Water transport and evaporation

[FlouretAlex]

Hello PHREEQC user,

I would like to know how to model the impact of a solution moving through a layer of soil and its impact on the soil water composition, driven by evaporation. Till (now see script below) I'm using the trick of creating a new solution and mix it with the cell where the soil solution is to only remove the water that has been evaporated.

Code: [Select]

#Contaminated soil ==> RIZO
SOLUTION 3 # ==> stagnant cell
units mol/L
pH 7.5
Cl 1.00E-05
K 1.00E-05
Mg 7.50E-04
S(6) 7.50E-04
Cs 1.00E-6
use surface 3 #number of surface and exchange need to be the same as the cell here solution 3 ==> cell 3
use Exchange 3
EQUILIBRIUM_PHASES   3
Fix_H+ -7.5 HCl  10

END

SOLUTION 0-1 # inlet solution 0 # define solution 1 since PHREEQC wants at least 1 cell in the colum for transport ==> mobile cell.
 units mol/L
pH 7.5
Cl 1.00E-05
K 1.00E-05
Mg 7.50E-04
S(6) 7.50E-04

Solution 100 #for evaporation
END

MIX 3 # simulate % water evaporation of solution 3 then replaced by solution 0
0 0.1
100 -0.1
3 1
END

TRANSPORT
-cells 1
-lengths 0.01 # 1 cm sol
-shifts 576 # number of time_steps total time is 24 days
-time_step 3600 # each time_step (s) the MIX is performed
#-flow_direction  diffusion_only
-boundary_conditions constant closed
#-diffusion_coefficient 2.3E-08
-stagnant 1
-punch_cells 3
-punch_frequency 1 # sample every time step
END
Like this is working fine but I don't really have any effect of the water transport through the layer as the MIX mix the solution into the cell. Thus to ad more complexity, I would like to model the evaporation at the surface of the soil and the movement of the soil solution in the soil layer (scheme in attachment). The main problem is that I only want to remove water from this cell and for this, I don't know how to do it. If someone has some idea how to model this that would be really nice!

Thank you for your help!

[dlparkhurst]

Your scheme keeps a constant volume in cell 1, which could be helpful for transport.

Alternatively, you can use REACTION or KINETICS to simply remove water, but then the volume of water in the cell decreases.

Code: [Select]

REACTION n
H2O 1
-5.5 # ~10 percent of the volume

From your diagram, it looks like you are evaporating at the end of a TRANSPORT column, in which case the volume would not be as important; the solution in the last cell is advected out of the column and has minimal effect on the other cells (although dispersion will slowly decrease the volume in upstream cells.

[FlouretAlex]

Thank you for your help,

The thing is that I don't want to use MIX because advection and diffusion are not taken into account like that. First I was thinking of adding another cell on the top of the first one where water only will be removed with REACTION but I don't know if it's okay to do so.

P.S: Sorry for posting this question in the wrong place, this should have been posted on the transport topic.

Have a nice day !