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Scale dependence of groundwater flow and contaminant transport in fractured rock


Interest in groundwater resources is increasing in response to greater demands on water quality.
Fractures form a wide-spread and important source of subsurface heterogeneity which strongly influence groundwater flow and contaminant transport. There has been a growing awareness in recent years that outcrop scale measurements cannot simply be extrapolated to large scales and that for evaluating the fate of contaminants over significant times and distances, scaling aspects of fracture systems are crucial. Recent research has revealed that fracture size distributions commonly approximate power laws. Recent theoretical and field studies have shown that the connectivity of the fracture system (i.e. the way in which fractures link to form continuous networks) is sensitive to the exponent in this power law. Fracture connectivity is a major controlling factor for flow and contaminant transport. These finding have implications for the bulk rock permeability and contaminant transport properties of fractured rock which remain to be explored. The proposed project is a study of the scale dependence of permeability and dispersivity in fractured rock masses, with the specific aim of evaluating these properties from outcrop scales up to scales important for long term contaminant transport.

In the proposed project, scale dependence in flow and transport in fractured rocks will be studied through a number of approaches. Field studies will be aimed at characterizing natural fracture systems with reference to fluid flow. Data will be collected on different scales through outcrop investigations, aerial photography and satellite imagery. Constitutive rules will be developed through experimental studies of flow and transport properties in fractures under realistic in situ conditions (pressure, temperature). A perGolation theory approach will be used to derive analytical expressions for the percolation threshold, bulk rock permeability and dispersivity in the case of simple but geologically realistic fracture systems. Using both statistically based models and models that simulate the physical fracturing process, geometrical fracture system models which reflect the connectivity and other geological features of natural systems will be generated.
These will be used as input to numerical models of flow and contaminant transport in fractured rock masses and the results will be compared to the analytical expressions. These models will be used to study the existence and size of homogenization length scales for transport properties, and to develop techniques for determining meaningful equivalent porous media. Finally, the results of the project will be applied to a number of field sites.
The benefits of the proposed project arise from an increased understanding of groundwater flow and contaminant transport which will be of direct use in improving predictions of contaminant distribution, particularly of long term contamination. This will include an improved fundamental knowledge of the scale dependence of flow and dispersivity properties of fractured rock and the development of numerical modeling techniques that can utilize such information.

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Participants (3)