Inverse modelling for surface water in laterite terrain

[Chinaechem]

I want calculate the inverse model for surface water in laterite terrain. With the input file below, I can get 10 models with uncertainty of 0.7. Please is there anything I can do to reduce the uncertainty?  I will be grateful for any help. Thanks in advance.

TITLE  Simple inverse model calculation for surface water in Ni laterite terrain
SOLUTION 1
   units mg/kgw
   pH 7 charge
SOLUTION 2
   units mg/kgw
   pH 6.53
   temp 27.2
   Al 0.06
   Ca 2.06   
   Cl 1.67
   K  0.15
   Mg 5.38   
   Mn 0.80
   Ni 0.29
   Na  0.96
   Si  4.3
   S(6) 6.34 as SO4
   Fe 3.51

Phases

Pimelite
Ni3Si4O10(OH)2:H2O + 6.000 H+ = 3.00 Ni++ + 4.00 SiO2 + 5.00 H2O
   
log_k 11.46

   INVERSE_MODELING 1
        -solutions 1 2
        -uncertainty 0.7
        -balances
         Alkalinity 1
        -phases
                Magnesite  dissolve
                Antigorite
                Calcite    precipe           
                Quartz   
                Halite
                Goethite precipe
                Sepiolite precipe
                Gibbsite
                Magnetite     
                Forsterite 
                Gypsum   dissolve
                Manganite dissolve
                Pimelite    dissolve
                CO2(g)
             
         -range                       
END

[dlparkhurst]

I ran your SOLUTION definition, and I got a 60 percent charge imbalance. To use inverse modeling, you really need to have an analysis that has better cation/anion balance. There should be some alkalinity at pH 6.5. Is it the predominant anion?

If you have other analyses or can re-analyze the the water, then you need to consider the source of each element. In particular, consider the anions. If bicarbonate is the predominant anion, there is a ready source from soil zone CO2 reacting with minerals. If Cl and SO4 are predominant, the source is not obvious. Gypsum and halite are evaporite minerals that not consistent with laterites, which presumably formed under high rainfall and rapid weathering. The low concentrations of SO4 and Cl indicate there are not large quantities of these evaporite minerals, which would generate high concentrations. So, if SO4 and Cl are the predominant anions in the solution, what is the ultimate source of Cl and SO4, precipitation?