Fig. 4. (a) Schematic of the two-dimensional diffusion simulation system in which diffusion occurs from the dark area on the left of the soil thin section to the right. (b) and (c) represent enlarged parts of the thin section as displayed within the gray lines of (a) and (b). (d) shows the unstructured mesh adopted for the finite element (FEM) simulations, including a typical enlarged 6-node triangular element (indicated by arrow) used in the simulations. (e) shows nodes of the computational mesh. (f) gives a contour plot of the concentration field at time t = 150 s. (g) example of concentration distribution (t = 150 s) along the paths (x = 1, 3, and 5 cm).