different inv model input (solution 1), exact same results....?

[deso]

Hi PHREEQC users,

I'm a bit stumped with some inverse modelling results I have. I'm running two inverse models (see code below). The only change is the input solution (solution 1)...solution 2 stays the same. Under one scenario I get 8 models....if i change the input, I get four models....but for all four of these the phase mole transfers are exactly the same for four models in the other scenario. I don't understand how that can happen... Both solution 1's are similar...but surely it would produce exactly the same result. Any ideas. The code for each scenario is also exactly the same (see code for scenario 2 below. if you change the solution 1 from scenario 2, to scenario 1....4 of the 8 models will be exactly the same as the 4 from scenario 2. 

Solutions for scenario 1. (produces 8 models)
Number   temp   pH   Si   Ca   Mg   Na   K   Alkalinity   Cl   S(6)   F
1   21.1   8.114285714   12.71428571   22.925   9.767857143   244.3285714   3.507142857   253.5128806   252.6892857   18.63214286   0.348214286
2   21.1   8.122641509   31.13207547   74.14716981   90.74150943   655.6962264   3.428301887   393.5756263   1031.256604   155.2603774   0.140754717

Solutions for scenario 2 (4 models)
Number   temp   pH   Si   Ca   Mg   Na   K   Alkalinity   Cl   S(6)   F
1   21.1   8.2064   31.136   40.2056   49.4168   393.5072   2.4544   393.0354098   495.4256   74.6976   0.22304
2   21.1   8.122641509   31.13207547   74.14716981   90.74150943   655.6962264   3.428301887   393.5756263   1031.256604   155.2603774   0.140754717

example of code
TITLE Example 11.--Scenario 2
SOLUTION_SPREAD
        -units  mg/l
Number   temp   pH   Si   Ca   Mg   Na   K   Alkalinity   Cl   S(6)   F
1   21.1   8.2064   31.136   40.2056   49.4168   393.5072   2.4544   393.0354098   495.4256   74.6976   0.22304
2   21.1   8.122641509   31.13207547   74.14716981   90.74150943   655.6962264   3.428301887   393.5756263   1031.256604   155.2603774   0.140754717
INVERSE_MODELING 1
        -solutions 1 2
        -uncertainty 0.025
     -minimal
     -multiple_precision
        -balances
                pH   0.1
          Ca      0.025   
          Mg   0.025   
           Si   0.025   
          Na   0.025   
                K      0.025   
          Cl   0.025   
          S(6)   0.025   
         -phases   
          Halite   dissolve
          Calcite   
          dolomite
          gypsum   dissolve
          fluorite
          Kaolinite precipitate
                Ca-montmorillonite precipitate
          illite     precipitate
             gibbsite precipitate
          K-feldspar dissolve
          Fe(OH)3(a)
          CO2(g)
          CH2O
          H2O(g) 
          CaX2   precipitate      
          NaX   dissolve
          MgX2   precipitate
          Quartz  precipitate
          Plagioclase dissolve
          Trona    dissolve
          albite   dissolve
           -range       
PHASES
Plagioclase
   Na0.7Ca0.3Al1.3Si2.7O8 + 5.2 H+ + 2.8H2O = 0.7Na+ + 0.3Ca+2 +1.3Al+3 + 2.7H4SiO4
   log_k      0.0
Trona
   Na3H(CO3)2:2H2O + 1H+ = 2HCO3- + 3Na+ + 2H2O
   log_k      0.0
END

[dlparkhurst]

Look at the mixing fractions for the first and last solutions. I suspect the mixing fraction for solutiion 1 is 0.0 for all models, which means the water for the target solution came from minerals, particularly H2O(g).

You can remove H2O(g) or turn off the mineral water switch, but I think you will find no models with the remaining minerals. So, you will need to go back and reconsider  sources and sinks for each mineral. You may also remove some of the dissolution/precipitation constraints and look at the models that result; it may give you a clue as to get to a plausible model.