Processes > Inverse modelling

Preventing Mineral Dissolution/Precipitation

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ztwatson:
Hello,

I am currently trying to do some inverse modeling to determine fractional contributions of fluids to a final solution. However I cannot figure out a way to just mix the solutions and prevent the dissolution or precipitation of minerals. Can anyone help me with a command for this? It seems that PHREEQC accounts for increased NaCl concentrations by just dissolving halite.

Furthermore the solutions that I am working with are supersaturated with carbonate minerals but during their transport time I would like to prevent any precipitation of these minerals.

Any help is welcome!
TIA

-Zach

Tom:
Hi Zach,

Someone else will likely be able to provide a more appropriate response but have you attempted the use of "pre" or "diss" on the phases line of INVERSE_MODELING ?

Or are you attempting to prevent both precipitation and dissolution of the same phase under different conditions?

An extract from an example (PHREEQC manual) is as follows:

INVERSE_MODELING
-phases
 Calcite     force  pre  -1.0   1
 Anhydrite   force  dis  13.5   2

The explanation is:

"dissolve or precipitate--The phase may be constrained only to enter the aqueous phase, " dissolve", or leave the aqueous phase, " precipitate". Any set of initial letters from these two words are sufficient to define a constraint"


See:

http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/html/final-44.html#pgfId-254827

Apologies if I misunderstood your question.

ztwatson:
I am trying to do the inverse modeling based solely on the solutions. I dont want phreeqc to think it can dissolve a mineral to account for the elemental difference in solutions as there is no interaction with any minerals along the flow path. Its a relatively rapid mixing process. Furthermore I dont want anything to precipitate out to account for a change in solution. But its more important to prevent dissolution.

dlparkhurst:
You can enter just the solutions (-solution n1 n2 n3 n4 ...) with no entries for -phases. However, the phases are used by default to determine the elements for which mole balance equations are included. So you get no mole balance equations unless you use -balance to include the major elements (usually Ca, Mg, Na, (K), C, S, and Cl).

It still can be difficult to establish the mixing fraction. If you find no models, try upping the uncertainties a little (-uncertainty). Also include any stable istotope information that you have. 

ztwatson:
Thanks dlparkhurst,

I ran the model as shown in the image. This worked but the uncertainties are pretty high. High enough to where I wonder if the output that I get is even reasonable or realistic.

I only received one solution for the modeling. Ideally this is what you would want. But I still am skeptical about the results.

Any criticisms about how my model is set up?

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