How can I implement transport through layers with different comp. and porosity?

[Sidol]

Hello!

I would like to modeling transport of rainwater through a set of different layers (sand, soil, clay and concrete) with different thickness, mineral composition and porosity.
I can build the framework of the layers with pore solutions, eqilibrium phases, etc. but in the TRANSPORT data block only one time step can be set up. However the different layers have different hydraulic conductivity result differrent velocity (v) values in time step equation (Δt)A = Δx/v (equation 108 in Users guide to PHREEQC (version 2) (Equations on which the program is based)) . It follows that should be counted with different time step in each layer. How can I implement it in PHREEQC?

Thank You for Your help in advance.

[dlparkhurst]

The time step is the same, so the thicknesses need to vary to simulate different velocities in the different layers. Each cell should have the same volume of water; so, it does not come into the problem definition, but the cross sections of the cells also vary.

[Sidol]

Thank You, it helps a lot.

[Sidol]

Another question: If I would like to use denser grid mesh (shorter cells) in a section within a given layer, will it mean that the shorter cells contain same volume of pore water and solid phases as the longer cells?

[John Mahoney]

You might want to consider using PHAST, as it has a lot more features than the TRANSPORT option in PHREEQC. 

[dlparkhurst]

If you refine part of the mesh, you need to refine the entire mesh such that each cell contains the same volume of water.

I agree with John, you may want to consider PHAST. It has a beautiful interface (Phast4Windows) written by Scott Charlton that allows rapid development of a reactive transport model for saturated flow and constant density. It does not have the restrictions on cell length, cell volume, and time step, although you need to pay more attention to numerical dispersion.

Other simulators that use PHREEQC are PHT3D, VS2DRT (2D variable saturation), OpenGeoSys, and Hydrus, and there are others.