How to get -totals for SOLUTION_MODIFY?

[dlee05291]

Hi all,

I'm trying to couple COMSOL with PHREEQC. An issue that I have is getting -totals in SOLUTION_MODIFY. In my problem, transport is calculated using COMSOL, and chemical reaction and equilibrium is calculated using PHREEQC. COMSOL uses molarity so that I converted that into molality, and then the difference between current and previous molality is obtained. The mass_water calculated from the previous solution is multiplied to the difference in molality to get the change in the mass of solutes. Then the mass is subtracted from the current mass_water to get new mass_water. Finally, the current molality is multiplied by the new mass_water to obtain -totals for SOLUTION_MODIFY. This is o.k. as far as the flux is low enough. But when the flux become large, significant deviation is observed. Could anyone tell me whether my method is right? If not what is the correct way to get -totals for SOLUTION_MODIFY?

Thank you for your time!
Dooyoul Lee

[dlparkhurst]

There will always be a discrepancy generated by the operator splitting. However, for the first step, I would take the molarity from COMSOL, and multiply by the volume of water (porosity * saturation) to get the moles of elements in the solution.

If there is no reaction, then you should be able to recreate the molarity by using the number of moles in solution (TOTMOL) divided by the solution volume. With some reaction, you will get some deviations that you have to decide how to deal with. Some strategies iterate to account for the differences, some simply ignore it.

[dlee05291]

Dear David,

Thank you for your quick reply. What you have mentioned is much simpler than I did. Thank you so much!

Dooyoul