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Re: phreeqc

> In  attempting to conduct an inverse model I am receiving an error
that states the model cannot work for the uncertainty of the second

> Is there a simple solution for this problem? Any further advice you could
give will be very appreciated.

There are two reasons you may have gotten this error. (1) You may have a
large charge imbalance in the second solution. (check the Initial Solution
output for solution 2). PHREEQC allows uncertainty in the analytical data
and attempts to adjust concentrations so that charge balance is achieved
with the adjusted concentrations. If the charge imbalance is large, then a
large uncertainty will be needed to achieve charge balance. In general, the
uncertainty must be at least half of the percent error in charge balance to
avoid the error you got. (2) PHREEQC automatically includes mole-balance
equations for the elements in the phases that you select for inverse
modeling. However, elements not included in the phases are ignored unless a
mole balance equation is specifically included by the "-balances" option.
So if you did not include a phase that contains one of the major
constituents in the water analyses, PHREEQC will ignore that element and
try to adjust the other elements, for which mole-balance equations are
included, to achieve charge balance. If chloride is a major ion and you do
not include a phase with chloride, you would very likely get this error.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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