  Using the advection-diffusion equation, we analytically study contaminant
transport in a sharply contrasting medium with a diffusion barrier due to
localization of a contaminant source in a low-permeability medium. Anomalous
diffusion behavior and crossover between different transport regimes are
observed. The diffusion barrier results in exponential attenuation of the
source power, retardation of the contaminant plume growth, and modification of
the concentration distribution at large distances.
