PrintPéclet number (Pe) is often used to describe the relative importance of advection and dispersion/diffusion in terms of their respective time scales $\tau_{a} \text { and } \tau_{d}$:
where L is the length of the domain of interest (m), u is the average Darcy flow velocity in the direction of interest (m/s), $D_{h}$ is the dynamic dispersion coefficient (m2/s). There are also some mathematical equations to define the time scales of these processes with similar concepts, mostly depending on the selected characteristic length [Elkhoury et al., 2013; Huysmans and Dassargues, 2005; Steefel and Maher, 2009; Szymczak and Ladd, 2009]. For example, L can also be the grid spacing (m) or correlation length (m) [Huysmans and Dassargues, 2005]. As shown in Figure 3, increasing Pe values indicate increasing dominance of advective transport and sharper front in breakthrough curves.
