DIFFUSION, DISPERSION, AND ADVECTION IN FRACTURED ROCK

In porous media, diffusion is generally thought to be easily distinguished from the influence of advection and hydrodynamic dispersion because their effects occur over very different time scales. In fractured rock, however, this distinction is not always so clear. Heterogeneous velocity fields can cause advection and dispersion to occur over a wide range of time scales, sometimes on the order of the rate of diffusion. One of the expressions of this behavior is that the tails of tracer breakthrough curves collected in the field may be dominated by diffusion or advection, depending upon the formation characteristics and experimental design. As tracer breakthrough curves are the primary tool for investigating field-scale transport, this confusion can lead to error or misinterpretation of the influence matrix diffusion in transport of contaminants in a fracture rock system. The relative role that diffusion, hydrodynamic dispersion, and heterogeneous advection play in fractured rock contaminant hydrology will be explored through conceptual models and field breakthrough data. Implications of these roles for the measurement and prediction of matrix diffusion will be discussed, particularly with respect to the design of field tracer experiments.