Convergence issues calculating speciation

[caitlin.selfe]

Hi there,

I am trying to calculate CO2 speciation. I have a large data set and get the below error at a particular point (solution 96) in the data set and it cannot be completed.

Initial solution 96.    LK21_3

WARNING: Maximum iterations exceeded, 100

WARNING: Maximum iterations exceeded, 100

WARNING: The program has failed to converge to a numerical solution.

The following equations were not satisfied:
ERROR:           Alkalinity has not converged. Total: 5.995336e-05   Calculated: 6.917939e-04   Residual: -6.318406e-04

ERROR: Is non-carbonate alkalinity greater than total alkalinity?

ERROR:                   Ca has not converged. Total: 2.894597e-05   Calculated: 3.504272e-05   Residual: -6.096752e-06

ERROR:                   Cl has not converged. Total: 1.314591e-03   Calculated: 1.300904e-03   Residual: 1.368695e-05

ERROR:                   Fe has not converged. Total: 1.790845e-07   Calculated: 1.756569e-07   Residual: 3.427583e-09

ERROR:                    K has not converged. Total: 3.043727e-05   Calculated: 3.012045e-05   Residual: 3.168206e-07

ERROR:                   Mg has not converged. Total: 1.176530e-04   Calculated: 1.200371e-04   Residual: -2.384085e-06

ERROR:                   Na has not converged. Total: 1.113685e-03   Calculated: 1.104218e-03   Residual: 9.466943e-06

ERROR:                 S(6) has not converged. Total: 6.965034e-05   Calculated: 6.891489e-05   Residual: 7.354492e-07

ERROR:                   Si has not converged. Total: 1.664550e-06   Calculated: 1.632220e-06   Residual: 3.232994e-08

ERROR:                   Mu Ionic strength has not converged.    Residual: -4.560002e-04

ERROR:               A(H2O) Activity of water has not converged.    Residual: 1.053571e-05

Here is the input (I have excluded the lines of data that run fine) :

SOLUTION_SPREAD
    -units    mg/l

modelling No       Descriptor    Temp     pH      Ca      Mg       Na       K          Alkalinity    S(6)       Cl
                                                                    as Ca0.5(CO3)0.5                
1   LK1_1   5.2   8.1   3.14   4.32   27.4   1.01   13   7.23   46.8

....
96   LK20_3   9.4   7.62   1.99   3.09   22.9   1.09   7   5.59   41.9
97   LK21_3   7.7   11.43   1.16   2.86   25.6   1.19   3   6.69   46.6
98   LK22_3   9.1   10.9   1.63   2.67   22.4   1.16   5   4.27   41.1
99   LK23_3   8.5   10.88   6.73   3.61   25.4   0.97   21   6.39   45.8
100   LK24_3   13.7   8.77   6.59   4.23   29.4   0.4   22   6.37   53.9
101   LK25_3   7.81   8.77   7.78   3.9   31.4   0.4   23   6.42   56.7
102   LK26_3   10.8   6.98   9.82   2.33   29.5   0.31   31   7.78   46.1
103   LK27_3   7.39   7.12   4.01   3.76   29.8   1.06   11   7.56   54.5
104   LK28_3   7.7   6.6   1.1   2.56   21.4   0.89   2   6.3   39.8
105   LK29_3   8.1   7.15   3.79   3.53   26.9   1.12   12   6.38   49.2
106   LK30_3   7.8   5.68   1   2.67   23.3   0.92   1   6.6   43.3
107   LK31_3   9   159.2   1.47   3.28   28.7   1.12   4   7.14   52.8
108   LK32_3   9.1   6.34   1.35   2.56   20.8   0.86   3   5.55   39.2
109   LK34_3   12.1   8.82   3.22   3.21   24.8   0.56   10   5   44.9
110   LK35_3   8.8   193.8   5.1   4.6   35.2   1.12   15   8.56   65.4
111   LK36_3   9.6   6.82   3.23   4.63   37.6   1.31   7   9.83   70.7
112   LK37_3   10.5   6.47   1.86   4.94   46.6   1.85   2   10.6   88.8
113   LK38_3   10.3   6.07   1.4   3.67   33   1.35   2   8.97   61.2
114   LK39_3   9.1   7.55   3.06   4.19   26   0.26   13   5.69   48
115   LK40_3   10.4   6.78   1.37   3.08   26.4   1.05   2   6.75   49.8
116   LK41_3   8.7   6.91   2.14   3.17   25.5   0.99   6   6.09   48
117   LK42_3   10   5.52   1.14   3.02   25.8   1.07   1   6.89   49.3
118   LK43_3   9.2   6.61   1.54   3.19   28.3   1.18   3   6.02   53.8
119   LK44_3   9.7   6.42   1.61   4.26   36.5   1.61   3   8.25   67.8
120   LK17_4   9.1   7.43   2.85   3.63   23.4   0.7   11   4.36   42.1

PRINT                                    
-saturation_indices true                                    
-selected_output    true                                    
SELECTED_OUTPUT
    -file                 DIC calcs.xls
    -percent_error        true
    -totals               C(4)
    -molalities           CO2  CO3-2  HCO3-
    -saturation_indices   Calcite Dolomite CO2(g)
end            

Can someone help me to understand how these values are wrong or what I need to change?

Thanks,
Caitlin

[dlparkhurst]

Your error messages have Fe included, whereas your SOLUTION_SPREAD does not. Something is not consistent.

Make a SOLUTION for the offending solution definition and post it.The posts in the forum lose the tab characters in SOLUTION_SPREAD, so it is difficult to reproduce your problem.  I'll take a look, but my first guess is that the error message is correct; the alkalinity from species of the other elements exceeds the defined Alkalinity. Also, state which database you are using.

[caitlin.selfe]

Hi,

Thank you for your response. The database I am using is PHREEQC.DAT

SOLUTION

modelling No   96
Descriptor    LK20_3
Temp     9.4
pH     7.62
Ca      1.99
Mg       3.09
Na      22.9
 K     1.09
Alkalinity as Ca0.5(CO3)  7
S(6)     5.59 
Cl   41.9
Si   0.5
Fe    0.119

PRINT                                    
-saturation_indices true                                    
-selected_output    true                                    
SELECTED_OUTPUT
    -file                 DIC calcs 96.xls
    -percent_error        true
    -totals               C(4)
    -molalities           CO2  CO3-2  HCO3-
    -saturation_indices   Calcite Dolomite CO2(g)
end            

I have included the Fe values and have run with and without them and still encounter the same issue.

[dlparkhurst]

I cleaned up your input file and it runs.

However, the error messages you show are for the solution with the discriptor LK21_3, not LK20_3.

I am almost certain that the error message is correct. The alkalinity from OH- and perhaps Fe and Si is greater than the specified Alkalinity. PHREEQC would need to calculate a negative alkalinity for C, and it cannot do that. You can either remove Alkalinity, in which case there will be no C in your solution, or if the pH is in the range of 7 to 9, specify TDIC equal to the value for Alkalinity. If the sample was in contact with air, you could specify C(4) 1 CO2(g) -3.4. There are probably other options, but the SOLUTION definition will not run as currently defined.

To see the alkalinity contributions for a solution, use PRINT; -alk.

Code: [Select]

SOLUTION 96 LK20_3
-units mg/L
Temp     9.4
pH     7.62
Ca      1.99
Mg       3.09
Na      22.9
K     1.09
Alkalinity 7 as Ca0.5(CO3)0.5 
S(6)     5.59
Cl   41.9
Si   0.5
Fe    0.119
END