Contaminant transport in a fractured porous medium can be modeled, under appropriate conditions, with a double porosity model. Such a model consists of a parabolic equation with a coupling term describing contaminant exchange between the fractures, which have high permeability, and the matrix block, which has low permeability. Interpreting the coupling term as an operator, pseudo-differential in time, we obtain a method for calculating this term which is rapid in comparison with standard discretization methods.