Re: Inverse modeling of brine formation

["David L Parkhurst" <dlpark@xxxxxxxx>]

Inverse modeling really only requires the correct total concentrations of
the elements. The speciation may be entirely erroneous and the saturation
indices unreliable, but it should not matter. You are looking for mole
transfers that account for the changes in concentrations between two
solution compositions that have been (mostly) measured. There may be
problems with total carbon if pH and alkalinity are used to determine total
carbon; this would be model dependent. Also, if you perform any mineral
equilibrium calculations in the course of defining the solution composition
of any of your inverse modeling solutions, you could have problems.
However, if you can feel confident about the total concentrations in the
solutions, then you should be able to proceed with INVERSE_MODELING.

David

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


                                                                                                          
                      Michael                                                                             
                      Zilberbrand               To:      'David L Parkhurst' <dlpark@xxxxxxxx>            
                      <w_mzilberbrand@x         cc:                                                       
In-Reply-To: <01C1CD9A.8141DF00@xxxxxxxxxx>
                      ail.gov.il>               Subject: Inverse modeling of brine formation              
                                                                                                          
                      03/17/02 02:59 AM                                                                   
                                                                                                          
                                                                                                          



Dear David,
Could you please help me? I have a question.
Is it possible to use the inverse modeling with PHREEQC for explanation of
formation of brines with very high ion strength (through evaporation,
mixing and reactions)? What may be the arguments?
For studying the influence of complexes at low ion strengths, I tried to
model addition of Na2CO3 to concentrated Gypsum solution (formed in
equilibrium with Gypsum). At Na2CO3 amount required for the complete CaCO3
precipitation (without taking into consideration complexes), (CaSO4)0
concentration in the final solution was not nil. It became very small only
at strong Na2CO3 excess. Does it mean that inverse modeling is impossible
without taking into consideration complexes?
I"ll very appreciate your help.
Sincerely,
Michael

------------------------------------------
Dr. Michael Zilberbrand
Hydrological Service of Israel
Research Division
P.O.B. 36118, Jerusalem 91360
Israel

TITLE Complexes and Calcite precipitation
SELECTED_OUTPUT
-reset             false
-file        Complexes-Calcite_prec_res.prn
-molalities        Ca+2 CaSO4
SOLUTION 1
Temp 20
pH           7.0
EQUILIBRIUM_PHASES 1
Gypsum 0.0
SAVE SOLUTION 2
END
USE SOLUTION 2
REACTION 1
Na2CO3 1.0
0.01532 moles
SAVE SOLUTION 3
END
USE SOLUTION 3
EQUILIBRIUM_PHASES 1
Calcite 0.0
END
# RESULTS at Na2CO3 1 mol
# m_Ca+2                 m_CaSO4
# 0.0104                       0.0049      After equilibration with CaSO4

# 0.0011                       4.53E-05    After addition of Na2CO3

# 2.89E-07                     1.21E-08          After equilibration with
Calcite
# RESULTS at Na2CO3 0.01532 moles (0.0104+0.0049)
#  1.0365e-02              4.9486e-03
#  4.9106e-03              2.2413e-03
#  1.4375e-04              9.0900e-05