> 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: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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