Fig. 4. Gas density profiles for gas samples taken at 5, 15, and 25 cm from the column inlet of (a) Tachikawa loam and (b) Toyoura sand at 40% air content in horizontal experiments. The plots show the average of three replicates. The solid and dashed lines show the Fickian model predictions (Eq. [1]) with retardation factor R = 1.0 (no gas adsorption) and R = 2.0, respectively. Coefficient of variation of three replicates at each plot was <5%.