Boundary Conditions for Displacement Experiments through Short Laboratory Soil Columns1
- M. Th. van Genuchten and
- J. C. Parker2
This paper presents a discussion of the physical and mathematical significance of various boundary conditions applicable to one-dimensional solute transport through relatively short laboratory soil columns. Based on mass balance considerations, it is shown that a first-type or concentration-type condition at the inlet boundary incorrectly predicts the volume-averaged or resident concentration inside both semi-infinite and finite systems. A third-type or flux-type inlet boundary condition preserves mass in semi-infinite systems, but underpredicts effluent concentrations from finite columns unless a local transformation is used to convert volume-averaged concentrations into flux-averaged concentrations. This transformation leads to an expression for the effluent concentration that is identical to the solution for the semi-infinite system using a concentration-type boundary condition. For column Peclet numbers greater than about five, the resulting analytical expression for the effluent curve is shown to be nearly identical to the analytical solution for a finite system based on a flux-type inlet boundary condition and a zero-concentration gradient at the exit boundary. Both solutions correctly preserve mass in the system; other solutions of the convective-dispersive transport equation are shown to be inappropriate for analyzing column effluent data.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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