Experimental Investigation of Solute Transport in Stratified Porous Media: 2. The Reactive Case

Laboratory experiments and mathematical simulations were used to investigate the transport of a reactive solute,85Sr, through a sand layer located between two silt layers. The experimental results showed breakthrough curves that were similar in form to those reported in the preceding paper for a nonreactive solute, but were delayed in the time of appearance, had a lower peak concentration, and were more highly dispersed. A mathematical model that accounts for longitudinal advection in the sand layer, transverse diffusion in the silt layers, and retardation in both the sand and silt layers was used to simulate the experimental results. With a trial-and-error adjustment of the retardation coefficicents, the mathematical model gave a good representation of the experimental data; however, different parameter values were required in order to simulate breakthrough curves measured at different values of the pore-water velocity in the sand. The use of measured retardation parameters in the model produced a much better fit to the data compared to that which was obtained when retardation was neglected. Nevertheless, significant discrepancies existed between the measured and simulated results, with the discrepancies becoming greater at lower velocities. In view of the excellent agreement between measured and simulated results obtained in part 1 for nonreactive solutes, the less satisfactory agreement obtained here suggests that there is some physical or chemical aspect of the retardation process that is not adequately represented in the model.