||Groundwater pollution studies generally involve dispersivity as a parameter. The methods used to obtain its value are tedious and need the use of computer programs or nomographs. Moreover, Dieulin (1980) has shown that dispersivity may depend on time. For a model of a fractured medium (parallel fractures with various apertures and frequencies), we have worked out two functions C(x,t) including intrinsic parameters, for which determination is directly worked out from an experimental graph. These parameters being determined, it will be possible to make forecasts about the propagation of groundwater pollution. The method has been tested against the result of two tracer experiments carried out in chalk and seems to be utilizable for any kind of porous or fractured medium.