In this paper the geometric and hydraulic characteristics of fractures and fractured rock masses are reviewed to assess the current state and future direction of fracture hydrology research. Laboratory data suggest that the parallel-plate analogy for flow through a single fracture is valid. Fracture-flux is a function of the cube of the fracture aperture. Flow through fractures is a function of normal stress, shear stress, and fracture surface characteristics such as roughness. Flow through fractured rock masses is determined by fracture orientation, spacing, fracture interconnection, and the stress field. These factors must be considered in assessing the directional permeabilities of fractured rocks. Consideration of these factors and the structural characteristics of fractured rock leads to the formulation of a conceptual framework for flow in fractured rock masses that is a form of coupled discrete-fractured porous media model. In this conceptual framework, the shear zones and fracture zones are described as discrete hydrogeologic features; individual fractures (joints) are defined as discrete features near zones of interest, such as underground excavations; and the fractured rock mass in general is described as a form of equivalent fractured porous medium. I propose that the properties of the equivalent continuous porous medium must be developed from discrete fracture properties, reflecting the dependence of fracture permeability on the stress tensor, fracture geometry, distribution of fracture apertures, and degree of fracture interconnection.
The sparse data base, lack of agreement on the appropriate conceptual models, and difficulty of conducting field studies in low-permeability fractured rocks require that calculations of total-flux and transit times in such rock masses be supported and confirmed by other lines of evidence, including geochemical and isotopic data.