TITLE Example 3, part A--Calcite equilibrium at log Pco2 = -2.0 and 25C. SOLUTION 1 Pure water pH 7.0 temp 25.0 EQUILIBRIUM_PHASES CO2(g) -2.0 Calcite 0.0 SAVE solution 1 END TITLE Example 3, part B--Definition of seawater. SOLUTION 2 Seawater units ppm pH 8.22 pe 8.451 density 1.023 temp 25.0 Ca 412.3 Mg 1291.8 Na 10768.0 K 399.1 Si 4.28 Cl 19353.0 Alkalinity 141.682 as HCO3 S(6) 2712.0 END TITLE Example 3, part C--Mix 70% ground water, 30% seawater. MIX 1 1 0.7 2 0.3 SAVE solution 3 END TITLE Example 3, part D--Equilibrate mixture with calcite and dolomite. EQUILIBRIUM_PHASES 1 Calcite 0.0 Dolomite 0.0 USE solution 3 END TITLE Example 3, part E--Equilibrate mixture with calcite only. EQUILIBRIUM_PHASES 2 Calcite 0.0 USE solution 3 ENDThe input for part A (table 7) consists of the definition of pure water with SOLUTION input, and the definition of a pure-phase assemblage with EQUILIBRIUM_PHASES input. In the definition of the phases, only a saturation index was given for each phase. Because it was not entered, the amount of each phase defaults to 10.0 mol, which is essentially an unlimited supply for most phases. The reaction is implicitly defined to be the equilibration of the first solution defined in this simulation with the first pure-phase assemblage defined in the simulation. (Explicit definition of reaction entities is done with the USE keyword.) The SAVE keyword instructs the program to save the solution following the final (and only in this example) reaction step as solution number 1. Thus, when the simulation begins, solution number 1 is pure water. After the reaction calculations for the simulation are completed, the composition of the water that is in equilibrium with calcite and CO2 replaces pure water as solution 1.
Part B defines the composition of seawater, which is stored as solution number 2. Part C mixes ground water, solution 1, with seawater, solution 2, in a closed system in which is calculated, not specified. The MIX keyword is used to define the solutions and mixing fractions. The SAVE keyword causes the mixture to be saved as solution number 3. The MIX keyword allows the mixing of an unlimited number of solutions in whatever fractions are specified. The fractions need not sum to 1.0. If the fractions were 7.0 and 3.0 instead of 0.7 and 0.3, the mass of water in the mixture would be approximately 10 kg instead of approximately 1 kg, but the concentrations in the mixture would be the same as in this example. However, during subsequent reactions it would take approximately 10 times more mole transfer to equilibrate with the phases, that is, to produce the same concentrations as in this example.
Part D equilibrates the mixture with calcite and dolomite. The USE keyword specifies that solution number 3, which is the mixture from part C, is to be the solution with which the phases will equilibrate. By defining the phase assemblage with "EQUILIBRIUM_PHASES 1", the phase assemblage replaces the previous assemblage number 1 that was defined in part A. Part E performs a similar calculation to part D, but uses phase assemblage 2, which does not contain dolomite as a reactant.