Distinguishing Advection, Dispersion, and Diffusion in Fractured Bedrock

Mass spreading of non-reactive tracers in fractured bedrock can be described by a combination of molecular diffusion, hydrodynamic dispersion, and heterogeneous advection. These spreading mechanisms are fundamentally different in their behavior and, therefore, must be measured separately for reliable predictions of transport to be made. To separate the influence of these mechanisms, to the extent possible, specially designed tracer experiments can be conducted. To isolate the effect of molecular diffusion, tracers of different diffusion rates can be compared. Because molecular diffusion occurs at a rate independent of fluid velocity, repeating experiments at different velocity will also highlight the influence of molecular diffusion. Distinguishing hydrodynamic dispersion and molecular diffusion is best accomplished by isolating molecular diffusion and heterogeneous advection. Heterogeneous advection is conceptualized here as spreading caused by variable transport rates in media of different hydraulic conductivity. Heterogeneous advection is distinguished from hydrodynamic dispersion, therefore, by reversing the flow field during a tracer experiment. This should result in the nullification of the influence of heterogeneous advection and leave only the influence of dispersion and diffusion on the breakthrough curve. A flow-field reversal results during a push-pull tracer experiment, for example. It is important to realize, however, that advection, dispersion, and diffusion are coupled processes so that experimental separation is never complete and is meaningful only with respect to a particular theoretical transport model.