Boundary effects on solute transport in finite soil columns

This study investigates the influence of inlet and outlet disturbances and formulated boundary conditions on the estimation of the dispersion coeffici ent and retardation factor for short soil columns. Unsaturated miscible dis placement experiments utilizing a Br- tracer were carried out on undisturbe d columns of a fine-textured Ultisol. Solutions were applied using either a fritted plate or an array of dispensing tips that produced droplets at a p rescribed flow rate. One- and two-layer analytical solutions of the advecti ve-dispersive equation were fitted to effluent concentrations using nonline ar least squares parameter optimization. Comparison of two-layer simulation s with experimental data indicated that the analytical solution with a semi -infinite interface boundary best approximated effluent concentrations unde r the conditions of this study. This solution corresponds to a continuous f lux concentration and a macroscopically discontinuous resident concentratio n at the interface between the soil and porous plates. Parameter estimates were not significantly different with respect to the application method use d at the inlet. This may be attributed to a less uniform distribution of so lution onto the soil surface by the drip apparatus and/or by the presence o f stagnant regions within the inlet reservoir and hence increased dispersio n within the inlet platen apparatus. Two-layer simulations indicated that t he dispersion coefficient was underestimated by 14-27% when the influence o f the inlet and outlet apparatus were not included in the fitted solution o f the advective-dispersive equation. In addition, use of one-layer analytic al solutions caused the retardation factor to be overestimated by no more t han the fractional increase in pore volume imparted by the platen apparatus .