Jökull - 01.12.1987, Page 5

contain several hundreds of ppm of total carbonate.
These waters are high in magnesium compared with the
sodium-chloride type waters and seem to have low Cl/B
ratios. Na/K ratios appear to be similar to those of the
associated rocks and are probably controlled by stochio-
metric dissolution rather than equilibrium with second-
ary minerals. Some of the bicarbonate waters deposit
travertine and approach saturation with amorphous sil-
íca but do not otherwise seem to approach closely equi-
librium with other minerals.
The acid sulphate waters are, like the bicarbonate
waters, low in chloride, and this feature together with
low pH and high sulphate are the distinguishing charac-
teristics. These waters may be very low in sodium and
potassium and have low Na/K ratios. Sulphate is always
the dominant anion and for waters low in sodium and
potassium, hydrogen ion is the dominant cation.
It is considered that the bicarbonate waters tend to
form when steam containing little or no hydrogen sul-
phide mixes with shallow non-thermal water. Such
steam could originate by boiling of water which has
equilibrated at a depth of a few hundred metres (there-
fore of relatively low temperature) or that the steam has
reacted extensively with the rock in the upflow, either
due to slow or long passage, loosing its hydrogen sul-
phide in the process. The acid sulphate waters form, on
the other hand, by condensation of hydrogen sulphide
bearing steam in oxygenated surface water and sub-
sequent oxidation of the sulphide into sulphate, as has
been demonstrated worldwide.
In Hrafntinnusker boiling hot springs occur contain-
ing very low chloride indicating that they are steam
heated surface waters. Yet they are distinctly alkaline,
low in dissolved carbonate and possess many other char-
acteristics of the sodium-chloride waters described be-
low. These waters are considered to be surface waters
heated by secondary steam, which is, therefore, low in
gas.
The chloride concentrations in the sodium-chloride
water in the Landmannalaugar area are variable due to
mixing with cold water in the upflow and steam loss by
boiling (Arnórsson 1985). The highest reported chloride
concentration is 535 ppm (Table 1). This is to be con-
trasted with chloride levels of 10-100 ppm in geothermal
waters in Iceland associated with basaltic rocks (and not
affected by sea-water mixing). Sigvaldason and Óskars-
son (1976) showed that acid rocks are higher in chloride
than basaltic ones. The relatively high chloride concen-
TABLE 1. Major element analysis of selected hot spring waters from the Torfajökull field’ (Concentrations in ppm).
Location Temp °C pH/°C SIO, B Na K Ca Mg Li LCO, so4 LH,S C1 F
1 Eyrarhver3 95 9.96/14 271.8 4.21 413.9 16.1 0.83 0.077 0.228 68.7 26.2 17.5 416.0 25.2
2 Grænagil8 3 Landmanna- 94 9.27/14 207.1 6.14 363.6 17.3 12.3 0.059 0.114 11.4 18.2 4.58 534.7 9.6
laugarb 4 Landmanna- 82 6.10/23 258.4 1.72 252.6 35.3 10.9 2.28 0.604 280.3 70.0 0.07 307.0 7.2
laugarb 24 6.57/13 91.8 0.25 57.6 8.74 6.14 2.23 0.386 19.9 36.2 <0.01 79.3 2.2
5 GrænagiF 45 6.64/14 160.6 1.25 151.6 14.0 15.8 5.17 0.097 99.1 37.7 <0.01 192.0 4.5
6 Gilshver' 75 6.60/75 168.0 0.16 69.3 12.6 8.3 19.4 - 118.9 25.3 <0.1 8.5 0.1
7 Jökulgil' 70 5.90/70 149.0 1.05 116.6 8.40 17.8 5.8 - 403.0 106.2 29.0 19.1 7.2
8 Strútslaugc 9 Hrafntinnu- 67 6.60/67 138.0 1.41 318.0 41.0 4.2 7.3 448.8 40.9 <0.1 30.2 2.0
sker'1 10 Hrafn- 87 9.00/87 234.0 0.32 100.9 7.60 0.50 0.08 ~ 20.6 75.3 8.3 1.0 18.0
tinnuhraune 75 2.60/75 93.0 0.10 6.4 2.70 14.7 7.1 0 41.7 <0.1 3.1 1.2
a Boiled reservoir water. b Mixed Na-Cl water. c Steam heated bicarbonate water.
Steam heated water by low gas steam. ' Steam heated acid shulphate water.
Sample locations are shown in Fig. 1.
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