Dispersion denotes the macroscopic tendency of a solute plume to spread and to be diluted as groundwater moves away from a source location. It is often invoked as a concentration attenuation mechanism for contaminants in groundwater modeling and conceptual site models and the standard view of dispersion envisions two components: molecular diffusivity and hydrodynamic dispersivity. Both dispersivity and diffusivity are superfi cial characteristics, in which the behavior we observe is an average of processes occurring at much smaller scales. In the case of molecularlevel diffusivity, we observe system behavior at scales several orders of magnitude larger than that at which the process occurs and the superfi cial characterization reliably represents the underlying process. Hydrodynamic dispersivity is a more conceptual process, based on the expectation that solutes will spread as a result of pore-scale bifurcation of groundwater fl ows and bulk water mixing. The conventional view in hydrogeology has been that hydrodynamic dispersivity is the dominant contributor to overall plume spreading and diffusivity makes only a small contribution at the large scales of contaminant plumes. However, detailed solute concentration mappings, whether for plumes arising from persistent sources or from short-term tracer pulses, typically do not follow patterns predicted by standard dispersion models.