Inverse modeling of brine formation

[Michael Zilberbrand <w_mzilberbrand@xxxxxxxxxxx>]

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