| Numerous laboratory column experiments of transport suggest that the concentration C of an
inert solute is ruled by the advective-dispersion equation (ADE) with longitudinal dispersivity
αL. The latter is constant for granular media and, for the typically high Peclet numbers
encountered in applications, of the order of αL ~ 10-2 ÷ 10-3m. There is an
increasing evidence from field tests that spreading of solutes at the aquifer scale is much
larger than that occurring at the laboratory scale, by orders of magnitude; for that
reason αL is often denoted as macrodispersivity. Transport of a nonreactive solute in
natural aquifer is indeed deeply influenced by the spatial distribution of the hydraulic
conductivity K. Experimental evidence and the numerous studies carried out in
the last decades show that the dynamics of transport is affected by the degree of
heterogeneity present in the groundwater system, and in particular when in presence of
highly heterogeneous aquifers. Different and interesting transport features have
been observed or postulated from theoretical analysis, like e.g. the time and scale
dependency of αL, which is no longer constant in natural aquifers, the non-Fickian
behavior of macrodispersivity, distributions of the concentration field far from the
Gaussian one implied by the ADE, anomalous transport characterized by an ever
increasing or decreasing αL, significant and persistent tailing of the breakthrough curve
measured at given control planes, to mention some. Under such circumstances,
which seem to suggest a significant departure from the ADE, the overall meaning of
macrodispersivity is questioned. Thus, new approaches for modeling solute transport in
heterogeneous formations have emerged in the last years, in the attempt to overcome the
ADEÂlimitations. Nevertheless, most of the numerical codes used in applications employ the
concept of dispersivity and implicitly assume that ADE holds true. The significance of
macrodispersivity, its main features and its relevance in applications are discussed by
means of recent theoretical work and experimental evidence, as function of the
particular scope for which transport is analyzed and the heterogeneous structure of K. |