This is what I spent all day working on. It finally came to me after I took a couple hours off. 

Here is the problem statement: "How would you set a up a simulation to track pH and calcite saturation as a function of pressure?" 

There was an example where the two varied with temperature, but pressure is tricky. First, I had to search a cryptic manual for information. In truth, it is decent in terms of manuals. 

I knew, for example, that the increase in pressure should increase diffusion of CO2 into the water, which will affect pH by making it more acidic (lower pH). 

Also, the increase of CO2 in the water will affect the Ionic Strength, which causes Calcite (CaCO3) to dissolve. This is seen by the negative Saturation Index (SI). If SI< 0, dissolution; SI == 0, equilibrium; and SI > 0 precipitation. As you can see, Log (Calcite SI) decreases. As such, Calcite is dissolving. 

I am learning. This will be important when I model CO2 mineralization processes in ex-situ and in-situ.