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different results between GWB and PHREEQC


B_Kimball
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I am trying to estimate dissolved fluoride concentrations assuming a water sample is in equilibrium with both fluorite and calcite. This calculation gives me notably different total fluoride concentrations in GWB (~17 mg/kg F) and PHREEQC (~11 mg/kg F) when I use the wateq4f database for both programs. I have double checked to make sure the log K for fluorite solubility at 25 C is the same in both the GWB and PHREEQC versions of the database. Does anyone know why this discrepancy exists? Relevant files are attached. 

Thanks.

fl_ca_1.txt

fl_ca_1.sp8

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Hi Brynn,

I’m not too familiar with PHREEQC, but it looks like your PHREEQC input is conceptually different from your GWB input. In PHREEQC you’re defining an initial fluid composition (the SOLUTION block) with a pH of 8.43 and .001 mg/l dissolved Ca++ and F-. The minerals aren’t actually present here, though. As far as I can tell, EQUILIBRIUM_PHASES is used to dissolve enough calcite and fluorite into the solution you’ve already defined to achieve equilibrium with those minerals. There are thus two blocks of output, because you’ve set up a reaction path model. One pre-dissolution (note the pH and concentrations of calcium and fluoride equal your constraints), and one post-dissolution (note the pH has changed to 8.069 and the F- concentration to ~11 mg/kg). 

In the GWB, you can find the equilibrium state of a water-rock system directly. You swap minerals into the basis to set them as part of the initial equilibrium system. Since you’re limited thermodynamically to one constraint per basis entry, you set mineral masses (arbitrary here) in place of dissolved calcium and fluoride concentrations. The GWB will find the equilibrium system that honors your initial conditions. In SpecE8 (or React with precipitation disabled), you’ll notice one block of output that satisfies your specified system exactly.

You can certainly set up the same type of titration in GWB that PHREEQC appears to be doing. You’d use React instead of SpecE8, since this is a reaction path. Your basis would have Ca++ and F- set to the dissolved concentrations used in PHREEQC. Then, you’d add calcite and fluorite as simple reactants. When both minerals dissolve and reach equilibrium with your fluid, your results should match what you get in PHREEQC. 

The way you described the problem (a water sample of known composition in equilibrium with fluorite and calcite), the swap procedure in GWB seems closer to what you’re after. It honors the fluid composition you’ve specified. Determining the composition of a known fluid after reaction with fluorite and calcite seems like a different process.

By the way, since CO3—is the master species in your PHREEQC dataset (and the basis species in the equivalent GWB dataset), I don’t think it is necessary to swap HCO3- into the basis in place of CO3-- Your results won’t be affected too much, but I thought the concentration in mg/l you specify in PHREEQC assumes CO3--‘s mole weight.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC
 

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