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Specifying HCO3- with the alkalinity = <value> command

Tom Meuzelaar

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[admin note: this post represents recovered forum content after a critical hard drive failure - it is not in it's original form]



Apr 6 2007, 02:23 PM

I'm trying to use SpecE8 to model multiple data sets that include a bulk acid neutralizing capacity that happens to be reported in mg/kg as CaCO3. I want to minimize assumptions about carbonate speciation before the model is run.


I'm using a Tcl script to read in and speciate several data sets using the MINTEQ thermodynamic database. Prior to reading any data, the script swaps HCO3- in for CO3--, which is the only carbonate basis species in the MINTEQ database. I do not include a column for HCO3- concentrations, but of course there's a column called alkalnity with values in mg/kg (as CaCO3). Within an elseif command, when the script recognizes "alkalinity" in the column header, the code is supposed to execute the following command:


alkalinity = x mg/kg


where x is the current alkalinity value. The problem is that I get the following error:


"Cannot apply alkalinity constraint because cannot find CO2(aq) or H2CO3."


This is unexpected, because neither CO2(aq) nor H2CO3 are part of the MINTEQ database, either as basis species or aqueous complexes. Why did this happen?


When I click on the Basis tab of the SpecE8 window, I see that HCO3- has been swapped in and is defined as the last alkalinity value in the input data file, so it appears the script is executing the swap and alkalinity commands. But, none of the output files are populated with the model results (they are created). I tested the order in which the swap, data entry, and alkalinity commands can be made and found that, at the SpecE8 command line, the following order worked:


1. swap HCO3- for CO3--

2. alkalinity = x


which is how it's set up in the script, which I've attached with a simple input data file. Any ideas why this isn't working?



Attached File(s)

gwb_tcl_script.txt ( 2.06k ) Number of downloads: 5

input_data.txt ( 180bytes ) Number of downloads: 2



Apr 9 2007, 08:25 AM

Hi Aaron:


The alkalinity feature works only when HCO3- is in the basis. It cannot be used directly with the Minteq database which

does not follow the convention. To use this dataset you would need to calculate carbonate concentration externally, in your script.


Hope that helps,



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