Using flow interruption to identify factors causing nonideal contaminant transport

The transport and fate of many contaminants in subsurface systems can be influenced by several rate-limited processes, such as rate-limited sorption, diffusional mass transfer, and transformation reactions. Identification of the controlling process in such systems is often difficult, and is confounded by the possible influence of additional factors such as nonlinear or hysteretic sorption. We present a relatively simple method, flow interruption, that can be used to discriminate between various sets of processes. The application of the method is illustrated with results obtained from experiments performed for selected systems. Specific process-pairs investigated include physical nonequilibrium vs. physical heterogeneity, rate-limited sorption vs. nonlinear sorption, and sorption vs. transformation reactions. The results show that, while both physical nonequilibrium and physical heterogeneity can cause enhanced spreading or dispersion, only the former causes a noticeable concentration perturbation upon imposition of flow interruption under typical conditions. In addition, while both rate-limited sorption and nonlinear sorption can cause breakthrough curves to exhibit tailing, only rate-limited sorption induces a concentration perturbation upon imposition of flow interruption. The information obtained from applying flow interruption can be used to assist in the planning of additional, process-specific experiments and to help identify appropriate mathematical models to be used for transport simulation.