Final Report: Investigation of the Entrapment and Surfactant Enhanced Recovery of Nonaqueous Phase Liquids in Heterogeneous Sandy Media

EPA Grant Number: R825409
Title: Investigation of the Entrapment and Surfactant Enhanced Recovery of Nonaqueous Phase Liquids in Heterogeneous Sandy Media
Investigators: Abriola, Linda M. , Dane, Jacob H. , Pennell, Kurt D.
Institution: University of Michigan , Auburn University Main Campus , Georgia Institute of Technology
EPA Project Officer: Stelz, Bill
Project Period: November 1, 1996 through October 31, 1999
Project Amount: $449,938
RFA: Environmental Fate and Treatment of Toxics and Hazardous Wastes (1996) RFA Text |  Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management , Safer Chemicals


The remediation of aquifers by conventional pump-and-treat technologies is often an inefficient and costly undertaking, particularly when nonaqueous phase liquids (NAPLs) are present. The failure of this technique can be attributed, in large part, to the low aqueous solubility of most NAPLs and their relatively slow rates of mass transfer into the aqueous phase. To overcome such limitations, surfactants have been proposed as a means for enhancing the performance of pump-and-treat systems, based on their ability to increase the aqueous solubility of hydrophobic organic compounds via micellar solubilization and mobilize entrapped NAPLs due to interfacial tension reductions.

Mathematical models are needed to integrate the complex processes involved in surfactant-enhanced aquifier remediation (SEAR) and to serve as design tools in the field implementation of this technology. Based upon the available data, mathematical models for the SEAR process have been developed primarily from laboratory scale observations in homogeneous systems or as extensions of enhanced oil recovery models. Although results from the available SEAR field studies are quite promising, they have tended to be qualitative in nature. In addition, field trials are extremely costly to conduct. Thus, there is a clear need for controlled, larger scale observations of SEAR performance under heterogeneous conditions more representative of the field. Such studies are necessary to refine and validate existing mathematical models for this technology and to explore performance limitations and remediation system design under heterogeneous conditions.

The primary objectives of this research were to: (1) investigate the influence of scale and formation heterogeneity on the entrapment and surfactant-enhanced recovery of NAPLs in saturated sandy aquifer systems; and (2) refine and validate numerical simulators, which may be used for the design and prediction of SEAR performance at the field scale.

To accomplish these research objectives, the project was divided into four tasks and involved the coupling of mathematical modeling with controlled laboratory measurements and experiments. These tasks are:

  • Task I: Laboratory measurement and development of functional expressions of system parameters, not quantified in previous investigations.

  • Task II: Refinement and validation of an existing numerical simulator for enhanced solubilization processes.

  • Task III: Laboratory investigation and numerical simulation of NAPL infiltration and entrapment processes in 2D laboratory sand tank systems.

  • Task IV: Laboratory investigation and numerical simulation of SEAR processes in 2D laboratory sand tank systems.

Two representative NAPLs were selected for study?a dense nonaqueous phase liquid (DNAPL) tetrachloroethylene (PCE), and a light nonaqueous phase liquid (LNAPL) dodecane. A LNAPL and DNAPL were selected for study to allow for evaluation of density effects on NAPL mobilization behavior and plume migration. Ethanol was used as a cosolvent. Three ethoxylated nonionic surfactants representative of the sorbitan, nonylphenol, and linear alcohol families were selected for study. Nonionic surfactants are attractive due to their biodegradability, low toxicity, and relative insensitivity to pH and electrolyte concentration. In addition, they typically can enhance the solubility of organic liquids at relatively low concentrations. An anionic surfactant formulation consisting of a 4:1 mixture of Aerosol MA/OT was used in the mobilization experiments.

Summary/Accomplishments (Outputs/Outcomes):

Research performed under this grant has led to an improved understanding of enhanced solubilization processes in heterogeneous systems and an improved ability to implement field surfactant SEAR projects through the direct measurement of fundamental parameters, laboratory studies of SEAR processes in 2D sand tanks systems containing macro-heterogeneities, and the advancement and application of numerical simulation tools.

Measurement of Fundamental Parameters. Parameter measurements were conducted to quantify the PCE solubilization capacity under varying concentrations of surfactant and alcohol amendments, and to quantify aqueous density and viscosity under varying composition and temperature.

