Dissolution of calcite is a function of the CO2 pressure in the air:
CaCO3 + CO2 + H2O
↔ Ca2+ + 2 HCO3-
for which the law of mass action yields:
K = 10-6.0 =
[Ca2+] [HCO3-]2 /
[PCO2]
This equation can be reworked to:
[Ca2+] = ( 10-6.6
[PCO2] ) (1/3)
Accordingly, the Ca concentration in water in equilibrium with calcite is a parabolic function of the
CO2 pressure.
Mixing of waters with different CO2 pressures results in subsaturation (the mixture lies below the equilibrium curve).
Mixing is the prime explanation for cave formation in karst areas and operates even when seawater (that is supersaturated for calcite)
mixes with fresh water with a higher CO2 pressure. The process accounts for the attractive caves
in the coastal zones of Southern Portugal, Italy, Greece and Mexico.
The PHREEQC input file Example 5.5 calculates the equilibrium Ca concentration
and a mixing line.
First, CO2 is added to distilled water (keyword REACTION) while maintaining equilibrium with
calcite (keyword EQUILIBRIUM_PHASES) to obtain the equilibrium line. The x-axis gives the vol% of CO2 in air,
which is equal to 100 * PCO2 / atm.
Next, 2 specific water compositions on the equilibrium line are calculated and a 1 : 1 mixture of the 2 waters (keyword MIX).
The mixture is subsaturated.