A network model based on a semi-analytical solution method was used to simulate the flow of groundwater and migration of solute in a fracture network. The solution domain consisted of a major fracture of large aperture that bisected a stochastically generated set of finer, interstitial fractures. The fracture and matrix parameters used in the simulation were representative of an accidental release of petroleum in a crystalline rock environment. The orientation, aperture, and density of the interstitial fractures were systematically varied, to determine the influence of the arrangement of the interstitial network on the transport process. The porosity was altered at a given fracture density to determine the relative effect of matrix diffusion. The results were determined using breakthrough concentration calculated at the down-gradient end of the major fracture. An instantaneous pulse was used to simulate the source condition. Results indicated that the orientation and mean aperture had significant effects on the breakthrough concentrations. An increase in the interstitial fracture density resulted in a decrease in peak concentration and an increase of mass in the tail of the curve. Matrix diffusion was observed to cause tailing even at low porosity. In addition, at later times in the breakthrough curve, the concentrations became increasingly coincident, an effect that became more apparent with higher porosities.
In fractured crystalline bedrock, the discrete fracture network and unfractured rock matrix form an intricate system for the flow of groundwater and transport of contaminants. Stochastically generated networks of discrete fracture are often used in the simulation of the natural fracture systems. Recently, flow and transport models have been developed that simulate the migration of a contaminant within such a network to a high degree of accuracy [Rouleau, 1984; Dverstorp, 1992; Bogan, 1996]. Using these models, an improved understanding of solute transport in fracture systems is achieved. This is necessary in order to enact efficient cleanup of contaminated sites or to determine the effectiveness of selected sites for the containment of hazardous waste in fractured rock. Matrix porosity and fracture density have recently been shown to be significant factors in the attenuation of the solute in discrete fracture networks [Rouleau and Raven, 1992; Toran et al., 1995]. In this paper, this is examined by using a recently-developed model for solute transport in an analytically derived fracture network. The model was derived by Bogan [1996] and is based on a semi-analytical solution of the transport equations in a network of fracture elements. Using this model, an investigation is conducted into 1) the role of the matrix in solute transport in densely fractured crystalline rock, 2) the influence of the spatial arrangement and density of fractures within the network, on the migration of solutes. In such environments, shallow, flat lying fractures of large lateral extent often predominate the groundwater flow system. However, these fractures are often interconnected by a network of finer, interstitial fractures that limit, and perhaps govern, the transport process. In this paper, we investigate the importance of the network of interstitial fractures.
