Your setup runs KCl into the column for 2 years, which should move the front 1 meter. Then you introduce 0 Cl water for 1 year, which should move the interface between 0 Cl and 50 Cl a distance of .5 m. The velocity you are using is .05/.1 = .5 m/yr. Dispersivity is 0.1 m. If all of these are correct, PHREEQC should be solving the transport correctly. Choosing a constant or flux boundary for the inflow boundary condition will make a difference. In the first transport you are printing results at approximately 0.5 m. The second transport the punch_frequency is greater than the number of shifts. You have not defined SELECTED_OUTPUT, so nothing is written to the selected-output file anyway. Here is a plot for dispersivity of 0.1 m at the end of each transport simulation. I think it looks ok, the large dispersivity has reduced the peak concentration to less than 40 mmol/kgw at the end of the second transport simulation. The flux boundary condition at the lower end may be part of the difference with CXTFIT. It may be simulating a different condition. The peak at the end is asymmetric because the length of time for dispersion for the leading edge is longer than for the trailing edge. (Embedded image moved to file: pic29755.pcx) If you decrease the dispersivity to 0.1, the peak concentration is not diminished. (Embedded image moved to file: pic05445.pcx) I think PHREEQC is doing the problem you are requesting. Check that all of the parameters and the velocity is correct. Perhaps the bottom boundary condition is not consistent with CXTFIT and you should lengthen the column to avoid it. Discretization could also cause differences. Otherwise, I don't see any problem. We have tested PHREEQC against analytical solutions, and the calculation approach the analytical solution as the cell size decreases (see manual). Let me know if you still think that the PHREEQC calculations are incorrect. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled TITLE Flow in column SELECTED_OUTPUT -file disp1.sel -reset false -dist -time -total Cl SOLUTION 0 # 50 mmol/l KCl units mmol/l K 50.0 Cl 50.0 SOLUTION 1-40 # Column without Cl units mmol/l K 0 Cl 0 TRANSPORT -cells 40 -shifts 20 -flow_d forward -timest 3.15e6 -bcon constant flux -length 0.05 -disp 0.1 # -punch_cells 10 -punch_frequency 20 END SOLUTION 0 # Original solution reenters units mmol/l K 0 Cl 0 END TRANSPORT -shifts 10 -punch_frequency 10 # -punch 1-40 END
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