Processes > Inverse modelling

Preventing Mineral Dissolution/Precipitation

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dlparkhurst:
20 percent uncertainty is on the high side, on the other hand you are trying to match at least seven mole balances with essentially just one mixing fraction.

I am a little suspicious of your analyses that have identical pH and alkalinity for all three waters, and some missing data for the brine (Ca, Mg). Alkalinity should be a pretty stable number, but pH can be affected by CO2 degassing before sampling, depending on the water and the sampling procedure. I think you could reasonably add CO2(g) as a phase unless you are absolutely confident in your measuring procedure.

ztwatson:
Well part of my decision to unify the values is so the model actually runs. Changing alkalinity and pH makes it so the model produces no solutions.

Is it possible to remove pH and alkalinity from the solution?

Two of the fluids come from depth (~250 m) and the ending fluid is seen at the surface. Thus the pH and alkalinity are also a function of the pressure regime at depth. This is a study of a CO2 driven geyser, so the system is quite dynamic with respect to CO2, alkalinity and pH. While all of these things change and have some impact on the system the major ions can be considered tracers of the mix which occurs as the fluids migrate up the conduit and produce the eruption at the surface.

dlparkhurst:
You cannot really remove mole balance constraints with PHREEQC. Netpath does allow you to pick your constraints. You can always up the uncertainties of pH, Alkalinity, C to 1.0, but I think you are loosing some information. While pH and TDIC vary with loss of CO2, alkalinity does not, unless there are additional mineral reactions (calcite precipitation most likely). Including CO2(g) as a phase should, in theory account for gain and loss of CO2.  Bottom line is that it is probably difficult to make all the mole balances work out. Perhaps you should just calculate mixing fractions with individual elements, and then see if there is relative agreement.

ztwatson:
Can anyone explain what this error in the output means?

I did not receive any potential models.

dlparkhurst:
The post-mortem printout is sometimes useful and sometimes not. I think here it is not particularly informative, but looks like there is a problem with redox transfomations. However, I ran the file and got a model; so, I am not sure what the difference is between your run and mine. I may have a typo. I did remove aragonite from the list of phases. Adding more than one mineral with the same stoichiometry just generates more models.

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