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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

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  • $\begingroup$ What type of model do you use? The samples you have are real-world samples -- or from your model? $\endgroup$ – daniel.neumann Nov 15 '17 at 11:05
  • $\begingroup$ they are real groundwater samples i collected. i used default database in phreeqc. $\endgroup$ – chrisuw92 Nov 15 '17 at 11:07
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A geochemical model, like phreeqc calculates the saturation index (SI) for minerals based on the composition of the solution, temperature, and possibly pressure. These are used to calculate the activities of the ions, based on the activity coefficients. These are then compared to the activities that would be present at saturation as defined by the solubility product (Ksp). The saturation index is < 0 if the solution is undersaturated with respect the mineral, = 0 at saturation, and > 0 if supersaturated.

So when you say the model shows you are saturated with respect to gypsum, it indicates that by definition you are in equilibrium with solid phase gypsum. It does not indicate if you got there by dissolving gypsum or by precipitating gypsum. It is highly unlikely (but theoretically possible) that your SI is exactly 0 without one of those things happening.

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  • $\begingroup$ Thank you so much for your answer. It is very helpful. When you say "It does not indicate if you got there by dissolving gypsum or by precipitating gypsum" don't you mean both are happening? Isn't that what maintains equilibrium (SI = 0) with the solid phase? From my understanding, chemical equilibrium with a mineral means both reactions (i.e. dissolution and precipitation) should be happening reversibly, so there is zero net precipitation or net dissolution. $\endgroup$ – chrisuw92 Nov 17 '17 at 6:13
  • $\begingroup$ I was talking about how the solution approaches equilibrium with the solid. You could have a solid containing gypsum and an undersaturated solution so that the gypsum dissolves until saturation is reached. Or you could, for example, have a solution containing calcium and not much sulfate in contact with sulfides that are oxidizing to provide sulfate causing gypsum to precipitate. $\endgroup$ – haresfur Nov 18 '17 at 21:25

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