Hydrodynamic dispersion; dispersivity কী?

Hydrodynamic dispersion is the spreading of a solute in flowing groundwater by the variations of the velocity of the water from pore to pore and by molecular diffusion. Along the flow it is described by a dispersion coefficient $D = \alpha_Lv + D^*$, $v$ the seepage velocity, $D^*$ the effective molecular diffusivity, and $\alpha_L$, a length, the longitudinal dispersivity of the medium.