Transport "Charge" component in phreeqcrm

[Haohan]

Hi,

When coupling with phreeqcrm with our transport model (Advection Diffusion Equation with FEM), I need to transport "Charge" component which can be "negative". This somewhat contradicts the ADE where concentrations should not be negative. How should I deal with the possible negative "Charge" concentrations (and Dirichlet  boundary condition)?

I checked the transport part of PHAST code and I didn't find any special treatment to "Charge" component.

The question https://phreeqcusers.org/index.php?topic=2141.0 is similar to this. But I just need more clarifications.

Thanks

[dlparkhurst]

PHAST simply transports charge whether positive or negative. There is nothing in the transport equation s to prevent transporting a negative values.

If you must, you can add a constant to charge values to make them positive, and then subtract it after transport.

My coauthor implemented PhreeqcRM in FEM without issues.