The aqueous solubility of PCE increased by orders of magnitude (150 ppm to 27, 000 ppm) in the presence of 4 percent Tween 80 solution. The addition of ethanol further enhanced the aqueous solubility of PCE.

The aqueous density of 4 percent Tween 80 solutions containing PCE was found to increase linearly with increasing PCE concentration. The aqueous density of 4 percent Tween 80 solutions is greater than pure water at all PCE concentrations, indicating a potential for density controlled downward migration of the solubilized plume. Plume plunging can be a potentially adverse result in field applications, leading to the inadvertent migration of contaminants into regions outside of the capture zone or into low permeable lenses, where they would be more difficult to remove.

The viscosity of 4 percent Tween 80 solutions over a realistic natural temperature range of (10?25?C) was not substantially larger than the viscosity of water, suggesting this surfactant solution can be readily injected into most unconsolidated aquifers. The solution viscosity increased with increasing cosolvent (ethanol) concentration, and with decreasing temperature.

Laboratory Column and Sand Experiments. Enhanced solubilization experiments conducted in laboratory soil columns showed that enhanced solubilization of PCE by solutions of 4 percent Tween 80 is strongly controlled by rate-limited interphase mass transfer. Effluent data from enhanced solubilization experiments in laboratory soil columns were successfully used to develop mass transfer correlations as a function of aqueous velocity under flowing conditions.

Laboratory sand tank experiments were conducted to investigate the influence of surfactant-enhanced interfacial tension reduction on the migration and entrapment of PCE in saturated sandy media. Experimental observations showed that surfactant facilitated interfacial tension reductions substantially lowered capillary resistance to vertical downward migration of PCE, and enabled PCE to enter finer grained, less permeable media that were not penetrated in the absence of surfactant. This result indicates that the use of surfactants to reduce interfacial tension and to enhance mobilization of organic liquids in field settings can potentially result in the migration of organic liquids into finer grained media, where they may be more difficult to remove.

Partitioning tracers were successfully used to estimate average PCE saturation values in the large sand tank system, as verified by gamma radiation measurements of the PCE saturation distribution. A non-equilibrium partitioning tracer inverse method was developed to estimate residual NAPL saturations, and was found to permit a better evaluation of NAPL saturations than the traditional moment analysis, especially in the presence of missing tail data.

Enhanced solubilization experiments were conducted in laboratory sand tank systems. PCE recovery ranged between 65?80 percent for surfactant flush volumes ranging between 5?8 pore volumes. Gamma radiation measurements collected during these experiments in the large tank system provide quantitative data of PCE saturation distributions during SEAR experiments. Fluid density clearly impacted the efficiency of surfactant delivery and contaminant removal in the experiments. Co-solvents (ethanol and NaBr) were effective in adjusting the density of the surfactant solution and improving contact between the surfactant and the free-phase PCE. Ethanol co-solvents also were found to increase the solubilization capacity of the 4 percent Tween 80 solution. These results indicate surfactant enhanced solubilization can be effective in removing organic liquid contaminants from the subsurface, and that the use of co-solvents for density control potentially can improve SEAR efficiency.

Numerical Simulation. A numerical multiphase flow model was used to simulate the laboratory experiments of PCE migration under surfactant-enhanced interfacial tension reduction. Predicted PCE distributions and saturations were compared to experimental observations and measurements. Comparisons indicate that in the absence of surfactant, the immiscible flow model could successfully simulate the general migration behavior of the organic liquid. Model predictions under reduced interfacial tension conditions exhibited poorer agreement with observed migration pathways. At very low interfacial tensions, predicted PCE migration pathways were very sensitive to the capillary pressure scaling factor, grid resolution, and packing variability. These results indicate that numerical simulation models potentially can be used to predict PCE migration pathways under reduced interfacial conditions; however, model accuracy is highly dependent on the ability to characterize fluid and heterogeneous soil properties, as well as on the accurate specification of parametric properties. Accurate predictions also may require the need for very fine domain discretization.

