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GSS Mineral Saturation problem


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Hi, I am trying to calculate mineral saturation indices from GSS with the YMP thermodynamic database. The calculation fails because the speciation cannot be completed with the following error.


Ionic strength out of range.
Largest residual(s):
                       Resid     Resid/Totmol   Cbasis
 Al+++               2.247e+06            1    4.256e-07


If I run the same in SpecE8 it gives a similar result until I change the Al from Al+++ swapped to AlO2-, then it works. It also works if there is a lower concentration of Al+++. I understand that I am entering a component amount so I am not sure why there would be an error since it is just the total Al for the system. Anyway, attached is the GSS (this just has one water but my sample set is much larger and I would like to be able to plot saturation indices).

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  • 2 weeks later...

Hello Dirk,

You are correct in that you are specifying the total concentration for the component in the basis pane, as long as the free option is not selected. When a concentration is assigned to the component, the program begins to figure out the distribution of mass of species that makes up that component. The program starts with the first guess of the solution assuming that most of the concentration entered for the initial constraint is in the form of the basis species and solves the mass action law for secondary species concentration. The program uses the Newton-Raphson iteration method to solve for the unknown concentration of all species. If the species you choose to represent as the system component is too small, the program might have issues converging on a solution by mass action law. In this case, the predominant Al-bearing species is unlikely to be in Al+++ ion form. Therefore, when the program tries to calculate Al secondary species concentration, the program could not converge on a solution. It would help the program to converge if the aqueous species in the basis is also in an abundant form in the system. If you would like more details regarding the numerical method, please refer to section 4.3.4 Newton-Raphson iteration and 4.3.7 Optimizing the starting guess sections in the Geochemical and Biogeochemical Reaction Modeling text.

If you only have a few samples for calculating the saturation indices, then you might want to consider swapping in AlO2- for Al+++ in SpecE8 for each sample. Alternatively, if you are working with a  large dataset, you might want to create a version of the Yucca Mountain Project dataset such that you exchange Al+++ and AlO2- so that you can enter AlO2- as a basis species in your GSS spreadsheet. A convenient way to do this would be to open the Yucca Mountain Project thermo dataset in TEdit and go to Al+++ in basis species, click the exchange arrows next to the species name and select AlO2-. For more information, please refer to section 9.2.9 Exchanging species in the GWB Essentials User Guide.

Hope this helps,
Jia Wang

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