[en] Variation in the velocity field results in dispersion of a tracer cloud relative to the mean advective transport of the tracer. A major cause of variation in the velocity field is variation in the hydraulic conductivity field in clastic aquifers is stratification, whereby the rate of change in the hydraulic conductivity is much greater in the vertical direction than in the horizontal direction. Dispersion, under these circumstances, is not governed strictly by a Fickian flux, but by a more complicated integral expression involving the gradient of the mean concentration. Because a pulse input of conservative tracer is assumed in the investigations summarized in this paper, it is possible to derive both spatial and temporal moments; these moments are compared with those from a classical Fickian flux where a macrodispersivity has been adopted. By numerical Laplace inversion, it also is possible to obtain concentration profiles of the mean tracer as it moves downgradient through an imperfectly stratified aquifer. These results generally indicate that a classical Fickian flux provides a good simulation of the mean concentration after the center of mass of the cloud has moved at least 20 length scales from the point of injection. (Author) (10 refs., 2 tabs., 10 figs.)