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Comparison of Spec8 vs React Output

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Hi Jia - I'm interested in determining mineral saturation in shallow groundwater at a well in Colorado.  I entered water-quality data from a well using Spec8 and React, making sure to enter everything exactly the same for both input screens, then compared the saturation indices of the supersaturated minerals for the Spec8 vs React output. While the supersaturated minerals are mostly the same, their saturation indices are different at the higher end and then converge as the saturation index gets lower.   Why might that be and is it something to be concerned about? See below:

                     Spec8                    React

CoFe2O4       29.2                    23.9

Hematite       23.8                     18.5

Jarosite         20.2                     12.2

Ferrite-Mg    13.0                       7.7

Ferrite- Ca   11.4                        6.1

Gothite          11.4                      8.7 

Alunite            7.4                      7.4

Fe(OH)3        6.2                       3.5

Dawsonite     3.4                      3.5

Diaspore        3.4                      3.4

Boehmite     2.9                        2.9

Gibbsite       2.8                        2.8

Corundum   2.6                        2.6

Barite          1.05                       1.0   

Dolomite      0.331                    0.39

Gypsum       0.16                       0.16

Well 3B React_output.txt Well 3B SpecE8_output.txt Spec 8 model Well 3B.sp8 W-3B React.rea

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Hello Gregg,

I compared the two input files attached and I see some slight differences between them. In your SpecE8 input file, the concentration for Fe+++ is set as a free quantity but in React, a bulk concentration is set for the same component. A free quantity sets only the concentration of the the free Fe+++ in solution and the software will calculate additional dissolved concentration of Fe+++ in other aqueous species at equilibrium. Please see section 7.2 Equilibrium models in the GWB Essentials Guide for explanation on free vs bulk constraints.

Another difference is that Cl- is set as the charge balancing ion in the React script, while SpecE8 has charge balancing disabled. In React, Cl- is set as the balancing ion by default when present, you can turn off charge balancing by selecting on the unit and check "Do not balance". Alternatively, you can set Cl- as the charge balancing ion in SpecE8. Please see 2.3 Initial system of the GWB Reaction Modeling guide for more information.

Note that in React, if the initial system contains supersaturated minerals, and the precipitation option is enabled, there will be two blocks of output in your text output file. In the first block, the calculation represents the fluid's metastable state. In the second block, the program continues to calculate the equilibrium state of the system assuming supersaturated minerals were allowed to precipitate. SpecE8 does not calculate mineral precipitation. Therefore, when comparing results between React and SpecE8 using the same input constraints, you should look at the initial block of the React output file.

Hope this helps,
Jia Wang
Aqueous Solutions LLC


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