Nonequilibrium sorption in soil/bentonite barriers

The installation of soil/bentonite (SB) slurry walls is a popular strategy for containing subsurface contamination. An important aspect of performance assessment for SB systems is an evaluation of the rate of contaminant transport across the barrier. The one-dimensional (ID) advective-dispersive-reactive equation is often applied to this analysis, under the common assumption of linear local equilibrium sorption. For cases in which sorption to barrier materials is not rapid, application of the local equilibrium assumption in the design and analysis of barrier systems will be nonconservative. Computational strategies are presented for incorporating first-order kinetic sorption into predictions of ID contaminant transport in vertical walls. In particular, an extension to the popular finite-layer approach is developed that allows treatment of nonequilibrium sorption while retaining the computational advantage associated with the semianalytical framework. Application of the method is illustrated through sensitivity analysis and simulation of published experimental results. While it is unlikely that nonequilibrium sorption will influence the performance of installed slurry walls, it is suggested that kinetic effects may complicate the interpretation of laboratory experiments for some systems.