvatnsheidi region is
the probable recharge area for the Reykholt ther-
mal system. There numerous active northeasterly
and easterly faults (Saemundsson 1967) form
favourable geological conditions for percolation
of meteoric water. This is best demonstrated in
the easterly trending Urdhaedir-fault, where
open fissures are observed for several km. The
water reaches 1-3 km depth and driven by the
hydrostatic gradient, it flows laterally for about
50 km towards the lowlands in southwest.
By using natural discharge and a mean base
temperature of 125°C, the thermal power of the
Reykholt system can be estimated about 210
MW. No thermal gradient holes have been dril-
led in the region, but according to Pálmason et al.
(1979) the regional thermal gradient can be
expected to be 100-150°C/km. Using 125°C/km as
an estimate and a thermal conductivity of 1.7 W/
m°C (Pálmason et al. 1979) the energy dissipation
corresponds to a total conduction heat flow of an
area of 1000 km2. However, total thermal drain-
age is impossible to accomplish. Thus it can be
assumed, that the Reykholt thermal system needs
a recharge area that is at least twice that size.
It is of interest to compare these results with
calculations based on time-dependent heat
sources. Bödvarsson (1982) calculated the energy
balance of the Reykholt thermal system based on
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