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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
thank you very much.
Dr. Xiaomin MAO
Institute for Infrastructure and Environment
School of Engineering and Electronics
The University of Edinburgh
Edinburgh, EH9 3JN

Tel: +44 131 650 5810
Fax: +44 131 667 9238

The input file is as follows,

10 Cu_conc=Tot("Cu")
20 H_conc=ACT("H+")
30 Org_xcue1=4.722*

33 Org_xcue2=0.8811*

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
35 rate1=get(1)
40 moles=-rate1*time
50 save moles

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
 -Formula  H -2.0 Cu +1
 -m0 0.00
 -m 0.00
 #-Parms 0.1
 -Formula H 0
 -m0 0.1
 -Parms 0.1

  -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

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

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


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