Re: New Phreeqci (interface)

["David L Parkhurst" <dlpark@xxxxxxxx>]

> 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