Rit (Vísindafélag Íslendinga) - 01.06.1976, Síða 156

ferent deep water systems which to some extent mix as they ap-
proach the snrface.
Assuming that these waters have precipitated under climatic
conditions similar to those of today, the mean recharge area of the
Laugarnes and the N-Reykir deep water should then lie somewhere
in the mountainous area north of the lake Þingvallavatn, approxi-
mately 50 km north-east of Reykjavík. The mean value for the lake
Þingvallavatn is 8D = — 65%0. The Elliðaár and the westerly S-
Reykir water, on the other hand, should have the center of their
recharge area somewhere in the area Mosfellsheiði, probably 25 km
east of Reykjavík.
According to Theodórsson (1967 and personal communications)
the water at S-Reykir already shows increase in its tritium content
since 1967. This increase in tritium shows that the deep water con-
tains significant amount of precipitation deposited after the hydro-
gen-bomb tests started in 1952. Consequently it is reasonable to
assume the thermal water at S-Reykir to he relatively young.
15. 10.4 Seltjarnarnes.
The thermal water recovered from the wells on Seltjarnarnes
S—1 to S-3 (samples No. 106-108) differs from that in the other
water systems in the district both in deuterium and Cl' concentra-
tion. The mean Cl“ concentration is approximately 500 mg/1 and
the mean 8D = — 74.5%0. The low silicate content Si02 ~ 110 mg/1
and other chemical composition of the water does not indicate that
this water has aquired its CL by reaction with rocks at higher tem-
peratures. The ratio mg Na+/mg Cl“ = 0.52 also suggests that most
of the Cl“ is of marine origin.
Whether the water at Seltjarnarnes has acquired its Cl“ by direct
mixing with sea water or by washing out marine sediments, is of
no importance as regards its 8D-value, since in neither case should
it have changed significantly. The much lower 8D-value of the Sel-
tjarnarnes water compared with other water systems in this area,
however, indicates that the deep groundwater system at Seltjarnar-
nes is quite different in origin from other deep groundwater sys-
tems in this area.
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