Charge Balance issues

[Gregg W. Jones]

Hi Jia - I'm using react to model mineral saturation in groundwater at a contaminated site.   I've entered in the chemical analytes determined from samples collected from a monitor well into the basis. I have attached the react file so you can see the inputs. When I run the simulation, it doesn't run and says residuals too large and shows the largest residuals as Cl-.   

I know that the simulation is balancing charge on Cl-, so I go into the basis, click on Cl- and select do not balance. When I do that, the simulation runs just fine. However, when I check the output, there is a charge balance error of -24.3%.  Here are my questions:

1) Why doesn't the simulation run when I balance on Cl-?

2) When the simulation doesn't run and says, residuals too large, largest residuals as Cl-, what does that mean?

3) Why the large charge balance error? Is it because the simulation is ignoring Cl- because I told it not to balance on Cl-?

4) is the fact that I get a large charge balance error invalidate the results of the simulation in any way?        

W-3B React Run.rea

[Jia Wang]

Hello Gregg,

Your specific example shows a negative charge imbalance, which indicates that there is an excess of anions. Balancing on Cl- means that you are allowing the program to adjust the amount of Cl- in solution to maintain charge balance. The program uses the concentration as a starting point for iterating to a solution but it is not a true constraint on the system’s composition. The program will vary the concentration as needed to charge balance. Given that you are already in excess of anions, can't remove enough of Cl-  to achieve charge balance. If I go back to use the most abundant cation for charge balance (Na+), your script converges with no issues. You can also change your balancing ion to SO4-- and that will converge with no issue. In your calculation with balance off, you can plot the results as a bar chart to see that there's a lack of cations and that there are a lot more SO4-- than that of Cl- in terms of meq/kg. Turning charge balance off asks the program to not enforce charge balance in its speciation calculation. The program will constrain the Cl- component with what you entered.

Whether or not the charge imbalance error indicates an issue is difficult to say. For some, a large charge balancing error may indicate a problem with the fluid's analysis, perhaps there's a missing component that you are not accounting for. Typically, the charge balance species chosen is the species concentration with the least certainty. Cl- is traditionally given in laboratory analysis based on charge balance from other measured components and therefore a common charge balancing species in simulation. You do not have to stick with that if it doesn't make sense for your simulation. Any charged species can be set as the charge balancing species.

You always perform a quick search on charge balancing to see what other users posted questions about regarding this topic or scenarios they have experienced. 

Hope this helps,
Jia Wang
Aqueous Solutions