Notes
The capabilities available through the ADVECTION data block are a simplified version of a more complete formulation of 1D advective-dispersive-reactive transport that is presented by Appelo and Postma (1993) and implemented in the TRANSPORT data block. Calculations using the ADVECTION keyword are sufficient for initial investigations, and in comparison to other problems that include dispersion, the calculations are fast. For many systems with limited data, the kinds of calculations available with ADVECTION are adequate and appropriate. The TRANSPORT data block allows modeling of the additional processes of diffusion, dispersion, and diffusion into stagnant zones. The transport capabilities of the ADVECTION keyword in PHREEQC version 2 are equivalent to the capabilities of the TRANSPORT keyword in PHREEQC version 1.
In the example data block given in this section, a column of five cells ( cells ) is modeled and 5 pore volumes of filling solution are moved through the column ( shifts / cells is 5). Unless kinetic reactions are modeled, no explicit definition of time is required, only the number of shifts. Also, no distance is explicitly specified for advection calculations, only the number of cells.
The
-time_step identifier is required if kinetic reactions (
KINETICS data block) are defined for at least one cell in the column. If kinetic reactions are defined, then an integration is performed for each cell that has kinetic reactions for each advective shift. Kinetic reactions significantly increase the run time of a simulation because the integration of the rates of reaction imposes 1 to 6 (or possibly more) additional batch-reaction calculations for each cell that has kinetic reactions for each advective shift. The total time modeled in the example data block simulation is 25,000 seconds (
).
By default, the composition of the solution, pure-phase assemblage, exchange assemblage, gas phase, solid-solution assemblage, surface assemblage, and kinetic reactants are printed for each cell for each shift. Use of -print_cells and -print_frequency, will limit the data written to the output file. The -print_cells identifier restricts printing in the output file to the specified cells; in the example data block, results for cells 1, 2, 3, and 5 are printed to the output file. The identifier -print_frequency restricts printing in the output file to those advection shifts that are evenly divisible by print_modulus . In the example data block, results are printed to the output file after each integer pore volume (5 shifts). Data written to the output file can be further limited with the keyword PRINT (see -reset false). The USER_PRINT data block can be used to calculate quantities to be printed to the output file.
If the SELECTED_OUTPUT data block has been defined (recommended), then data specified in the SELECTED_OUTPUT and USER_PUNCH data blocks are written to the selected-output file. Use of -punch_cells and -punch_frequency in the ADVECTION data block will limit what is written to the selected-output file. The -punch_cells identifier restricts printing to the selected-output file to the specified cells; in the example data block, results for cells 2, 3, 4, and 5 are printed to the selected-output file. The identifier -punch_frequency restricts printing to the selected-output file to those advection shifts that are evenly divisible by punch_modulus . In the example data block, results are printed to the selected-output file after each integer pore volume (5 shifts). All printing to the selected-output file can be switched on or off through the -selected_output identifier of the keyword PRINT.
Most of the information for advection calculations must be entered with other keywords. This advection calculation assumes that solutions with numbers 0 through 5 have been defined using the SOLUTION or SAVE data blocks. Solution 0 is the infilling solution and solutions 1 through 5 are the initial solutions in the cells of the column. Other reactants may be defined for each of the cells. Pure-phase assemblages may be defined with EQUILIBRIUM_PHASES or SAVE, with the number of the assemblage corresponding to the cell number. Likewise, an exchange assemblage, gas phase, solid-solution assemblage, or surface assemblage can be defined for each cell through EXCHANGE, GAS_PHASE, SOLID_SOLUTIONS, SURFACE, or SAVE data blocks, with the identifying number corresponding to the cell number. Note that ranges of numbers can be used (for example SOLUTION 1-5) to define multiple solutions, pure-phase assemblages, exchange assemblages, gas phases, solid-solution assemblages, or surface assemblages and that SAVE allows a range of numbers to be used.
The REACTION data block can be used to define a stoichiometric reaction that applies to a cell at each shift, with the reaction number corresponding to the cell number. This capability is not very useful because it represents only zero-order kinetics, the reaction rate is constant throughout the advection simulation. The KINETICS data block provides a better definition of time-varying reactions for individual cells.
The MIX keyword can be used with ADVECTION modeling to define simplistic dispersion or lateral inflow to the column. At each shift, solution 0 is moved to cell 1, any stoichiometric reaction or mixing for cell 1 is added, kinetic reactions are integrated while maintaining equilibrium with the contents of cell 1; solution 1 (before mixing and reaction) is moved to cell 2, reaction or mixing for cell 2 is added, kinetic reactions are integrated while maintaining equilibrium with the contents of cell 2; and so on until solution cells-1 is moved to cell cells . The moles of pure phases and kinetic reactants, and the compositions of the exchange assemblage, surface assemblage, and gas phase in each cell are updated with each shift, but only after mixing for the next cell has been accomplished.