This talk concerns the mathematical treatment of solute
transport when the mean velocity is perpendicular to a sharp interface.
The mathematical model is based on the Fickian laws of
advection-dispersion with continuity of the concentration and the flux
across the interface. In deriving the solution we extend methods of a
recently published paper (Ramirez, Thomann, Waymire, Haggerty, Wood.
2008) involving the theory of skew Brownian motion for describing solute
transport parallel to a sharp interface to the case of orthogonal flow.
An application to breakthrough curves is given which explains
emperically discovered asymmetries under a mirror symmetric change in
the orientation of the flow from coarse to fine and fine to coarse.