PhreeqcUsers Discussion Forum
Reactive Transport => Reactive Transport Modelling => Topic started by: Pak on January 07, 2015, 04:39:26 PM

I have coupled COMSOL with iPHREEQC using Matlab.
So far, the comparison between iPHREEQC and the COMSOL/iPHREEQC has been reasonable when the transport has advection and dispersion. Although, when I use just diffusion (no advection), I am having some problems for the following system:
Solution 0
units mol/kgw
pH 7 charge
Solution 1
units mol/kgw
C 6.470e06
pH 12.468 charge
Cl 7.27e4
Ca 2.028e02
Equilibrium_Phases 1
Calcite 0 130.7e2
TRANSPORT
cells 1
lengths 0.055
shifts 500
time_step 2.5208e3 #other dt= 1260.4, 630.2, 315.1
flow_direction diffusion_only
boundary_conditions constant, closed
dispersivities 0.0
correct_disp true
diffusion_coefficient 3e7
SELECTED_OUTPUT
water true
high_precision true
pH true
time true
totals Ca C Cl
equilibrium_phases Calcite
COMSOL/iPHREEQC gives similar values to iPHREEQC when the used dt is close to the von Neumann criterion. If I decrease the value of the dt, iPHREEQC keeps on giving the same results, but the coupling decrease the amount of Calcite. So, Even though, I still have similar concentration of the master species in the comparison, the amount of the mineral varies a lot.
I was wondering If somebody has had a similar problem or if somebody could give me a hint about where I could look for.
Thank you for your time.

For TRANSPORT, the velocity is the length divided by the time step. if you change dt then you must also change the length of the cells (and the number of cells) to be consistent.
If you have made these changes to refine the TRANSPORT calculations, then I think you have correct results to test your COMSOL/IPhreeqc connection. The trickiest part of the interface is to convert between the transport units (mg/L?) and PHREEQC (mass of each component in solutionH, O, charge, each element). Looks like you are using charge balanced solutions, but it is possible that the transported concentrations are not exactly charge balanced. So check your charge balance.
Another issue may be the solution volume. PHREEQC has approximately 1 liter of water in each cell, so the moles of calcite are relative to ~1 liter of solution. You could have a different representative volume in your COMSOL/IPhreeqc interface, which would produce different calcite mole transfers.

I will check that, thanks a lot.