QUANTIFYING THE DIFFUSIVE MASS-TRANSFER OF NONREACTIVE SOLUTES IN COLUMNS OF FRACTURED SAPROLITE USING FLOW INTERRUPTION

Subsurface transport processes of low-level radioactive contaminants a t hazardous waste sites are complex due to a nonuniform distribution o f pore regions in heterogeneous media. During transport, contaminant m ass is continuously transferred between the various pore regions via h ydraulic and concentration gradients. Our objective was to quantify th e diffusive mass transfer of a nonreactive solute between the matrix p orosity and preferential flow paths in fractured subsurface media. A l arge, undisturbed soil column (41 cm long by 17 cm diameter) of weathe red, fractured shale was acquired from a proposed waste site on the Oa k Ridge Reservation in eastern Tennessee. We injected a nonreactive tr acer, Br-, under saturated conditions, interrupting tracer Bow for a d esignated time, and then reinitiating Bow. Experiments considered trac er injection and displacement, variations in duration of flow interrup tion, and variations in Bur. Transport of Br- through the undisturbed medium was characterized by asymmetric breakthrough curves (ETC), indi cative of preferential flow coupled with movement into the matrix. Int errupting Bow resulted in decreased and increased Br- concentrations d uring tracer infusion and displacement, respectively, when Bow was rei nitiated. More pronounced concentration perturbations were observed at high fluxes and long interrupt durations. These perturbations were dr iven by solute concentration gradients established between pore region s as a result of preferential flow. This confirmed the importance of a diffusion contribution to the overall physical nonequilibrium mechani sm that affects salute transport. Observed effluent concentrations wer e adequately simulated with a two-region flow interrupt model by using independently determined parameters and adjusting the mass transfer c oefficient. Estimated mass transfer coefficients increased linearly wi th increased flux, probably due to enhanced nonequilibrium that was es tablished between pore regions as flux increased.