Transport of petroleum hydrocarbon vapor components in the subsurface; a laboratory soil column study

The diffusive transport of volatile organic compounds (VOCs) in the subsurface at petroleum spill sites can significantly affect vapor migration from sources to buildings and ground surface. Thus, knowledge of compound-specific vapor transport and bio-attenuation is of great interests to those who must identify risks and make corrective action decisions for petroleum spill sites. In this work, the vapor transport of individual compounds in complex petroleum vapor mixtures is being studied for idealized lithologies in 2-m (6-ft) tall laboratory soil columns. Six columns, representing different geological settings were prepared using 40-60 mesh sand (medium grained) and 16-minus mesh crushed granite (fine-grained). The contaminant vapor source is composed by twelve petroleum hydrocarbons that typify weathered gasoline. The liquid hydrocarbon mixture is placed in a chamber at the bottom of each column, and the vapors are allowed to diffuse upward through the soil to a chamber at the top of the columns, which is swept with humidified gas. The contaminant source vapor concentration is maintained constant throughout the experimental period by periodic replacement of the liquid. The experiment is conducted for two cases: i) anaerobic conditions, in which the sweep gas at the top of the column is nitrogen and no biodegradation is expected to occur; and ii) aerobic conditions; in which, air is the sweep gas; this phase will performed in the near future. Soil diffusion coefficients, and oxygen, carbon dioxide and hydrocarbon vapor concentration of each chemical are monitored over time in the column and the effluent sweep gas. The data allow determination of compound-specific flux and times for steady profiles to be achieved. The anaerobic phase of the experiment is ongoing. Results show that vapor transport is highly influenced by the chemical and physical properties of the chemicals, soil moisture, soil effective diffusion coefficients and geological settings.