Context for SI Change

[lourdsa]

Hi there,

I was wondering if anyone has context for changes in SI. I understand where the SI is >0 , the solution is supersaturated with the mineral and is expected to precipitate. Where the SI is < 0, the solution is under saturated with the mineral and the mineral will dissolve. Finally, where the SI equals zero, the mineral is in equilibrium with its ionic constituents.

However, if I have modeled various samples with saturation indices of gypsum to range from -1.28 to -0.06, is it acceptable to say it's indicative to the groundwater being close to equilibrium ?

Does anyone have a scale for SI ? e.g. If I have a mineral with a -200 SI, does that mean it's significantly undersaturated and unlikely to precipitate unless groundwater conditions change drastically.

I'm keen to understand if the magnitude of modeled SI means anything or is it only significant relative to 0 (=equilibrium).

Any advice, comments will be greatly appreciated.

Thank you,
Kind regards,
LA

[dlparkhurst]

First, SIs may have no information at all, say if an SI indicates undersaturation, but the mineral is not present in the system, or SI indicates supersaturation, but the mineral does not form readily (say dolomite). An SI near zero may be due to rapid reaction and maintaining near equilibrium conditions, or it may be fortuitous. So, knowledge of mineralogy and kinetics is important.

The magnitude of the SI is indicative of nearness to equilibrium, but again there are caveats. The more ions in a mineral, the larger the SI tends to be. If CaCO3 is undersatured, CaMg(CO3)2 tends to have twice the SI, just because there are four ions compared to two. There are also more uncertainties in log Ks for some minerals than others, and mineral stability may vary between more and less ordered crystallinity of the same mineral.

Ideally, you have a sequence of samples that indicate a trend, either a consistent change in SI, or constant near-zero SI. For example, gypsum is undersaturated in the recharge area of the Floridan, but SIs increase downgradient and approach equilibrium as the water dissolves more isolated grains of anhydrite. As a result, calcite precipitates and dolomite dissolves in the dedolomitization process, which can be demonstrated as a regional trend.