Large-Scale Solute Transport in Fracture Networks

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Contaminants emanating from industrial and waste sites in bedrock aquifers typically migrate rapidly and widely due to the pathways provided by discrete fractures and fracture interconnections. Transport processes include the tortuosity of pathways within individual fractures, the variability of matrix porosity, the variability of fracture interconnections,*lateral migration through preferential pathways, temporal effects in the hydraulic gradients,*and the dispersive effects that occur at fracture intersections.*In the proposed study we will assemble several data sets from existing well characterized*sites and the limited number of tracer experiments conducted*at large scale to provide a basis to investigate through numerical analysis the dominant transport mechanisms. Initial studies will*focus on the processes which occur in discrete fractures, and the complexity of pathways developed through the variability in fracture aperture, and the effects due to matrix diffusion are expected to dominate. In the full fracture network study, numerical analysis will again be used to evaluate the principal transport mechanisms. In this case, the dominant mechanism is expected to be the complexity of the fracture interconnections, and the dispersive effects due to the fracture intersections. Between the discrete*fracture study and the full-network analysis, two Masters students will be required. The analysis of the network experiment poses a very significant*challenge computationally and a significant component of the research will be conducted in comparing Eulerian (spatially fixed frame of reference) and Lagrangian (frame of reference which moves with the fluid) approaches for this setting that will require the fulltime*effort of one PhD*student. The comparison will be conducted at both local scale (150 m).*In order to support the numerical studies, local-scale*investigations of discrete fractures and fracture intersections will be conducted in a*crystalline rock field setting. The Applicant has developed a unique state-of-the art field site with eight boreholes drilled in an area of*approximately 20 by 30 m, in a granite gneiss. Preliminary hydraulic testing results have shown several wells which are connected by one*or more fractures, some with intersections. Tracer experiments using both solute and solute and heat will be conducted between these wells to specifically explore the dispersive effects played by fracture aperture heterogeneity and fracture intersections. This component of the study will involve two PhD students and one Masters student.*The outcome of this research will provide an assessment of the*fundamental processes governing contaminant migration in fractures, and will have practical application by hydrogeologists and engineers in the prediction of risk from waste facilities, and the management of contaminated sites in these settings.