Quantifying the Diffusive Mass Transfer of Nonreactive Solutes in Columns of Fractured Saprolite using Flow Interruption

Subsurface transport processes of low-level radioactive contaminants at hazardous waste sites are complex due to a nonuniform distribution of pore regions in heterogeneous media. During transport, contaminant mass is continuously transferred between the various pore regions via hydraulic and concentration gradients. Our objective was to quantify the diffusive mass transfer of a nonreactive solute between the matrix porosity and preferential flow paths in fractured subsurface media. A large, undisturbed soil column (41 cm long by 17 cm diameter) of weathered, fractured shale was acquired from a proposed waste site on the Oak Ridge Reservation in eastern Tennessee. We injected a nonreactive tracer, Br⁻, under saturated conditions, interrupting tracer flow for a designated time, and then reinitiating flow. Experiments considered tracer injection and displacement, variations in duration of flow interruption, and variations in flux. Transport of Br⁻ through the undisturbed medium was characterized by asymmetric breakthrough curves (BTC), indicative of preferential flow coupled with movement into the matrix. Interrupting flow resulted in decreased and increased Br⁻ concentrations during tracer infusion and displacement, respectively, when flow was reinitiated. More pronounced concentration perturbations were observed at high fluxes and long interrupt durations. These perturbations were driven by solute concentration gradients established between pore regions as a result of preferential flow. This confirmed the importance of a diffusion contribution to the overall physical nonequilibrium mechanism that affects solute transport. Observed effluent concentrations were adequately simulated with a two-region flow interrupt model by using independently determined parameters and adjusting the mass transfer coefficient. Estimated mass transfer coefficients increased linearly with increased flux, probably due to enhanced nonequilibrium that was established between pore regions as flux increased. Joint contribution from Oak Ridge National Lab., the Univ. of Tennessee, and Louisiana State Univ. This research was supported by the Office of Environmental Management, Oak Ridge National Lab., managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy, under contract DE-AC05-96OR22464. Publication no. 4564, Environmental Science Division, ORNL.