A two-dimensional enhanced solubilization simulator was developed through the modification of an existing multiphase flow and transport model. The model was verified by simulation of the enhanced solubilization experiments conducted in the laboratory column and sand tank systems.

Simulation of the column experiments showed that the accurate prediction of enhanced solubilization requires the incorporation of rate-limited solubilization processes, the quantification of system-dependent interphase mass transfer rates, and the explicit representation of interfacial contact area or a saturation-dependent mass transfer coefficient.

Results from the simulation of enhanced solubilization experiments in the laboratory sand tanks exhibited reasonable agreement with measured surfactant and PCE effluent concentrations and PCE recovery. These results indicate that a column measured, system-dependent mass transfer coefficient can be successfully used to simulate enhanced solubilization processes in sand tank systems containing macro-heterogeneities. Model prediction accuracy was most sensitive to the mass transfer correlation for rate-limited mass transfer, and to the specified PCE distribution, which is controlled by soil heterogeneities. Numerical simulations also showed that composition dependence of surfactant solution properties should be taken into account in numerical simulation models used for the design of field scale SEAR systems. Collectively, simulation results suggest the enhanced solubilization simulator can be used to aid in the design of field scale SEAR systems.

Journal Articles on this Report : 4 Displayed | Download in RIS Format

Other project views: All 23 publications 5 publications in selected types All 4 journal articles
Type Citation Project Document Sources
Journal Article Jalbert M, Dane JH, Abriola LM, Pennell KD. A nondimensional evaluation of tracer sensitivity to density effects. Groundwater 2000;38(2):226-233. R825409 (Final)
  • Full-text: University of Michigan-Full Text PDF
  • Abstract: Wiley-Abstract
  • Journal Article Jalbert M, Dane JH, Bahaminyakamwe L. Influence of porous medium and NAPL distribution heterogeneities on partitioning inter-well tracer tests: a laboratory investigation. Journal of Hydrology 2003;272(1-4):79-94. R825409 (Final)
  • Full-text: ScienceDirect-Full Text HTML
  • Abstract: ScienceDirect-Abstract
  • Other: ScienceDirect-Full Text PDF
  • Journal Article Rathfelder KM, Abriola LM, Taylor TP, Pennell KD. Surfactant enhanced recovery of tetrachloroethylene from a porous medium containing low permeability lenses: 2. Numerical simulation. Journal of Contaminant Hydrology 2001;48(3-4):351-374. R825409 (1998)
    R825409 (1999)
    R825409 (Final)
  • Abstract from PubMed
  • Full-text: ScienceDirect-Full-Text
  • Abstract: ScienceDirect-Abstract
  • Other: ScienceDirect-Full Text PDF
  • Journal Article Taylor TP, Pennell KD, Abriola LM, Dane JH. Surfactant enhanced recovery of tetrachloroethylene from a porous medium containing low permeability lenses: 1. Experimental studies. Journal of Contaminant Hydrology 2001;48(3-4):325-350. R825409 (1999)
    R825409 (Final)
  • Abstract from PubMed
  • Full-text: ScienceDirect-Full-Text HTML
  • Abstract: ScienceDirect-Abstract
  • Other: ScienceDirect-Full Text PDF
  • Supplemental Keywords:

    groundwater, soil, sediment, adsorption, water, drinking water, solvents, organics, DNAPL, NAPL, surfactants, remediation, cleanup, restoration, environmental chemistry, engineering, soil science, geology, mathematics, modeling, measurement methods, analytical chemistry, gamma radiation measurement, Great Lakes, Midatlantic., RFA, Scientific Discipline, Toxics, Waste, Ecosystem Protection/Environmental Exposure & Risk, Remediation, Environmental Chemistry, Chemistry, HAPS, Fate & Transport, Hazardous Waste, Hazardous, Environmental Engineering, SEAR technology, fate and transport, fate and transport , NAPL, contaminant transport, surfactant enhanced aquifer remediation, transport contaminants, dual energy gamma radiation system, chemical contaminants, pump and treat systems, geochemistry, saturated porous media, ecological impacts, hazardous chemicals, assessment methods, heterogenous sandy media, NAPLs, porous media

    Progress and Final Reports:

    Original Abstract
    1997 Progress Report
    1998 Progress Report
    1999 Progress Report