Benzene vapor transport in unsaturated soil: Adequacy of the diffusion equation

Experimental data for unsteady state benzene vapor transport in large (10.5 cm x 100 cm) columns packed with dry and wet soil were used to evaluate the adequacy of the diffusion equation. It was shown that the diffusion equation and local equilibrium, accounting for water phase partitioning and linear sorption, adequately described vapor transport in dry soil. In wet soil, however, possible benzene biodegradation resulted in deviation of the diffusion equation from the experimental data. At steady-state, the dimensionless vapor concentration versus distance profile for the dry soil was linear, as opposed to the same profile in the wet soil column. The best fit retardation factor of benzene vapor for wet soil ( R = 12) was lower than that for dry soil ( R = 46), because of a reduction in vapor sorption capacity, due to competition with water molecules. A vapor phase sorption coefficient, K′ d= 5.05 cm 3/g, was computed for the dry soil and K′ d= 0 for the wet soil.