EXCHANGE_MODIFY in COMSOL-PHREEQC coupling

[Jeonghwan Hwang]

Hi,

I am conducting research that links COMSOL and PHREEQC.
Currently, my PHREEQC model includes cation exchange reactions.

At each time step, the concentration changes calculated in PHREEQC are passed to COMSOL, where the chemical species flow is then calculated.
For each time step, I use SOLUTION_MODIFY for the PHREEQC calculations.

From my observations, it seems that considering SOLUTION_MODIFY alone automatically takes the EXCHANGE information into account.
Moreover, since the cation exchange sites do not flow separately, I have not considered EXCHANGE explicitly.

However, the model results exhibit abnormal oscillations during the calculations.
On the other hand, when I include EXCHANGE_MODIFY, the model works well.

Is it necessary to explicitly set EXCHANGE_MODIFY as well?

Thank you
Sincerely,

Jeonghwan Hwang

[dlparkhurst]

SOLUTION_MODIFY only accounts for the dissolved concentrations of elements. Usually, only the dissolved concentrations are transported, and the exchanged species are assumed to be stationary; so, it is sufficient to transport the dissolved concentrations, update the solutions with SOLUTION_MODIFY, and then run reactions to equilibrate the new solution composition in a cell with the exchanger in the cell. No EXCHANGE_MODIFY definition is necessary.

Perhaps your transport solution is introducing oscillations. In 1D, to avoid oscillations using centered-in-space weighting, delta_x/alpha must be less than 2 (alpha is dispersivity), and for centered-in-time, alpha*v*delta_t/delta_x^2 must be less that 1. Try decreasing your time step and grid spacing, or use backward-in-time and upstream-in-space differencing, which will not produce oscillations, but may introduce numerical dispersion.

I think I have suggested before that you should use PhreeqcRM. It is designed to couple geochemical reactions with transport models.

[Yongqiang]

Hi David,

It seems difficult to use PHREEQCRM in MATLAB to connect with COMSOL. The data type is quite different and difficult to handle in MATLAB for PHREEQCRM as an external C library.

Thanks

[dlparkhurst]

There is a C version documented here https://water.usgs.gov/water-resources/software/PHREEQC/documentation/phreeqcrm-html/index.html.

The latest version is available at https://github.com/usgs-coupled/phreeqcrm/releases/.