Rain precipitation into a soil

[Max1]

Hi all,

I'm trying to model a molybdenum pollution in the subsurface using PHREEQC. So far I've managed to model a 100 cm soil column (with surface complexation), but I ran across some problems, so my question is: 1) how do I model rain precipitation in to the soil and the subsequent downward movement of the molybdenum pollution? Can anybody help me with this? This is my code so far:

TITLE 100 cm soil column

SURFACE_MASTER_SPECIES
     Hfo_s  Hfo_sOH
SURFACE_SPECIES
     Hfo_sOH + MoO4-2 + H+ = Hfo_sMoO4- + H2O
          log_k       9.5   
     Hfo_sOH + MoO4-2 = Hfo_sOHMoO4-2
          log_k       2.4
     Hfo_wOH + MoO4-2 + H+ = Hfo_wMoO4- + H2O
          log_k       9.5
     Hfo_wOH + MoO4-2 = Hfo_wOHMoO4-2
          log_k       2.4
SOLUTION_MASTER_SPECIES
     Mo       MoO4-2  0.0     Mo 95.94
SOLUTION_SPECIES
        MoO4-2 = MoO4-2
          log_k        0.0
SOLUTION_SPECIES
        MoO4-2 + H+ = HMoO4-
             log_k          4.2988
                delta_h          20 kJ
        MoO4-2 + 2H+ = H2MoO4
             log_k          8.1636
             delta_h          -26 kJ
        7MoO4-2 + 8H+ = Mo7O24-6 + 4H2O
             log_k          52.99
             delta_h          -228   kJ
        7MoO4-2 + 9H+ = HMo7O24-5 + 4H2O
             log_k          59.3768
             delta_h          -218   kJ
     7MoO4-2 + 10H+ = H2Mo7O24-4 + 4H2O
             log_k          64.159
             delta_h          -215   kJ
     7MoO4-2 + 11H+ = H3Mo7O24-3 + 4H2O
             log_k          67.405
             delta_h          -217   kJ
SOLUTION 0-100 #initial solution in a 100 cm column in the subsurface
     units ppm                           # mg/L
     pH          6.86         
     Mo         1.335                     
     Fe            12                          
     N(-3)      4.1 as NH4+
     Alkalinity    480 as HCO3-   
     Cl          62       
     N(+5)      1 as NO3-
     S(6)      400 as SO4-2
     C(-4)       0.013 as CH4
     Ca         30 charge                   # Charge balance
EQUILIBRIUM_PHASES 0-100                     
     Goethite             0 0.0674               # 0.0674 mol goethite per 1 m3 soil, pb = 2.65, n = 0.3
SURFACE 0-100
         Hfo_w Goethite         0.174689 53400            # 0.174689 = site density, on the given surface in mol/mol (1.97 sites/nm2),
         Hfo_s Goethite          0.004367225             # 53400 = specific area in m2/mol, with 600 m2/g surface area and 89 g/mol
     -equil with solution 0                  # ratio between weak sites and strong sites is 40:1
END
USER_PRINT
-start
10 PRINT "Amount of Mo in solution", TOT("Mo")
20 PRINT "Total Hfo_wOHMoO4-2: ", MOL("Hfo_wOHMoO4-2")
30 PRINT "K_d (L/kg) = ", (MOL("Hfo_wMoO4-")+MOL("Hfo_sMoO4-"))/TOT("Mo")
-end
END
TRANSPORT                              
     -cells           100                  # 100 cells
     -shifts           100                  
     -time_step        720.0               # time of shift, 720 seconds, so total time = 100 x 720 s = 72000 s; 20 hours
     -lengths           100*0.01               # 100 cells of 0.01 cm each = 100 cm
END

[Tom]

Hi Max1,

From my experience, it looks like you need to define SOLUTION 0, the infiltrating solution, separately with the chemistry of the rainwater.

Currently you have SOLUTION 0-100 as your initial column solution. If you change this to SOLUTION 1-100 and define SOLUTION 0 (infiltrating solution) prior to this, you will get a step closer to examining the problem.

An example from the PHREEQC online manual linked below may assist you further:


http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/phreeqc3-html/phreeqc3-66.htm#50651151_46434

Regards,

Tom

[Max1]

Thanks Tom! Appreciate the help.