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-sorbin g solute in a single fracture is studied and expressions for the effective specific surface area, or flow wetted surface area, are proposed. All the k nown transport mechanisms are taken into account: advection and dispersion in a channel, molecular diffusion in the stagnant water zones and matrix di ffusion. In a first step, constant fracture aperture, it is shown that matr ix diffusion is multidimensional and that stagnant water is a major pathway for the diffusive mass transfer into the rock matrix. In a second step, th e 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 invers e of the arithmetic mean aperture for the channel and the inverse of the ge ometric mean aperture for the stagnant water. These expressions could be us ed in a site scale performance assessment model. (C) 1998 Elsevier Science B.V. All rights reserved.