geothermal resource potential of the safford-san simon basin, arizona

Silica concentration in Figure 24 is in excess of the concentration
predicted by equilibrium at well discharge temperature for solid silica
species, quartz, chalcedony or alpha-cristobalite.
Excess silica is determined
by subtracting pH-corrected silica from the largest predicted concentrations
of the three species that the pH-corrected concentration exceeds.
Silica
disequilibrium generally increases with carbon dioxide concentration.
Samples 24, 25, and 27 may have equilibrated with chalcedony.
These later
waters have low pC0 2
concentrations and high pH.
Waters with high carbon
dioxide concentrations readily attack alumino-silicate minerals, releasing
silica to solution faster than it (dissolved silica) can equilibrate with
quartz or chalcedony.
The dissolved carbon dioxide content of shallow sodium bicarbonate
waters is introduced by meteoric waters whose initial carbon dioxide content
is in equilibrium with the atmosphere.
Because the artesian ground waters
are in contact with gypsum (hence their high sulfate contents) and probably
carbonate minerals such as calcite, their dissolved carbon dioxide content
may be controlled by carbonate equilibria in a system closed to additional
atmospheric carbon dioxide where gypsum and calcite are both available in
sufficient quantities for saturation or near saturation to occur.
Ion exchange
on clay minerals between calcium and sodium may also influence carbonate
equilibria.
Meteoric water recharging shallow aquifers near the Pinaleno Mountain
front has high carbon dioxide content, which attacks alumino-silicate minerals
to form sodium bicarbonate chemistry water and a solid residue (kaolinite).
As the ground water flows deeper and outward into the confined aquifers in the
green clay facies, evaporite minerals in the clay dissolve to transform the
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