Large Percentage Error (up to 47%) in Saturation Index Calculation

[Fadeji01]

Hi all, I am calculating the Saturation Index for four solutions at standard Temperature and Pressure to determine what minerals will dissolve/precipitate. While the results were not unexpected, they had a percentage error that was too large. One of them had 47%, and another had 45%. Could something be wrong with the way I defined the solutions (cations-anions balance) using the SOLUTION_SPREAD block?

Code: [Select]

TITLE SI Calculation for four fluids at STP
SOLUTION_SPREAD
    -units    mg/kgw
     Mg      Ca     Na     Si        C     pH      F     Cl   N(5)   S(6)
  13.19   2.533    0.2 4.453 7.63933 10.15   0.41   1.31   0.16   0.87
 12.083    2.07    0.3 4.117    8.182 10.04   0.49   1.56   0.17   1.02
  3.733   0.049      0   0.15    1.565 10.06 0.000 0.638 0.000 0.000
      0 0.0009 0.133      0     0.17   5.95      0   0.28      0      0
END

[dlparkhurst]

I can't say what the problem is, other than you have more cations (dominated by Mg) than anions.