> My name is Kavita and I am presently finishing off my PhD in geochemistry. I downloaded the new phreeqci graphical interface and ran a test example through. When compared to the results i got from running the same example through wateq4f and netpath, i came upon some unusual errors. Firstly the charge balance, or percent error, was off by more then 8%. As well, there seemed be some errors in carbonate speciation(about 10x smaller). This then affected the IAP's of the calulated phases (mostly related to carbonate). With wateq4f and netpath bicarbonate is entered, however using the interface it looks like you enter carbonate concentrations. What confuses me is that when the input appears C(4) notation is used, which I thought was for bicarbonate. Am i missing something? I entered by bicarbonate concentrations and got errors, should i be entering carbonate concentrations? The aqueous models (aqueous species and log K's) are very similar for Wateq4f, PHREEQC, and Netpath. I suspect it is a question of conversion of the input data. In PHREEQC you can enter C(4), which is total inorganic carbon as would be measured by release of CO2 after acidification, or Alkalinity, which would generally be from an alkalinity titration. C(4) is essentially CO2(aq) + HCO3- + CO3-2 (plus ion pairs). Alkalinity is essentially HCO3- (+ 2CO3- + OH- etc, but these are usually small). You probably have alkalinity in mg/L as HCO3, so the next step is to get PHREEQC to convert the input value to equivalents correctly. The default database phreeqc.dat has 50 g/eq as the default, which corresponds to as CaCO3, which is common in the US, but not a unit I am fond of. To get PHREEQC to convert by using the gram equivalent weight of HCO3, you should use the "as" entry to generate a line that looks like this in the input file: Alkalinity 150 as HCO3 (replace 150 with measured concentration) The program will then convert to equivalents by 150/(61*1000) mg/L/(mg/meq*meq/eq) = eq/L You should compare the calculated alkalinity among wateq4f, phreeqc, and netpath to determine whether you are getting the carbon data in correctly. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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