A review of literature on the formation, characteristic properties, and flow of emulsion in subsurface environment, and its potential impact on the transport of nonaqueous-phase liquids (NAPLs) is provided. Synthesis of the literature indicates that emulsions can form during petroleum recovery as a result of production of natural surfactants by soil microorganisms, and the use of synthetic surfactants to clean up NAPL-contaminated soils and aquifers. The use of surfactants as a remediation technique to remove hazardous NAPLs from soil and groundwater has been explored in some detail. However, cost-effective and high-efficiency remediation approaches using surfactants have yet to be accomplished. This review suggests that the use of surfactants to remove NAPLs from soil may involve the formation and flow of emulsions in the subsurface environment. The flow of emulsions may influence the transport of hazardous NAPLs in the subsurface environment, depending on the type and stability of emulsion formed under specific conditions. In general, the flow of microemulsion enhances the transport of NAPLs through subsurface environments. Further investigation should focus on the formation, stability, microbial degradation, and flow of emulsion in subsurface environments when surfactants are used as a potential means for recovering NAPLs from soils and aquifers.
