I'm sorry, but I don't fully understand the question. Saturation indices, ie log (IAP/K), say whether a given mineral will probably dissolve in water or crystallize. SI can be calculated for each mineral for which analytical data are in the analysis and for which correct thermodymanic data exist. The solubility constant is really a thermodynamic function (K = e ^ -deltaGo / RT, ie function of schift of standard Gibbs energy in reaction). It is only an impression that SIs are preferentially evaluated only for calcite, gypsum and dolomite. Concentrations of SO42-, Alkalinity, Ca2 +, Mg2 + are determined in each surface water. They are also more interesting SI.

Dear Dalibor Matýsek,

many thanks for your answer but I want to know why we are choosing sulfate concentration to evaluate SIs for selected minerals in any studied aquifers as above figure 13

Dear Hussein Badr Ghalib,

I do not know exactly what the author of these figures wanted to document. I do not know this publication. There is no convention for the use of SO4 concentrations in graphs. The dependence between SO4 and SI gypsum is de facto functional, logarithmic. Deviations could be commented on, at high concentrations of SO4 as a common ion effect, perhaps the effect of calcite saturation. The second picture shows that this is basically no dependency. The dolomite SI does not depend on the SO4 concentration.