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GWB can't converge problem


JohnLiang
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Hello,
One of our client asked how to solve the following issue (they can't revise the initial parameters or value) and please refer to the attached file:
Solving for initial system.
Loaded:   76 aqueous species,
           70 minerals,
            2 gases,
            0 surface species,
           13 elements,
            0 oxides.
Newton-Raphson did not converge after 999 iterations, maximum residual =    1.13e-15, Xi = 0.0000
Largest residual(s):
                       Resid     Resid/Totmol   Cbasis
---------------------------------------------------------
K+                  5.421e-20     1.13e-15    4.777e-05
---------------------------------------------------------

Please help us.
Thanks a lot.
John

teat.rea

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Hi John,

Can you please explain the conceptual model for this input file? What type of process are you trying to simulate? What is the extent (size or volume) of the system? I'm especially curious about the Al2O3 and Quartz constraints in the Basis. Also, are the constraints for Fe++ and Fe+++ real (both are the same value)?

Thanks,

Brian Farrell
Aqueous Solutions LLC

 

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Hello Brian,
Thanks for your kind reply and support.
Our client wants to do the same work as the attached file.
Please see the paper "page 382" about Geochemical simulation in the granitic aquifer.
But now our client doesn't have the data of Al+++ and SiO2 (also doesn't have data of Al2O3 and H2SiO4), and it can't run without giving value of Al and SiO2.
On the other hand, she only has Total Fe value without Fe and Mn, without value of Fe2+ and Fe3+.
Finally she only knows Al2O3:kaolinite:SiO4:magnetite=10:1:5:1 and thermal database is minteq.
Hope you can help us.
Thanks a lot.
John

2008, Characterization of groundwater quality in Kinmen Island using multivariate analysis and geochemical modelling.pdf

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Hi John,

Thanks for the additional information. I read through the section of the paper entitled “Geochemical simulation in the granitic aquifer”. It’s really not clear from the paper how the model was constructed. It’s especially difficult to check because they only report the end-point values for a few parameters like Eh and pH. I would recommend writing to the corresponding author to ask for a more detailed explanation of the model, along with an input file for the PHREEQC calculations. Perhaps with this information we can help your client get the model up and running.

Regards,

Brian Farrell
Aqueous Solutions LLC

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Hi John, 

I tried to run the model as is. I think one problem is that the Al2O3 mineral that’s swapped into the basis is really unstable at the specified pH conditions. I also can’t understand why the Quartz and Al2O3 are both swapped into the basis and set as simple reactants. I’ve attached my interpretation of the model that is discussed in the paper. It runs fine, but I’m not getting the same answers as the paper, so obviously my interpretation isn’t quite right either. There are a few details that I’m pretty confident in, though. 

I think they are using the water chemistry data from Sample 1. The reported analysis is an average from several sampling periods, but in their modeling they state they use pH and Eh values from the second sampling period, which are 4.44 and 450 mV, respectively. 

They specify the concentration of nitrate (NO3-) as nitrogen (N). In other words, the mass of the oxygens is not counted. You can set concentrations as elemental equivalents in React by clicking the pulldown next to the unit for NO3- and selecting “as -> N”. 

I think all redox coupling reactions should be enabled here (specifically between Fe++ and Fe+++) so that Magnetite can form in response to the specified Eh.

They listed the stability of Kaolinite, Quartz, Magnetite, and Al2O3. The values for Magnetite and Al2O3 were slightly different from those in the thermo dataset, so I modified the 25 C values from the Config -> Alter log Ks dialog.

The original model was done in PHREEQC, which by default does not allow supersaturated minerals to precipitate. You can replicate this by going to Config -> Suppress, changing the “list” pulldown from “All” to “Minerals”, then hitting “suppress all”. Then, unsuppress the minerals they reported (Kaolinite, Quartz, Magnetite, and potentially Al2O3). 

The paper calls the mineral reactants, so instead of swapping them into the Basis, I set them as simple reactants on the Reactants pane. I needed to set a nonzero but trivial amount of H4SiO4 and Al+++ in the Basis pane. 

Hope this helps,
Brian
 

teat-modified.rea

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