Charge imbalance

[Stanislav]

Hi,

I apologize if that's a silly question. I am trying to calculate thermodynamic coefficients, elements speciation and saturation indices for bunch of aqueous samples. The concentrations of major ions does not vary significantly, although charge imbalance jumps from -90% to +90%. I was trying to manually manipulate values of Cl- or Na+ to figure out how sensitive the calculations. I was significantly changing numbers for major cations but the charge imbalance stayed the same. How is that possible? How charge imbalance is calculated in that case?

Thank you for your help,

Stanislaw

[Jia Wang]

Hello Stanislaw,

The program calculates the charge imbalance as average of anion-cation charge sum. GWB9 and later releases, at the request of users, use the calculation presented by Weight, W.D. (2008), Hydrology Field Manual, McGraw Hill, 751 p

{charge balance = 100 * [((sum cations - sum anions) / (sum cations + sum anions)) / 2]}

If this doesn't help, could you attach a couple of example scripts to demonstrate what you are observing?

Best regards,
Jia Wang
Aqueous Solutions LLC

[Stanislav]

I calculated charge imbalance in my samples manually and I've got less than 15% while GWB calculates it to be 96%. I must have something wrong with the data organization. Do maybe can spot where is the problem?

Thank you.

[Jia Wang]

Hello Stanislav,

It is not really possible to say for sure by just looking at your screenshot. The concentrations entered into GSS are bulk concentrations. The charge imbalance and charge imbalance error are calculated with all cation and anion species concentrations after chemical speciation. I suspect that this is the biggest difference between the software's calculation and your manual calculation. To perform a manual calculation of charge imbalance from the speciation result, I would suggest launching a SpecE8 instant for one sample in your GSS spreadsheet. Then run the calculation and calculate the charge imbalance by summing the total electrical equivalents for anions and subtracting it from the total equivalents of cations from concentrations, found under the variable type "Species concentration" in Gtplot. You can export the numerical values by going to Edit -> Copy as... -> Spreadsheet or Tab. I would suggest that you compare one sample by launching a SpecE8 instance and comparing it to your manual calculation.

For more information regarding speciation, please see section 7.2 Equilibrium Models in the GWB Essentials Guide.

Hope this helps,
Jia