# Gypsum saturation index

I did some geochemical modelling and found groundwater samples I have are saturated with respect to the mineral gypsum. I need some help with interpretation, more information is given below.

The groundwater comes from an engineered aquifer that is comprised of sand. Prior to construction, the sand was in contact with water containing high sulfate and calcium concentrations (dissolved gypsum). There was also some pyrite oxidation occurring which undoubtedly lead to increased sulfate concentrations.

Does my geochemical modelling work give evidence that the groundwater dissolved $Ca$ and $SO_4$ concentrations are in equilibrium with solid phase gypsum? meaning the mineral is present even though it was completely dissolved initially?

thanks

• What type of model do you use? The samples you have are real-world samples -- or from your model? – daniel.heydebreck Nov 15 '17 at 11:05
• they are real groundwater samples i collected. i used default database in phreeqc. – chrisuw92 Nov 15 '17 at 11:07