Re: phreeqc question

```
> Margaret here again, from CSM.  I'm still struggling with making
biodegradation work in phreeqc.  The decay equation I'm using is
benzene(final) = benzene(initial)*e^(-rate*time).

If C = C0e^-kt, then dC/dt = -kC, which is linear decay.

> I'm not sure what "TOT"
means (total moles of benzene in the system, moles of aqueous benzene?)

TOT("Benzene") is the total molality of benzene (i.e. mol/kgw, p. 128 of
manual).

> and
what the term "moles" represents and how it is recycled in the transport
function.

For a kinetic reaction, PHREEQC expects to know how many moles are reacted
over a period of time given by TIME, moles = dC/dt*kgw*TIME, but normally
the system contains 1 kgw, so that factor is dropped.

> The way I have the RATES block set up, biodegradation is occurring
to some extent.  However, the 1/2 life for benzene is a fraction of a day,
so
after the first time step (3 years), no benzene should exist and yet it
does.

> I've included my input deck and database.

How old are you? I have been trying to forget about card decks for many
years. You don't want to go back to that.

> If there's some obvious problem, can you let me know?

I think your input file should look like this. Given the fast degradation
rate, the long time step doesn't show you much. Note that there is -tol in
the kinetics definition, that controls how accurately you want to integrate
the rate along with a cutoff of 1e-15 that I've left in the rate
expression.

SOLUTION 0 Pulse solution leachate  #solution injected
units       ppb
pH          7.0
density     1.00
temp        10.0
Na          620000
Cl          752000
Ca          304000
Mg          300000
K           317000
Benzene     13.4
Benzdp      0.0
END
SOLUTION 1-6      Background solution initially filling column
units ppb
pH 7.0
density 1.00
temp  25
Na          1966000
F           390
Cl          3033000
Ca          468000
Mg          528000
K           15900
END
SELECTED_OUTPUT
-file             grid3.csv
-selected_out     true
-high_precision   false
# set value for all indentifiers to follow (lines 1 - 6)
-reset            true
-simulation true
-state            true
-solution         true
-distance         true
-time             true
-distance         true
-step             true
-percent_error    true
-totals     Cl Na Benzene Benzdp
-gases      Benzene(g)
END
SURFACE 1-6
Surfa_sH2O  0.03  300   0.03
KINETICS 1-6
Benzene
-formula    Benzene     -1.0  Benzdp      1.0
-tol  1e-15
#     -M    1.71E-7
#     -M0   1.71E-7
INCREMENTAL_REACTIONS         true
RATES
Benzene
-start
10    Benzene = TOT("Benzene")
20    if (Benzene < 1e-15)then goto 200
30    rate = 2.5/(60*60*24) * Benzene #day-1 to sec-1
40    moles = rate * TIME
50    if (moles > Benzene) then moles = Benzene
200   SAVE moles
-end
TRANSPORT Pulsing of Solution 0
-cells                        6
-shifts                       6
-time_step                    1.05E8
-flow_direction               forward
-boundary_conditions          flux  flux
-length                       6*11.0
-dispersivities               6*18.0
-correct_disp                 true
-diffusion_coefficient  0.3e-9
-stagnant                     0
-thermal_diffusion            1
-initial_time                 0
-print_cells                  6
-print_frequency        1
-punch_cells                  1 2 3 4 5 6
-punch_frequency        1
END

> If not, do you have any free time this week when we
could meet for perhaps an hour and discuss the intricacies of phreeqc?  My
schedule is open except for Thursday morning and Friday after 3:00 pm.

I'm feeling swamped right now, but I'll continue to answer your email.

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

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