CXTFIT

["David L Parkhurst" <dlpark@xxxxxxxx>]

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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