Rit (Vísindafélag Íslendinga) - 01.06.1976, Blaðsíða 155

ture of 84—86° C. Therefore it can be concluded that the two groups
of wells receive water from two different groundwater systems. The
easterly 86°C water was believed to be derived from a groundwater
system circulating down to a depth of 2.6 km, but the westerly
groundwater system delivering water of 99 °C should circulate to
a correspondingly greater depth. In his studies Einarsson assumed
the geothermal gradient in Iceland to be 1°C per 30 m in general.
More recent studies, however, have shown that the geothermal
gradient varies appreciably within the country. The highest values
are foirnd along the neovolcanic zone ~ 1°C per 5 m. From the
neovolcanic zone the geothermal gradient then decreases, in both
directions, with distance from the zone until it reaches the value of
~ 1°C per 40 m (Pálmason 1974).
Mixing of the two groundwater systems was verified in two wells
close to the boundary line which dehvered water at a temperature
of 90°C.
Since wells at Norður-Reykir, 2.8 km north-east of Syðri-Reykir,
also discharged water at a temperature of approximately 86° C, these
wells should belong to the easterly system.
Geological observation in the district as well as magnetic mea-
surements supported this idea, since they suggested that the two
systems were separated by a dyke located along the boundary line.
The results of deuterium measurements now available are in
excellent agreement with Einarsson’s model.
The wells at S-Reykir located east of the boundary line proposed
by Einarsson (samples No. 111 and 112), and the wells at N-Reykir
(samples No. 113 and 120) discharge water with 8D cz. — 65%0,
which is similar to the water in the Laugarnes area. The wells west
of the line (sample No. 114-117), on the other hand, discharge
water with 8D — — 60%0, which is about the same as the 8D-value
found in the southemmost wells in the Elliðaár area. The wells
located on the boundary line (samples No. 109-111 and No. 118-
119) discharge water ranging in 8D from — 60%o to — 64%0, which
apparently can be explained as a mixture of water from the two
groundwater systems.
The situation in the Elliðaár area, in the wells G-32 and G-33,
and at S-Reykir in the Mosfellssveit area seems to be very similar
in all cases. These places are apparently boundaries for two dif-
151