Lawrence Berkeley National Laboratory

Matrix diffusion is an important process affecting solute transport in fractured rock, and the matrix diffusion coefficient is a key parameter for describing this process. Previous studies have indicated that the effective matrix-diffusion coefficient values, obtained from a number of field tracer tests, are enhanced in comparison with local values and may increase with test scale. In this communication, we develop analytical expressions for the effective matrix diffusion coefficient for two simple fracture-matrix systems, and demonstrate that heterogeneities in the rock matrix at different scales contribute to the scale dependence of the effective matrix diffusion coefficient.