HYDROCARBON VAPOR TRANSPORT IN LOW MOISTURE SOILS

Gas-phase transport of hydrocarbons in unsaturated soils is important in the assessment and remediation of. volatile subsurface contaminants and other environmental applications. This paper presents models and measurements of diffusion, dispersion, and retardation parameters developed for several hydrocarbon vapors in unsaturated soils using laboratory column systems. Diffusion coefficients are determined in dry soils using steady-state methods. Retardation and dispersion parameters are found using transient methods and a range of soil moisture contents below the saturation soil moisture content. Media tested included sand, fill, loam, and glass beads as controls; contaminants included toluene, trichlorethylene, and methane, The experiments clearly demonstrate the role of humidity in absorption and transport of vapors in soils. Soil gas humidities below 30% result in considerable retardation of hydrocarbon vapors in all media. Retardation factors range up to 80 for toluene and to 46 for trichloroethylene, Retardation coefficients decrease but remain large with increasing humidity in organic-rich soils. No significant retardation is seen for methane. Based on soil-water isotherms, these results suggest competitive sorption between hydrocarbon and water vapors on soil surfaces, especially the mineral fraction.