Diffusive mobility of aromatic hydrocarbons in unsaturated and saturated soils

Contamination of soil and groundwater systems by petroleum hydrocarbons is a widespread concern. Use of organic chemicals by refinery and chemical industries has led to an abundance of environmental hazards due to such things as surface spills, leaking underground storage tanks, and broken pipes. The resulting contamination may be in the form of contaminants dissolved in the aqueous phase, sorbed to the soil solid phase, or as a component of a dispersed or continuous non-aqueous phase liquid (NAPL). Development of improved predictive models for contaminant transport in these multiphase systems would assist in human risk assessments and feasibility studies of remediation technologies.; In this study, diffusion of both volatile (benzene and toluene) and non-volatile (phenanthrene) aromatic hydrocarbons was examined. The effect of phase distribution (soil, aqueous, vapor, and NAPL) on contaminant partitioning among the phases was studied. Investigation of sorption from the NAPL to the soil solid phase indicated that contaminant mass distribution among the phases could be predicted by two-phase partitioning data, along with soil pore size distribution and specific surface area data. Adsorption from the NAPL to soil also was shown to be significant in some cases and to be related to the contaminant type, as well as soil organic carbon and specific surface area.; Diffusion through individual vapor and NAPL phases was independently studied in a multiphase system. Argon was used as a vapor phase diffusion tracer in unsaturated soils with a fixed NAPL relative saturation. Results showed that pore filling by NAPL had the same effect on the vapor phase effective diffusivity as pore filling by water. A previously developed parallel resistance model could accurately predict the experimental data. A tracer through a continuous NAPL phase also was studied as a function of NAPL content, water content, and soil type. Mercury intrusion and extrusion analysis were used to estimate pore connectivity. Using these results, a model was developed to predict the diffusion resistance through the NAPL and thus determine the NAPL phase effective diffusivity.; Simultaneous diffusion and sorption mechanisms were studied based on the mobility of phenanthrene through the aqueous phase in unsaturated soils. Phenanthrene diffusion tube experiments were carried out as a function of soil type, moisture content, and aging. Results indicate the phenanthrene diffusive mobility was most affected by dissolved organic carbon (DOC) concentration. This was found to be inversely related to the soil moisture content. Diffusive mobility of phenanthrene through unsaturated soil systems was much greater than that predicted for water saturated soil systems.; Diffusion controlled release of benzene and toluene from both saturated and unsaturated soil beds containing a fixed relative saturation of NAPL was studied. The soil beds were evaluated at various NAPL and water contents. Three soils were examined. A mathematical model, with parameters accounting for diffusion and sorption in each phase, was shown to accurately describe the experimental data.