Dispersion and transport of gas-phase contaminants in dry porous media: Effect of heterogeneity and gas velocity

The purpose of these experiments was to study the effects of physical heterogeneity and velocity on transport of gas-phase contaminants in dry porous media. Experiments were conducted at gas velocities ranging from 6 to 200 cm min^-1 to examine the contributions of longitudinal molecular diffusion, hydrodynamic dispersion, and rate-limited diffusive mass transfer to solute spreading. Methane was used as a nonreactive tracer, and trichloroethene, benzene and toluene were used as reactive tracers. Total dispersion of methane during transport through a homogeneous porous medium was the sum of longitudinal molecular diffusion and hydrodynamic dispersion. The latter was the major process contributing to total dispersion at gas velocities greater than 40 cm min^-1. The contribution of longitudinal molecular diffusion was negligible at gas velocities greater than 150 cm min^-1. Transport of tracers in the heterogeneous (macroporous) medium exhibited preferential flow and tailing at gas velocities greater than about 100 cm min^-1 as a result of rate-limited mass transfer between macropore and micropore domains. The spreading associated with rate-limited mass transfer between macropore and micropore domains was the main contributor to total dispersion at gas velocities greater than 120 cm min^-1. The transport of the reactive tracers was successfully predicted using data obtained for the nonreactive tracer.