Persistence and Interphase Mass Transfer of Liquid Organic Contaminants in the Unsaturated Zone : Experimental Observations and Mathematical Modeling

Abstract

Surface and subsurface releases of organic chemicals have resulted in widespread contamination of groundwaters and soils. Frequently, such chemicals are introduced into the subsurface as nonaqueous-phase liquids (NAPLs), which are only slightly miscible with water. These organic liquids tend to migrate downward through the unsaturated soil zone, displacing the pore gases under the action of gravitational forces. During its migration, a portion of the NAPL will become entrapped in the soil pores due to capillary forces, creating zones of persistent contamination in the soil matrix. Organic liquid saturation in such zones may range from approximately 4% to 10% of the pore space (Wilkins et al., 1995). This entrapped NAPL may serve as a long-term source of contamination to the aqueous and gaseous pore fluids through subsequent dissolution and volatilization. Soil vapor extraction (SVE) has evolved over the past decade as an attractive in situ remediation technology for unsaturated soils contaminated by entrapped volatile organic compounds (VOCs). This technology involves the induction of gas flow within the porous medium to enhance volatilization of entrapped contaminants (Hutzler et al., 1989). Based upon the success of a number of feasibility studies and the ease of implementation, SVE remediation technologies are currently employed at approximately 18% of Superfund sites (Travis and Macinnis, 1992). An extensive review of the literature pertaining to SVE and related technologies is given in Rathfelder et al. (1995). Although widely implemented, SVE systems are typically designed and installed with limited understanding of the processes that control their effectiveness. Clearly, the performance of SVE will be strongly influenced by contaminant volatility and effective gas-phase permeability (Pedersen and Curtis, 1991). Relatively little is known, however, about the physical and chemical processes that control contaminant vapor-phase mass transfer. The SVE systems characteristically exhibit large initial VOC recovery rates, followed by a rapid decline in effluent gas concentrations to a persistent low level (e.g., Crow et al., 1987; DiGiulio, 1992). Furthermore, a temporary increase in the produced gas organic concentration has often been observed following SVE shutdown periods (McClellan and Gilham, 1992). Such behavior suggests the presence of mass transfer limitations.