Chapter 13
Enhanced Removal of Dense Nonaqueous-Phase Liquids Using Surfactants Capabilities and Limitations from Field Trials
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John C. Fountain, Carol Waddell-Sheets, Alison Lagowski, Craig Taylor, Dave Frazier, and Michael Byrne Department of Geology, State University of New York, 772 Natural Sciences and Mathematics Complex, Buffalo, NY 14260 Results of two pilot field tests suggest that surfactant flushing can be successful under the following conditions: 1) free phase or residual DNAPL is present; 2) the hydraulic conductivity is moderate to high (> 10 cm/sec) and 3) an aquitard is present below the target zone to act as a barrier to vertical migration. These tests indicate that hydrogeologic parameters (aquifer heterogeneities) and contaminant distribution not surfactant performance are the variables that determine the ultimate level of remediation of DNAPL sites. Surfactant performance parameters affect the time and cost required to reach a specific remediation level. Surfactant performance depends upon surfactant type and concentration, contaminants present, water chemistry and aquifer materials (clay content and types, and organic content). Field parameters include the geometry of injection and extraction systems, contaminant distribution and aquifer heterogeneities. These results provide guidance for surfactant selection and for the determination of realistic remediation goals using surfactant enhanced technology. -3
The principal limitations of standard pump and treat remediation is their inability to mobilize most hydrophobic organic solvents or dense non-aqueous phase liquids (DNAPLs) except as dissolved phase. Because DNAPLs have relatively low aqueous solubilities (hundreds to several thousand ppm) and significant sorption within many sedimentary systems (1-9), these organic compounds are relatively immobile in the subsurface and hence are not conducive to rapid extraction by aqueous dissolution. In addition, as they slowly dissolve, DNAPLs act as continuous, long-lived contamination sources which will persist for decades if not centuries and thus will require extensive time frames for remediation unless these DNAPL sources can be removed (8,10-15). 0097-6156/95A)594-0177$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
Surfactants can address this problem by both aiding in locating and remediating DNAPL source zones. Surfactants increase the mobility of the contaminants by combinations of the following three mechanisms: 1) increasing the solubility of the contaminants (4,16-21); 2) reducing their sorption (22,23) and 3) lowering high interfacial tensions (TFT) between water and DNAPLs (19,24). After removal of the DNAPL source, the plume can then be remediated by standard pump and treat. Surfactant enhanced remediation is cost effective for DNAPL sites because the DNAPL zones are generally restricted in size. Due to their high densities, DNAPL motion is primarily controlled by the interaction between gravity and capillary forces, not by horizontal groundwater flow (11,25-28). Thus DNAPLs move downward until a low permeability zone is encountered. This results in minimal horizontal spreading and produces pools, lenses and isolated ganglia of DNAPL (10,29). The dissolved plume however, may extend thousands of feet from the original site of the spill or leak as it is transported by the groundwater flow. Field Demonstration of Surfactant Enhanced DNAPL Remediation To date, two successful field trials have been conducted using surfactant enhanced pump and treat remediation of DNAPL contaminated aquifers under radically different aquifer conditions. Thefirsttrial was conducted from June 1990 to August 1991 at the Canadian Forces Base Borden (CFB) in a clean sand with < 1% clay, < 1000 mg/kg organic content and fresh groundwater. This test occurred i n a 3 m x 3 m x 3 m cell which had been contaminated with 271 liters of PCE in a controlled release. The second trial was conducted from June 1991 to February 1993 at a chlorocarbons manufacturing plant in Corpus Christi, Texas in a fine-grained sand with variable smectitic clay content (115%), little organic carbon (250 - 310 mg/kg) and highly saline groundwater (12,000 ppm total dissolved solids). These sites share the following characteristics: 1) the presence of a well delineated DNAPL zone; 2) a small area of concern ( . 10" cm/sec) and 5) the presence of a thick clay aquitard beneath the target zone to maintain hydraulic control. Core and groundwater analyses from these tests suggest that when surfactants are used, DNAPL mass removal progresses at a rate considerably faster than what would be expected in standard pump and treat techniques. Core data in Table I suggests the following: 1) the surfactant solution rapidly reduced the amount of DNAPL present especially in the zones of higher hydraulic conductivity; 2) contaminated zones showed marked reductions in both maximum concentration and thickness of DNAPL; 3) pools of DNAPL remained at the same elevation throughout both tests indicating little vertical migration occurred due to lowered interfacial tensions and 4) remediation was incomplete at both sites because DNAPL remained in zones of low hydraulic conductivity (Table I). The upper PCE zone at Borden (0-1 m BGS) was perched on a layer within the sand less than 2 cm in thickness. This zone was 3
In Surfactant-Enhanced Subsurface Remediation; Sabatini, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
13. FOUNTAIN ET AL.
Enhanced DNAPL Removal Using Surfactants
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Table I. Core Data for Field Trials at CFB Borden and Corpus Christi, Texas Pore Volumes
Comments
2000
>2000
3
>2000
12.5
>2000
3
