CXTFIT, Tritium, Equlibrium or non-equlibrium?

A discussion forum for STANMOD users. STudio of ANalytical MODels is a software package for evaluating solute transport in porous media using analytical solutions of the convection-dispersion solute transport equation.

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CXTFIT, Tritium, Equlibrium or non-equlibrium?

Post by ericren » Fri Oct 15, 2010 3:49 am

Hi, I want to use CXTFIT to simulate tritium's transport through a ottawa sand column as a tracer.

The column was presaturated with background solution. At time zero, Then a pulse injection of tritium is introduced. After a certain time t, injection stops and background solution flow resumes.

I wonder in this case, from time zero, is it equilibrium scenario or non-equilibrium scenario?

From the book, it's said two kinds of non-equilibrium:physical and chemical. because tritium is non reactive and non absorbent, I think it shall be equilibrium scenario.

However, the example given in library (FIG79A), it states non-equilibrium. Could anybody explain it for me?

THank you!

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Post by Jirka » Fri Oct 15, 2010 5:26 am

From your description it is not possible to assess whether you need to use, or not, the nonequilibrium models. The term nonequilibrium refers to what is happening in the column (not to initial and boundary conditions). Physical nonequilibrium refers to nonequilibrium conditions caused by physical reasons, such as that solute is transported only in part of water, which is mobile, while other part is immobile. On the other hand, chemical nonequilibrium refers to conditions when the solid phase concentration is not in equilibrium with the liquid phase concentration (i.e., s=Kd*c), but that there is a kinetic (nonequilibrium) process. I would start with the equilibrium model, and only if you cannot describe your data with this model, then I would use the nonequilibrium model. But in any case, you should know what processes are occurring in the column. As tritium is likely not being sorbed, you can exclude chemical nonequilibrium from consideration.

I hope this helps a little.


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