  We consider a spatially homogeneous advection-diffusion equation in which the
diffusion tensor and drift velocity are time-independent, but otherwise
general. We derive asymptotic expressions, valid at large distances from a
steady point source, for the flux onto a completely permeable boundary and onto
an absorbing boundary. The absorbing case is treated by making a source of
antiparticles at the boundary. In both cases there is an exponential decay as
the distance from the source increases; we find that the exponent is the same
for both boundary conditions.
