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Spec8 Calculation Issues


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I am trying to run Spec8 on several water samples.

After running the program the electrical conductivity yielded is much lower than the input EC(experimentally determined)(approx. 30% lower). Can someone tell me why please? Or how the program calculates EC.

Also the saturation indices of gypsum are not what is expected. The samples are expected to be saturated or nearly saturated in terms of gypsum but the indices are giving values indicating sub-saturation. Does anyone know why this might be?

Also we are seeing a large component of the SO4- being allocated to the neutral CaSO4 species. This is unexpected and seems to be affecting the output EC given in Spec8. Could someone clarify the controls on this species within the program?


I have attached a copy of the data set. If anyone is able to help it would be greatly appreciated.


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I would recommend you take a look at conductivity.dat (File - View - conductivity.dat from within SpecE8) and the reference mentioned for explanation of calculating electrical conductivity. As it says, GWB uses the free species concentrations (i.e. Na+, Cl-, Ca++, SO4--) and not ion pairs or complexes in its calculation. The calculation is also temperature dependent, and limited to between 20 and 30 degrees C.


Compared to other minerals which can be formed from the basis, Gypsum is rather close to being saturated. Do you have a reason to expect it to be more saturated than it is? Perhaps you are ignoring the effects of activity coefficients in calculating saturation indices? Ca++ and SO4-- are both divalent ions, so the effect of activity coefficients can be substantial. The stability of minerals like Gypsum and aqueous species like CaSO4(aq) also depend on the logKs for their reactions with the basis species Ca++ and SO4--. It is possible that the CaSO4 is more stable than it should be in the thermodynamic dataset. Altering the logK for that species, or suppressing it entirely, will give you a slightly higher conductivity (more of the Ca++ and SO4-- components are distributed among the free Ca++ and SO4-- ions) and a higher Gypsum saturation. It is not unreasonable that a neutral CaSO4(aq) species exist, however, especially in solutions of higher ionic strength.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC

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