Surfactant-Enhanced Subsurface Remediation - American Chemical

Chapter 18

Modeling the Effectiveness of Innovative Measures for Improving the Hydraulic Efficiency of Surfactant Injection and Recovery Systems 1

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Downloaded by UNIV OF ARIZONA on June 13, 2013 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0594.ch018

Y. Chen , L. Y. Chen , and R. C. Knox 1

School of Civil Engineering and Environmental Science and Institute for Applied Surfactant Research, University of Oklahoma, Norman, OK 73019

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Vertical circulation wells (VCWs) can improve the hydraulic efficiency of surfactant injection/extraction systems used for remediation of subsurface contamination by dense non-aqueous phase liquids (DNAPLs). Simulations using a two-dimensional, glass plate sand tank allowed for visual observation of the flow mechanics of the VCW system and development of gross quantitative measures regarding performance. The VCW system was tested using both surfactant-based remediation mechanisms - solubilization and mobilization. The same tests were completed using a traditional two well (injection/extraction) system. The mobilization mechanism can remove more DNAPL mass per volume of surfactant than solubilization; however, to remain more effective, the mobilized DNAPL mass must be removed before it reaches any diffusion limited zones. The VCW system performs better than the two-well system using either mechanism. The VCW system also provides for more complete recovery of the injected surfactant solution. Concern about subsurface contamination by dense non-aqueous phase liquids (DNAPLs) is widespread because of their existence at a large number of sites, their persistence in the subsurface as trapped residual and/or separate phases, and their ability to contaminate a very large volume of ground water. DNAPLs are denser than water (specific gravity > 1), generally of low viscosity, and are only sparingly soluble in water (7). The Maximum Contaminant Levels (MCLs) for most DNAPLs are at least two orders of magnitude less than their aqueous solubility. The maximum volume of ground water that ultimately may be contaminated by any given spill increases as the ratio of solubility to the volume of contaminant released decreases (2). Some DNAPLs can be of higher viscosity than water and can have high water solubility (i). Due to such conflicting characteristics, DNAPL contaminated subsurface environments are extremely difficult to remediate.

0097-6156/95/0594-0249$12.00/0 © 1995 American Chemical Society

In Surfactant-Enhanced Subsurface Remediation; Sabatini, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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SURFACTANT-ENHANCED SUBSURFACE REMEDIATION

Efforts at remediating subsurface organic chemical (especially DNAPL) contamination have been characterized as being costly, time-consuniing and ineffective (3). The inefficiency of conventional pump-and-treat methods for remediating residual saturation or highly hydrophobic organics has been addressed by several recent reviews. Keeley (4) lists desorption of contaminants from media surfaces and liquid partitioning of immiscible contaminants as limiting factors. Haley et al. (5) determined that containment, rather than remediation, of organic contaminants was usually achieved using pump-and-treat methods. High sorption of organics and residual saturation were again cited as limiting factors. The authors recommended that research focus on methods to enhance extraction of these contaminants from the subsurface. Surfactants are one class of chemical agents that can alter the physicochemical properties of DNAPLs and aquifer materials by (6-7): (1) reducing the interfacial tension between the wetting (generally the aqueous) phase and the nonwetting DNAPL phase; (2) reducing the viscosity of the DNAPL thereby promoting favorable mobility ratios for increased mobilization; or (3) enhancing solubilization of the DNAPL into surfactant micelles. The first two mechanisms can mobilize the DNAPL by releasing trapped oil from residual saturation or by causing the DNAPL and ground water to form a middle phase microemulsion (microemulsification). The third mechanism (enhanced solubilization) can result in DNAPL solubilities several orders of magnitude greater than the normal aqueous phase solubility of the DNAPL. Surfiactant-enhanced remediation processes will require several basic hydraulic steps including: (1) introduction of surfactant solutions to the subsurface; (2) effecting intimate contact between the surfactant solution and the contaminant; and (3) extraction of the resulting sui^actant-contarninant mixture. During each of these steps, surfactants may be subject to losses due to physical and chemical reactions of the surfactant with subsurface materials. Significant masses of surfactant may be precipitated or sorbed in the subsurface (7-9). Also of concern in a surfactant-aided aquifer restoration program is the potential loss of surfactants to uncontaminated portions of the aquifer and the associated chemical costs (10).

