Input alkalinity and redox

[mlathouri]

Dear all,

I am writing as I will need your help on something simple, I can guess, but I don't know why I get an error. I have an input PHREEQC file (I use the PHREEEQC Interactive version) with the most common physico-chemical parameters and with alkalinity at 500mg/l as CaCO3, pH 8. I have changed the default concentration at mg/lt and I define the gwf next to alkalinity as Ca0.5(CO3)0.5.

My first question is about the redox choice; the water analysis that I have is from mine groundwater; unfortunately I don't have iron species to calculate pe so I want to ask what could be a typical pe value for mine waters so I can use this as redox.

Also, when I run the speciation with the default pe value, I get an warning message that "Maximum iterations exceeded, 100", "The program has failed to converge to a numerical solution", "pH will be adjusted to obtain desired alkalinity" and "Error: Is non-carbonate alkalinity greater than total alkalinity?"

I would very much appreciate if you could help me on that.

Thank you very much in advance.

Kind regards,
Maria

[dlparkhurst]

As for pe, it may not matter. It only affects a redox element in the SOLUTION definition for which you only have the total concentration. You do not have Fe, so what redox elements do you have for which only the total concentration is known? The pe will be only a guess in any event, and any saturation indices for the affected redox elements will not be reliable.

You do not include your input file, but my guess is that the error message is meaningful. It may be there is enough alkalinity from B, Si, P, etc that the sum of the alkalinity species for these elements exceeds the alkalinity that you have defined. PHREEQC is trying to find an amount of C(4) that balances the Alkalinity definition. In this case it may need to be a negative concentration, which causes PHREEQC to fail. Take out the Alkalinity definition and add -alk to PRINT and look at the sources of alkalinity.

[mlathouri]

Thank you very much for your message.

I have attached the phreeqc file; perhaps it will be of better help for you to check the warning and error.

Thank you.

Maria

[dlparkhurst]

OK, ignore my previous response.

Of pH, C, and Alkalinity, only two can be defined, and the other is calculated. You have defined both Alkalinity and C. In this case, PHREEQC tries to estimate a pH. You have 500 mg/L alkalinity, but only 4 mg/L C. The program fails, but you don't want to do this calculation anyway. This option is almost never used.

I think you should eliminate the definition of C (which seems inconsistent with the Alkalinity) and just use the Alkalinity and pH.

Note that you do have multiple redox states of N, which allows you to calculate 3 pe's. However, the pe only affects the distribution of Mn (and a few saturation indices), which is nearly always Mn(2) anyway.

[mlathouri]

Thank you very much. I followed what you said and when I run the file, I didn't get any errors or warnings.