Fig. 2. Relative concentration versus depth for a two-layered soil (L₁ = L₂ = 6 cm) based on analytical solution of Leij et al. (1991) and our numerical method, at time equals 0.4 d. The solid curve and closed circles are for a first-type boundary condition (BC) at the interface between the two layers, whereas dashed curve and open circles are for a combined first- and third-type BC.