Re: Questions regarding some PHREEQC output

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

I am writing this time regarding PHREEQC rather than PHRQPITZ
> 1.  I have a data set for injection well disposal.  Two aqueous streams
are mixed together and then injected.  In the attached file,
CoolingTowerMixA.out, the calculated errors for solution 2 are rather
large, >20%.  Does a calculated error this large suggest problems?

It's not great. Calcium and alkalinity errors will naturally affect the
SI(calcite) more than errors in sodium and chloride. A beneficial factor,
the SI's are log quantities, so 20% error in calcium translates to about .1
in SI(calcite) (log10(1.2)). However, a .2 error in pH translates to .2
error in SI(calcite).

> I am particularly concerned with the high calculated pH value.  Were this
to be
significantly in error all of the calculated saturation indices would be in
question.

I think the pH is probably about right if the water were to completely
equilibrate with air. You can check, but I don't think the calculated pH is
very sensitive to the analytical data for the initial solution.

> 2. later on in the process I will be equilibrating the mixed stream with
the
existing groundwater, which has a TDS of 34,000 mg/L with chloride 18,000
mg/L and sodium of 11,000 mg/L. Will PHREEQC handle this groundwater?  As
opposed to PHRQPITZ?

The ionic strength is a little high. I'd probably do a little checking with
PHRQPITZ, but I think because it is a sodium chloride dominated water,
PHREEQC and PHRQPITZ will be pretty similar. Also, if you are mixing the
waters, the resulting water will tend to have a lower ionic strength.

> Without documentation for PHRQPITZ PHREEQC is proving a lot easier to
deal with.

I can send the documentation for PHRQPITZ if you forward your address.

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225