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CO2 calculations

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I just finished the remote workshop.  Previous problems with GSS were corrected. (0.5 mg/L Al in a pH 5.7 sample blew up the GW calculations.  I had to change to 0.09 mg/L Al to get it to run.)   I now have a working data set which is what I am asking about.

My interest in the course involves carbonate chemistry at pH 5-7.  I currently have a very simple pH/alkalinity algorithm that is used to estimate CO2 atm for samples.  I also have a more involved USGS derived spreadsheet calculation (pH, alk, T, conductivity, calculated activity adjustments) that always provides similar results.  I asked a colleague to calc CO2 atm from the data in the GSS file using PHREEQ and the results were also similar to those calculated by me and USGS.  I am calculating CO2 partial pressure in GSS using the "calculate in SpecE8" option (+analyte).  GW is providing very different CO2 results.  As the conversion from GW's bar units to PHREEQ's atm unit is negligible, unit conversion does not appear to be problem.

I used the same "+analyte" option to add "carbonate alkalinity."  In my mine drainage world, this term is used to measure the sum of buffering capacity (samples with pH 4.5 - 8).  As alkalinity in this pH range is 99% HCO3, I estimated the HCO3 from this value and used it for the GSS input (HCO3- = field alk as CaCO3 * 1.22).  The value calculated by GW for "carbonate alkalinity" does make sense.  Interesting, I can make it return the "correct" carbonate alkalinity value if I input pH 7.  But not at the field pH values of 5-6.

I had GW calculate the charge imbalance error.  The GW result is not what I calculate in a spreadsheet or what PHREEQ calculates.  

I presume that there are units or data entry problems in my part.  Can you please advise?

I have attached the GSS spreadsheet and an excel spreadsheet that includes the GSS output and calculations made by PHREEQ and me.

Bob Hedin


HE sites with CO2 calcs by GW, HE, USGS, PHREEQ.xlsx HE_sites.gss

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Hello Bob,
I think there are a couple of things that might help here. The first being that the activity model for calculation activity and fugacity coefficients are determined in the dataset loaded. The default thermo dataset (thermo.tdat) in the GWB uses the bdot method for activity coefficients and the Tsonopolous method for fugacity coefficient. I think PhreeqC uses another form of the extended debye-huckel equation for aqueous activity coefficients and the Peng-Robinson method for fugacity coefficients. Additionally, you would also want to make sure that the programs are using comparable data for equilibrium constants of various reactions. For the best comparison to your collaborator's results, you can use the TEdit application to import the PhreeqC database he used and also use it for your calculations in the GWB. More about activity and fugacity models in sections 2.3 Thermodynamic datasets, 7.4 Activity coefficients, and 7.5 Gas partial pressure in the GWB Essentials Guide. Examples for importing datasets from PhreeqC to the GWB can be found in section 9.3 of the same guide.
You can specify a measured carbonate alkalinity (+analyte -> Chemical parameters, as long as carbonate alkalinity has not been added as a calculated species) as part of your data entry in GSS. It sounds like this might be better than using an estimate for a bulk HCO3- concentration.
I don't see any temperatures added to any of your samples. GSS assumes a temperature of 25C for the calculation unless you provide a temperature. Is that correct?
Just a reminder that the analytes here are representing a bulk concentration (except pH). I am not sure what you mean by using a different quantity for Al+++. Was the data just entered incorrectly?

Here are a couple of threads from previous users that provides the equation for how the software calculates charge imbalance error and carbonate alkalinity:


Hope this helps,
Jia Wang
Aqueous Solutions LLC
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