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Transport of sorbing solutes in fractured media : a numerical and experimental analysis of dispersion and retardation Wels, Christoph


The primary pathways for contaminant transport in lowpermeability fractured rock are likely to be through a network of hydraulically-connected fractures in the rock formation. Sorption of contaminants to the fracture walls may significantly retard their transport. The influence of sorption on solute transport is analyzed using both numerical and laboratory migration experiments. A random walk model is developed to simulate solute transport in a parallel plate fracture, assuming that fast, reversible, and linear sorption occurs in a small zone adjacent to the fracture wall. It is shown that a sorbing solute experiences greater longitudinal spreading than does a conservative solute. The magnitude of this enhanced dispersion reaches a maximum in the range of fluid velocities characteristic of Taylor dispersion. At the network scale, transport is simulated by tracking particles in discrete fracture networks, assuming that within each fracture, retardation varies in proportion to the product of a surface distribution coefficient and the specific surface area of the fracture. The results suggest that retardation at the plume scale is a non-uniform and anisotropic process. Different segments of the plume, or equivalently different breakthrough fractions at a downstream boundary, are retarded to a different degree. The degree to which various breakthrough fractions are retarded varies as a function of the orientation of the mean hydraulic gradient relative to the orientation of the fracture sets. This variation can be described in the form of a retardation ellipse. To test the surface retardation model used in the numerical analyses, the sorbing radionuclides strontium and uranium were injected in smooth-walled fractures in granite and steel, respectively. The retardation of uranium was inversely proportional to fracture aperture, providing qualitative support for the definition of the surface retardation factor. The influence of fracture aperture on the retardation of strontium was much greater than that predicted by the surface retardation factor. Strontium retardation was approximately an order of magnitude greater in a narrow fracture (b=450jim, Ra=45) compared to that in a wide fracture (b=780jim, Ra=3.5) . It is hypothesized that hysteresis in sorption, in conjunction with limited transverse mixing across the fracture, caused the apparent increase in sorption strength with a decrease in fracture aperture.

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