# Re: PHREEQC problem

```
Your rate expression is pretty complicated, so I have not completely

(1) The formulas for the kinetic reactions must be charge balanced. From
what I see, you have
H -2 Cu +1 OH -1. This does not look right to me. Should it be CuOH 2? or
just Cu+1 H -2? There does not appear to be a need for the second kinetic
reaction because it is the same rate as the first, couldn't you do the same
thing with an additional OH -1 in the first rate?

(2) I think another  problem may be the fraction of a kilogram of water in
your solutions. TOT returns molality. However, the SAVEd value is moles.
You probably need to multiply all TOT functions by TOT("water")  or the
final moles by TOT("water")  to keep the units consistent. TOT("water") is
the mass of water in kilograms.

I'd try a first order rate expression first to make sure everything is
working as you expect before you go to the full expression that you have.

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

xiaomin Mao
<xmao@xxxxxxxx>          To:       dlpark@xxxxxxxx
cc:
05/25/03 06:21 AM        Subject:  PHREEQC problem

Dear Parkhurst,

I am a research staff from Edinburgh Univ. in U.K. PHREEQC is a
programme I like, and recently we even coupled Modflow, Mt3dms with
PHREEQC for reactive transport modelling with dencity dependent flow.
Now, I have a trouble with PHREEQC modelling and need you help.
I am using PHREEQC for a transport modelling with kinetic sorption of Cu
by organic matter. The kinetic sorption was written based on Nica-Donna
model, and the parameter used is  from Christopher J. Milne (2003).
I am wondering why the simulation is so slow(or stopped) if the organic
matter get a little larger. Is it because the numerical convergence
problem? or, could you give me any suggestion on how to avoid such
problem?
thank you very much.
Xiaomin
----------------
Dr. Xiaomin MAO
Institute for Infrastructure and Environment
School of Engineering and Electronics
The University of Edinburgh
Edinburgh, EH9 3JN
U.K.

Tel: +44 131 650 5810
Fax: +44 131 667 9238
http://myprofile.cos.com/maoxm

The input file is as follows,

Rates
Org_xcu
-start
10 Cu_conc=Tot("Cu")
20 H_conc=ACT("H+")
30 Org_xcue1=4.722*
(1.8197*Cu_conc)^0.53/((1.8197*Cu_conc)^0.53+(218.78*H_conc)^0.66)
*((1.8197*Cu_conc)^0.53+(218.78*H_conc)^0.66)^0.59/(1+((1.8197*Cu_conc)^0.53+(218.78*H_conc)^0.66)^0.59)

33 Org_xcue2=0.8811*
(181970086*Cu_conc)^0.36/((181970086*Cu_conc)^0.36+(398107171*H_conc)^0.76)
*((181970086*Cu_conc)^0.36+(398107171*H_conc)^0.76)^0.7/(1+((181970086*Cu_conc)^0.36+(398107171*H_conc)^0.76)^0.7)

#0.0001*Cu_conc/(Cu_conc+H_conc)*parm(1)
35 rate1=-(0.0001*(Org_xcue1+Org_xcue2)-Kin("Org_xcu"))/10 #
0.0001--based on the quantity of organic matter
37 put(rate1,1)
40 moles=rate1 *time
45 if Tot("Cu")+moles < 0 then moles = -TOT("Cu")/3
50 save moles
-end
Org_xh
-start
35 rate1=get(1)
40 moles=-rate1*time
50 save moles
-end
end

SOLUTION 0  CCA concentration
units mol/kgw
pH 7.0 charge
pe 4
-water 0.03375
Na 1
Cl 1

SOLUTION 1  CCA concentration
units mol/kgw
pH 4.0
pe 4
-water 0.03375

Cu  0.123562
Na 1
Cl 1.3 charge

SOLUTION 2-16  CCA concentration
units mol/kgw
pH 4.0 charge
pe 4
-water 0.03375
Cu  0.00
Na 1
Cl 1

kinetics 1-16
Org_xcu
-Formula  H -2.0 Cu +1
-m0 0.00
-m 0.00
#-Parms 0.1
Org_xh
-Formula H 0
-m0 0.1
-Parms 0.1

TRANSPORT
-cells 16
-shifts  33
-time_step 30857  #10.5d/28*86400s/d
-flow_direction forward
-lengths 16*5e-3
-boundary_conditions flux flux
-dispersivities 16*0.012
-correct_disp true
-diffusion_coefficient 0.0000
-punch_cells 16
-punch_frequency 1
-warnings false

SELECTED_OUTPUT
-file C:\cca_tmp\cca1
-selected_out true
-user_punch true
-reset false

USER_Graph
-start
10  PUNCH ToT("Cu"), -LM("H+"),Kin("Org_xcu"),Kin("Org_xh")
-end

END

```

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