Release of DRP from a top layer.

[gurparshad]

Greetings Dr. Jirka,

I have a soil column with manure on the top. I am taking the manure layer of very small depth (0.5 cm) and trying to get it to act as a source of DRP release. You mentioned earlier that the manure either needs to be kinetically sorbed or in immobile water to release it as a function of time.

I am using a MIM model with chemical equilibrium. Also, my initial conditions are in terms of total concentrations. I am keeping the dispersivity of 0.5 cm layer = negligible. How could I achieve a kinetic release of manure?

Will assuming thlmob. + saturated water content and having all the sorption sites in contact with immobile water work?
or
Should I add all the solute mass in the graphical editor, in sorbed concentration 1 (Kinetically sorbed solute), keeping thlmob., omega, frac, frac_m, and alpha as optimized parameters?

Thank you.
Gurparshad.

[Jirka]

If you specify the initial condition in terms of the total concentration, then the code will distribute the mass between the mobile and immobile regions so that the system is in equilibrium. You will have thus some solute mass in the immobile region, from which solute will be released gradually into the mobile phase by mass transfer. If you fit sorption constants (e.g., Kd, Frac, etc), the initial mass will be unaffected by this.

You can also give the initial condition in terms of concentrations in the mobile and immobile phase directly. If you do that, you can place all the mass into the immobile region, and then you will get a mass transfer from this region into the mobile regions. In this case, the sorbed concentration will be automatically determined by Kd. In this latter case, if you run an inverse problem, the initial mass will change whenever you change (e.g., by fitting) the sorption constants (kd, Frac, etc).

J.

[gurparshad]

Thank you Dr. Jirka for your swift reply. I really appreciate that.

Sorry, I was not thorough in my question. I don't know the adsorption parameter values and thus will be optimizing kd and beta for the soil layer. Therefore, I cannot use liquid concentration in the initial conditions.

So I have to use total concentration. What would you suggest:
1. To have solute travel from immobile region to mobile region (after Cmean(mobile)=Cmean(immobile)).
2. to have all the solute be sorbed on the kinetic sorption sites of the mobile region, and released form there?

In the latter case, I input the concentration in the sorbed conc. 1 in the GUI?

The values of frac, frac_m, omega, alpha will be optimised and will not have a profound effect on the release of the solute.

Thank you for your time and consideration.

[Jirka]

You can perhaps divide the profile into two layers. One representing the source, for which you fix the parameters, to keep the right amount of solute there, and perhaps only fit the release constant (mass transfer from immobile to mobile zone) and the second representing the soil itself, for which you can fit all sorption parameters. J.

[gurparshad]

Thank you Dr. Jirka for your valuable time and suggestion.