calcite, dolomite and cation exchange

[grk1]

Hello
I'm trying to get the exchange composition of a rock with assumed CEC in an aquifer where there is also calcite and small amount of dolomite present.

I'm using this to get the exchange composition:

Code: [Select]

SOLUTION 1262
units mol/kgw
water 1.0
      temp 20.9
      pH 7.15
      C(4) 3.967213e-03  charge
      Ca 2.495000e-03 
      Cl 2.078984e-03 
      Na 1.589520e-03 
      Mg 4.525000e-04 
K 3.734000e-05 
      S 3.008380e-04 
EQUILIBRIUM_PHASES 1262
    Calcite 0 24
    Dolomite 0 0.02
SAVE solution 1262
END
EXCHANGE 1262
X 1.197e-01
-equil 1262
END
but when I use the exchange composition output from the above code, as follows,  my initial water and final water are not the same (different Mg and Ca, which is what interests me).
I thought the point was that the 1st code finds the exchange composition that, together with the minerals, remain the water quality unchanged. What am I doing wrong?

Code: [Select]

SOLUTION 1262
units mol/kgw
water 1.0
      temp 20.9
      pH 7.15
      C(4) 3.967213e-03  charge
      Ca 2.495000e-03 
      Cl 2.078984e-03 
      Na 1.589520e-03 
      Mg 4.525000e-04 
K 3.734000e-05 
      S 3.008380e-04 
EQUILIBRIUM_PHASES 1262
    Calcite 0 24
    Dolomite 0 0.02

EXCHANGE 1262
CaX2              3.872e-02
MgX2              2.026e-02   
NaX               1.562e-03   
        KX                1.888e-04   

   

[dlparkhurst]

The two calculations are not quite the same. The first calculates the initial solution, equilibrates the initial solution with calcite and dolomite, saves the solution, and then finds the exchange composition in equilibrium with the saved solution. 

The second calculates the initial solution, and then equilibrates the initial solution with the exchange composition and the equilibrium phases. So, the solution is not in equilibrium with the equilibrium phases when the reaction starts. That means that the solution has to change composition to be in equilibrium with the equilibrium phases, but, if the solution changes, then the exchange composition has to change as well.

I don't know which calculation makes the most sense for you. If you brought a new water into an aquifer, the second calculation seems best. If you are just trying to estimate initial conditions, I usually assume the analyzed water is in equilibrium with the aquifer, but that means I would expect the water to be in equilibrium with calcite and dolomite, and in turn, the exchanger would also be in equilibrium. If the water is close to equilibrium with calcite and dolomite, the two calculations would not be too different. If the water is not near equilibrium with calcite and dolomite, I would be curious as to why.

[grk1]

David, thank you for the reply.

I am trying to estimate initial conditions at the aquifer, a few years before new water arrives. So the water from the example is an average of a few samples from a few adjacent wells  and it's near equilibrium with calcite (SI = 0.07) and dolomite  (SI = -0.53).
The initial conditions are to be set in a reactive transport model on PHT3D, which requires equilibrium in the initial timestep.
We do have (good enough) data on the minerals, but no data on the exchange composition.  I'm trying to reach equilibrium between the three: water (known), mineral (assumed known), and exchanger (unknown, but constrained to literature CEC values in such type of rock). So I'm using the 1st code to get the exchanger composition. The 2nd code is a reproduction of what PHT3d does in the first timestep, so If I dont get the same solution, it means that my reactive transport model starts with a different solution and from there - everything is wrong.
The question is - how can I change the 1st code, so that the 2nd code (with the resulting exchanger from the 1st code's output) can reproduce the water? might be a matter of SAVE/USE/END keyword order?

[dlparkhurst]

Your average water is not in equilibrium with calcite and dolomite, so there will always be a reaction when you do the equilibration step.

I think you should use the first calculation, probably without dolomite because it is significantly undersaturated, to generate a new average groundwater that is in equilibrium with calcite. That water would be the SAVEd solution 1262. The exchanger is defined to be in equilibrium with that water. When you bring more of that water in contact with calcite in the EQUILIBRIUM_PHASES and the EXCHANGEr, there will be no reaction.