Influence of variable fracture aperture on transport of non-sorbing solutes in a fracture: a numerical investigation

The influence of fracture aperture variability on transport of a non-sorbing solute in a single fracture is studied and expressions for the effective specific surface area, or flow wetted surface area, are proposed. All the known transport mechanisms are taken into account: advection and dispersion in a channel, molecular diffusion in the stagnant water zones and matrix diffusion. In a first step, constant fracture aperture, it is shown that matrix diffusion is multidimensional and that stagnant water is a major pathway for the diffusive mass transfer into the rock matrix. In a second step, the impact of aperture variability on transport in the channel and transport in stagnant water is studied by means of Monte-Carlo simulations. The best expression of the effective specific surface area is found to be the inverse of the arithmetic mean aperture for the channel and the inverse of the geometric mean aperture for the stagnant water. These expressions could be used in a site scale performance assessment model.