Using GSS with SpecE8 and making a change to a species to apply to each sample chosen before launch

[Michael P]

Hi,

Thanks for you taking a look. I have a dataset that I'd like to really crank down on for mineral saturation indices. But when I highlight several samples in GSS and run SpecE8, none of them converge. With the resultant SpecE8 window that opens I can change/fiddle with Al3+ by setting it to kaolinite mineral phase at a nominal concentration (saw you do that in a youtube video). This actually works to produce a 'converging' result and I see the suite of minerals and their respeicve SI. 

1) I don't know that it is ideal to force Al to a certain mineral phase. These are pore waters from forested volcanic soils, but still...

2) I only see how to make this change in the SINGLE SAMPLE that pops up in SPecE8. There does not seem to be a pre-emptive handling of this Al-preference before the launch from GSS.  And so by this 'method' I would have to change/fiddle with Al for each of my 300+ samples. 

3) Is there another culprit that might be a better solution to my non-convergence?

I would attach a pic of my GSS line headers... to see if you would omit or change anything. 

Thank you!

[Brian Farrell]

I believe that the dissolved Al concentration vs. Kaolinite equilibrium example is intended to demonstrate how alternative conceptual models might be set up in SpecE8, not to indicate a general rule to follow in speciation modeling. For more info, please see section 6.2 Amazon River water in Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text.

It's probably a good idea to first launch SpecE8 for a few of your samples and look at the entire set of calculation results, but if you have a large group of samples that are pretty similar, you're probably better off calculating a few analytes of interest from within GSS. So, instead of going to Analysis > Launch..., try going to Analysis > Calculate with Spece8..., then selecting your analytes of interest. In other words, you might look at the complete distribution of species in solution and the saturation indices for a large set of minerals for a few samples at first, but then when you know what you're looking for (e.g. S.I. of calcite) you can pick and choose to calculate just that analyte for all of your samples, and add it directly to your GSS file.

Either way, you can specify trailer commands (for example, "swap Kaolinite for Al+++", "1 free g Kaolinite") in GSS that will be used for all samples when you launch SpecE8 or calculate values with SpecE8. Please see 3.3.7 Calculated values and 3.5 Launching SpecE8 and React in the GWB Essentials Guide as well as the SpecE8 chapter in the GWB Command Reference.

Regarding convergence, I'd check to make sure your units and constraints are correct, first of all. I'd also check whether settings for the extent of redox equilibrium/disequilibrium to use in the calculation are reasonable (see 7.3 Redox disequilibrium in the GWB Essentials Guide). It can sometimes be helpful to swap in species that are expected to be dominant under the conditions of interest, to help the program converge. Al+++, the default basis species for Al, is likely to be dominant at low pH, but not at high pH values, in which case it might be better to swap in Al(OH)4-, for example. This last option is straightforward in SpecE8, but not GSS, so let us know if you'd like to pursue it further.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC