Re: CCPP calculation using the PHREEQC program

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

> Metropolitan has been using the USGS program to calculate saturation
index
(SI) for our waters.  According to the "Standard Methods for the
Examination
of Water and Wastewater", this program also calculates the calcium
carbonate
precipitation potential (CCPP) which predicts the quantity of calcium
carbonate that may precipitate or dissolve in a given water.  Can we use
the
same program for SI to obtain data for CCPP?

PHREEQC can calculate saturation indices and also calculate the amount of
calcite that would dissolve or precipitate to achieve equilibrium with
calcite.

The following example uses a seawater analysis just because it was handy,
your analyses for potable water would be more dilute, but still could be
supersaturated or undersaturated with calcite. The SOLUTION keyword defines
the water composition and generates a solution speciation and saturation
indices. Calcite SI is 0.76.

The next block with EQUILIBRIUM_PHASES takes the water and precipitates
calcite to equilibrium in a closed system at the same temperature. 0.24
mmol/kg water of calcite precipitate.

The final block equilibrates with calcite in a system open to atmospheric
PCO2 and a temperature of 20 C. In this case, 0.75 mmol/kgw of calcite
precipitate.

You will have to choose which is appropriate, open or closed system, and
whatever temperature variation may occur.

David

TITLE Example 1.--Add uranium and speciate seawater.
SOLUTION 1  SEAWATER FROM NORDSTROM ET AL. (1979)
        units   ppm
        pH      8.22
        pe      8.451
        density 1.023
        temp    25.0
        Ca              412.3
        Mg              1291.8
        Na              10768.0
        K               399.1
        Cl              19353.0
        Alkalinity      141.682 as HCO3
        S(6)            2712.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
      calcite
END
REACTION_TEMPERATURE
      20.
USE solution 1
EQUILIBRIUM_PHASES 1
      calcite
      CO2(g) -3.5
END

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

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled