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. Ide ntification of the controlling process in such systems is often diffic ult, and is confounded by the possible influence of additional factors such as nonlinear or hysteretic sorption. We present a relatively sim ple method, flow interruption, that can be used to discriminate betwee n various sets of processes. The application of the method is illustra ted with results obtained from experiments performed for selected syst ems. Specific process-pairs investigated include physical nonequilibri um vs. physical heterogeneity, rate-limited sorption vs. nonlinear sor ption, and sorption vs. transformation reactions. The results show tha t, while both physical nonequilibrium and physical heterogeneity can c ause enhanced spreading or dispersion, only the former causes a notice able concentration perturbation upon imposition of flow interruption u nder typical conditions. In addition, while both rate-limited sorption and nonlinear sorption can cause breakthrough curves to exhibit taili ng, only rate-limited sorption induces a concentration perturbation up on imposition of flow interruption. The information obtained from appl ying flow interruption can be used to assist in the planning of additi onal, process-specific experiments and to help identify appropriate ma thematical models to be used for transport simulation.