The transport of volatile organic compounds in unsaturated porous media will be studied in the laboratory. The emphasis will be on processes that dominate transport over a broad areal extent. These include gas diffusion, gas advection, mass transfer between the soil water and soil air phases, and sorption onto soil particles. A bench scale apparatus will be constructed to study these processes individually. In the unsaturated zone, gas phase transport-- both gas diffusion and gas advection--is a primary pathway by which volatile organic compounds migrate. Five factors that affect gas transport in the unsaturated zone will be studied using the bench scale apparatus. These are the soil moisture content, the gradient in atmospheric pressure, the concentration of the diffusing gas, the temperature, and advective flow of water. In addition to gas diffusion and advection, transport in the unsaturated zone is influenced by the dissolution of the vapor phase compound into the water phase. This is true both in equilibrium systems--when Henry's law governs the distribution of the compound between the phases--and in nonequilibrium systems--for instance, during infiltration (Cho and Jaffe, 1990; Cho, 1992). The soil gas-soil water mass transfer rate coefficient will be quantified using the bench scale experimental apparatus over a range of moisture contents, advective fluid flow rates, atmospheric pressures, and temperatures. It has been observed that compounds bound to the solid matrix of a porous medium can act as long-term sources of contamination in groundwater aquifers (Cho, 1992; Smith et al., 1990). The sorptive and desorptive fluxes of such sorbed contaminants into the fluid phases will be quantified, again using the bench scale experimental cell. Finally, pilot scale experiments will be conducted using a pilot apparatus used in previous vadose zone experiments (Cho, 1992). In the pilot scale experiments, the individual transport processes described above will be examined in consort.
