Introduction

The application of speciation–solubility in geochemistry goes back to Garrels and Thompson (1962), who calculated the aqueous speciation in seawater and saturation states with respect to mineral solubilities. Since then, this subject has been treated extensively, and hundreds of codes are available for this kind of calculation. In principle, the concentration and activity coefficients of all aqueous species can be calculated if there are the same number of equations as unknowns. Given an equilibrium constant for each complex, plus an analysis giving the total quantity of each basis species, this is usually possible.

Speciation-solubility calculations provide a “snapshot” of the (assumed) equilibrium state of a dynamic system. That is, the chemical composition of a water sample is obtained, and assuming that the chemical species in the solution are at mutual equilibrium (homogeneous equilibrium), the concentrations and activities of the various ionic and molecular species present are calculated. Then, using these calculated quantities, the saturation states (i.e., whether superstaurated or undersaturated) of all possible pure solids and gases are calculated, as described in Chapter 3. Here, “all possible” refers to the fact that the range of solids and gases that can be calculated to exist in the system is limited by the chemical components in the water analysis. One cannot infer anything about the saturation state of calcite (or any other mineral containing calcium) if the water has not been analyzed for calcium.