Alkalinity and saturation indices

[jose981]

Hello,

I am trying to calculate saturation index for mineral gypsum using porewater chemistry data we have. In some of our data, alkalinity was not measured. Will this have impact on calculating saturation index for gypsum (we have calcium data)? The alkalinity we have is also reported as Na2CO3, how can I input this into phreeqc? It just says "alkalinity"

Should also add that the pH in our water is between 7-8.

Thanks

Jose

[dlparkhurst]

The alkalinity will have a couple of effects: it will increase the ionic strength a little, which affects the activity coefficients, and if you are using an ion-association model it will change the calcium activity by formation of ion pairs (Pitzer will have an effect throught the specific interaction terms). But the effects probably are not very large, especially considering that SI is a log quantity.

I haven't seen alkalinity reported as Na2CO3, but if it is you need to use the equivalent weight, so you could enter the following.

SOLUTION
Alkalinity  100 mg/L as Na(CO3)0.5