A Study of Nonaqueous Phase Liquid Characterization and Surfactant Remediation

The objectives of this research were to identify and evaluate environmentally acceptable surfactants that could be used for surfactant enhanced aquifer remediation and to develop a partitioning tracer test method for the detection, estimation, and remediation performance assessment of subsurface nonaqueous phase liquids. First, a systematic approach for selection of surfactants for the remediation of nonaqueous phase liquid contaminated subsurface was developed. The approach entails the observations of the classical Winsor type phase behavior of the surfactant-water-tetrachloroethylene mixtures in calibrated test tubes. The basic observations consist of how many and what type of phases form, how fast equilibrium is approached, and how much tetrachloroethylene is solubilized. The mixtures of sodium diamyl, sodium dihexyl and sodium dioctyl sulfosuccinates were found to yield both Winsor Type I and Type III phase behavior based on the diamyl or dihexyl to dioctyl sulfosuccinate ratio and calcium chloride concentration, which are ideal properties for enhancing the solubility of tetrachloroethylene and lowering the interfacial tension to mobilize the residual tetrachloroethylene phase.The effectiveness of these sodium sulfosuccinate mixtures were then tested in sand column experiments to recover trapped residual tetrachloroethylene. The results of these column experiments show that the selected surfactants have the capacity to simultaneously enhance the aqueous solubility of tetrachloroethylene and to mobilize tetrachloroethylene as a separate phase and remove more than 99% of residual tetrachloroethylene from sandpack columns. The mobilization of trapped residual nonaqueous phase liquid was investigated from theoretical and experimental perspectives. A trapping number which combines the effects of buoyancy and viscous forces was derived. This trapping number option was implemented in UTCHEM, a three-dimensional chemical flood simulator developed at The University of Texas at Austin. From a practical perspective, the trapping number provides a basis for assessing the potential for dense nonaqueous phase liquid mobilization during surfactant remediation operation and minimizes the possibility of the uncontrolled downward migration of mobilized dense nonaqueous phase liquids into uncontaminated parts of the aquifer. The most important contribution of this research is the development of a new partitioning interwell tracer test method for detection, estimation, and remediation performance assessment purposes. The method can be used in both saturated and unsaturated soils contaminated by either light or dense nonaqueous phase liquids. The key elements that need to be considered when designing and interpreting a field-scale tracer test are discussed. Validation of this technique is illustrated by both laboratory column experiments and numerical simulation results. Finally, the design of two field partitioning interwell tracer tests, an unsaturated zone partitioning interwell tracer at the site of the Chemical Waste Landfill at Sandia National Laboratories and a saturated zone partitioning interwell tracer in an isolation test cell within Operable Unit 1 at Hill Air Force Base in Utah is presented.