Hydraulic Control Measures The technical and economic feasibility of any surfactant-based remediation process will depend on the ability to achieve hydraulic control over the subsurface. Concurrently, it will be necessary to achieve hydraulic control while maximizing hydraulic efficiency. Hydraulic efficiency can be increased by: (1) rninimizing the volume of injected surfactant solution; (2) rninimizing the volume of fluid to be pumped to the surface (reducing treatment costs); (3) targeting injected chemicals to the contaminated zones of the aquifer; (4) preventing the movement of injected fluids towards clean portions of the aquifer; and (5) maximizing capture of resulting watersurfactant-contaminant mixtures. The hydraulic efficiency of surfactant-aided injection/extraction can be dramatically increased by strategically locating and/or operating impermeable and/or hydraulic barriers. Impermeable physical barriers (e.g. grout curtains, slurry walls, sheetpiling) can be used to deflectflowsinto or away from contaminated zones by creating zones of low permeability. Hydraulic barriers (e.g. injection wells,



18. CHEN ET A L

Improving Hydraulic Efficiency of Surfactant Injection

infiltration galleries) can be used to deflect flows into or away from contaminated zones by creating zones of increased hydraulic potential (head). Several authors have also proposed the use of certain operational measures such as cyclic (pulsed) pumping, push-pull pumping, and variable injection/extraction ratios to improve pump-and-treat efficiency (4). A recent numerical modeling study assessed the relative effectiveness of hydraulic and impermeable barriers for improving the efficiency of DNAPL remediation processes, both with and without surfactants (ii). Simple injection of water can improve DNAPL extraction efficiencies but is hydraulically inefficient. Impermeable barriers accompanying injection and extraction wells dramatically improve DNAPL extraction efficiency by increasing the gradient through the contaminated zone and by reducing the volume of fresh ground waterreachingthe extraction wells. The overall conclusion drawn from these results was that mass transfer of the contaminant from the sorbed phase to the "fluid" moving through the contaminated zone should be maximized,regardlessof whether the fluid is air, water, or a chemical solution. Using a surfactant solution as the fluid offers the potential for dramatically improved mass transfer processes. Simple upgradient injection of surfactants followed by downgradient extraction is tremendously inefficient because a significant mass of surfactant is lost to uncontaminated zones and/or does not move through the contaminated zone. Injection of surfactant solutions inside partially encircling impermeable barriers with downgradient deflector wells was found to be most efficient for the surfactant-based processes. The impermeable barrier "cuts off" upgradient water (eliminates dilution of surfactant solution) and prevents migration of surfactant solutions into uncontarmnated areas. The hydraulic barriers (deflector wells) provide increased gradient in addition to directional control. The volumes (mass) of surfactant solution required to exceed the critical micelle concentration (CMC) in the contaminated zone decreased significantly (up to 65%) with barriers over simple injection/extraction (ii). Pulsed pumping was first proposed by petroleum engineers to improve recovery from hydrocarbon reservoirs (12). However, since pulsed pumping has a resting phase, it may not increase the overall massremovalefficiency in remediation applications in terms of time. Disadvantages associated with pulsed pumping that have been identified in laboratory and field studies include increased remediation times, operation and maintenance issues, and lack of necessary hydraulic control (1315).

Vertical Circulation Wells Simultaneous injection to and extraction from a common vertical borehole creates a circulatingflowpattern (Figure 1) within a sphere or ellipsoid around the borehole. These systems arereferredto as vertical circulation wells (VCWs). The potential benefits of the VCW system are many and varied. The VCW system could be applied to DNAPL contamination by injecting a surfactant solution through one screened interval and extracting the svnfactant/ccntarninant mixture from the other screened interval. Some benefits of the VCW system include: (1)reducedcosts over


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SURFACTANT-ENHANCED SUBSURFACE REMEDIATION


18.

CHEN ET AL.

Improving Hydraulic Efficiency of Surfactant Injection

systems involving multiple wells; (2) effective hydraulic control is achieved over limited volumes of the formation; (3) ability to capture NAPL's that might sink when mobilized; (4) can apply the system to both Light NAPLs (LNAPL's or "floaters") and DNAPL's ("sinkers"); (5) minimal loss of surfactants; (6) reduced volumes of fluids requiring treatment produced at surface; and (7) induced mounding can remediate portions of the contaminated vadose zone around the well. Steady state flow induced by the VCW system in an aquifer with a regional gradient can be described using the complex potential, Ω,


Û = Φ +iΨ

(1)

where, Q is the complex potential, Φ is the hydraulic potential, and Ψ is the stream function. Lines of constant Φ are called equipotentials and they describe the head distribution within the aquifer. Lines of constant Ψ are called streamlines and they describe the flow paths of ground water within the aquifer. Referring to Figure 2a, the two screened intervals behave as a line source and a line sink, respectively. By superposition the complex potential for the line source and line sink can be combined, along with the complex potential for a regional gradient (horizontal flow), to produce the overall complex potential for a vertical slice of the aquifer. Using the equations for line sources/sinks andregionalgradient developed by S track (16), the complex potential becomes:

4π

[(Ζ - —) ln(Z - —) + (Ζ + - ) 1η(Ζ + - ) D D D D

- (Ζ - 1) ln(Z - 1) - (Ζ + 1) ln(Z + 1)]

(2)

Ρ - (z - Zj), ) 2 4

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Surfactant-Enhanced Subsurface Remediation - American Chemical

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