Jökull - 01.01.2010, Page 178

A. Stefánsson
In Figure 8 the results of the model calculations
are compared with natural low-temperature waters
in Iceland (Stefánsson and Arnórsson, 1999). The
natural water temperature ranged from 20 to 150◦C
whereas the calculations were carried out at 100◦C
with initial 1, 10 and 100 mmol/kg CO2. In the case of
Si, however, the isothermal model calculations do not
mimic the observed natural concentrations as in the
case for CO2, Na and Ca, for example. This demon-
strated that extent of reaction is not the predominant
factor in determining Si concentration whereas CO2,
Na and Ca seem to be largely influenced by the pH
and extent of reaction.
The effect of temperature on the same reactions
is shown in Figure 9 for a system initially containing
10 mmol/kg CO2. Interestingly, the reaction condi-
tions controlled by a pH <7 (high acid supply or low
reaction progress) show very different pH values com-
pared with the natural waters, whereas the model re-
sults of low acid supply or extensive reactions com-
pare well. This suggests that the natural waters in-
cluded are characterised either by low acid supply or
Figure 8. The relationship between the logarithm of the activity of dissolved H4SiO4, CO2, Na+/H+ and
Ca2+/(H+)2 as a function of pH. The lines represent the results of the model calculations at 100◦C and 1,
10 and 100 mmol/kg initial CO2 concentrations whereas the dots show natural geothermal waters in Iceland.
– Samband virkni á uppleystu H4SiO4, CO2, Na+/H+ and Ca2+/(H+)2 við pH-gildi vatnsins. Ferlarnir sýna
niðurstöðu líkanreikninga á samspili bergs og vatns við 100◦C og 1, 10 og 100 mmol/kg CO2 og punktarnir
sýna náttúrulegt jarðhitavatn á Íslandi.
178 JÖKULL No. 60