Rit (Vísindafélag Íslendinga) - 01.06.1976, Qupperneq 143

furthermore a shallow thermal system which derives water from
local precipitation and which is probably heated by steam and gases
ascending from the deeper thermal systems.
Tritium and Cl“ analysis carried out on the thermal water in
the Hengill area are in agreement with the above model (Árna-
son et al. 1969). The uppermost water system contains a consider-
able amount of tritiiun 200 tritium units) but very little Cl“
15 mg/1), indicating that this water is relatively “young”
groundwater and has not been in prolonged contact with rocks. The
deeper water is low in tritium (/-' 2-8 tritium units) but contains
considerable amounts of Cl~ (150 mg/1), which means that this
water has mainly precipitated before the beginning of the hydrogen-
bomb tests and has taken up considerable amounts of chloride.
Ámason et al. (1969) presented a schematic hydrological model
of the Hengill thermal area. This model is, however, not exactly
valid since it assumed that the Nesjavellir and the Hveragerði deep
water was derived from the same groundwater system. At that
time there existed deuterium measurements of only three samples
from Nesjavellir, which were believed to represent the deep water.
But in fact the sampling was improperly performed and the ob-
tained values had to be corrected for isotopic enrichment due to
evaporation. Samples collected later under well-controlled condi-
tions have shown that the deep water at Nesjavellir (ðD = — 71.7%0)
is significantly lower than that in Hveragerði (8r> = — 65.4%0).
Consequently the results obtained from deuterium measurements
point to the fact that the deep water at Nesjavellir and Hveragerði
is derived from two different groundwater systems. The same opi-
nion is expressed in recent geological studies (Björnsson et al. 1974).
The Nesjavellir water has a 8D-value similar to the preeipitation
deposited somewhere in the southernmost part of the Langjökull
glacier, but the 8D of the Hveragerði water is the same as that of the
precipitation in a mountainous area approximately 35 km to the
north-east of Hveragerði. It should also be mentioned, however,
that the 8D-value of the Hveragerði water is the same as that of the
outlet of the lake Þingvallavatn, north of Hengill.
The 8D-value of the water collected at a depth of 160 m in the
drill hole Vindheimar (sample No. 62) is practically the same
(8D = — 70.6%o) as that of the Nesjavellir water. The deep circu-
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