Environ. Sci. Technol. 2005, 39, 1778-1790
Pilot-Scale Demonstration of Surfactant-Enhanced PCE Solubilization at the Bachman Road Site. 1. Site Characterization and Test Design L I N D A M . A B R I O L A , * ,† CHAD D. DRUMMOND,‡ ERNEST J. HAHN,§ KIM F. HAYES,| TOHREN C. G. KIBBEY,⊥ LAWRENCE D. LEMKE,# KURT D. PENNELL,X ERIK A. PETROVSKIS,O C. ANDREW RAMSBURG,† AND KLAUS M. RATHFELDER× Department of Civil and Environmental Engineering, Tufts University, 113 Anderson Hall, Medford, Massachusetts 02155, HSW Engineering, Inc., 605 East Robinson Street, Suite 308, Orlando, Florida 32801, Latham & Watkins LLP, 633 West Fifth Street, Suite 4000, Los Angeles, California 90071, Environmental and Water Resources Engineering, University of Michigan, Ann Arbor, Michigan 48109-2125, School of Civil Engineering and Environmental Science, University of Oklahoma, 202 West Boyd Street, Carson Engineering Center Room 334, Norman, Oklahoma 73019, Department of Geology, Wayne State University, 0224 Old Main, Detroit, Michigan 48202, School of Civil and Environmental Engineering, Georgia Institute of Technology, 3230 Environmental Science & Technology Building, 311 Ferst Drive, Atlanta, Georgia 30332-0512, GeoSyntec Consultants, 8120 Main Street, Dexter, Michigan 48130, and GeoSyntec Consultants, 838 SW First Avenue, Suite 530, Portland, Oregon 97204
A pilot-scale demonstration of surfactant-enhanced aquifer remediation (SEAR) was conducted to recover dense nonaqueous phase liquid (DNAPL) tetrachloroethene (PCE) from a sandy glacial outwash aquifer underlying a former dry cleaning facility at the Bachman Road site in Oscoda, MI. Part one of this two-part paper describes site characterization efforts and a comprehensive approach to SEAR test design, effectively integrating laboratory and modeling studies. Aquifer coring and drive point sampling suggested the presence of PCE-DNAPL in a zone beneath an occupied building. A narrow PCE plume emanating from the vicinity of this building discharges into Lake Huron. The shallow unconfined aquifer, characterized by relatively homogeneous fine-medium sand deposits, an underlying clay layer, and the absence of significant PCE transformation * Corresponding author telephone: (617)627-3237; fax: (617)6273819; e-mail:
[email protected]. † Tufts University. ‡ HSW Engineering, Inc. § Latham & Watkins LLP. | University of Michigan. ⊥ University of Oklahoma. # Wayne State University. X Georgia Institute of Technology. O GeoSyntec Consultants, Dexter. × GeoSyntec Consultants, Portland. 1778
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products, was judged suitable for the demonstration of SEAR. Tween 80 was selected for application based upon its favorable solubilization performance in batch and twodimensional sand tank treatability studies, biodegradation potential, and regulatory acceptance. Three-dimensional flow and transport models were employed to develop a robust design for surfactant delivery and recovery. Physical and fiscal constraints led to an unusual hydraulic design, in which surfactant was flushed across the regional groundwater gradient, facilitating the delivery of concentrations of Tween 80 exceeding 1% (wt) throughout the treatment zone. The potential influence of small-scale heterogeneity on PCE-DNAPL distribution and SEAR performance was assessed through numerical simulations incorporating geostatistical permeability fields based upon available core data. For the examined conditions simulated PCE recoveries ranged from 94 to 99%. The effluent treatment system design consisted of low-profile air strippers coupled with carbon adsorption to trap off-gas PCE and discharge of treated aqueous effluent to a local wastewater treatment plant. The systematic and comprehensive design methodology described herein may serve as a template for application at other DNAPL sites.
Introduction The prevalence of dense nonaqueous organic-phase liquids (DNAPLs) at contaminated groundwater sites across the United States is well documented (e.g., refs 1-3). Aquifers contaminated with DNAPLs present some of the most costly and difficult remediation challenges. Entrapped and pooled DNAPLs typically serve as a long-term source of aquifer contamination, due to their limited aqueous solubility (on the order of hundreds to thousands of mg/L) and low regulated maximum contaminant levels (MCLs) (frequently on the order of a few to hundreds of µg/L). The region of a contaminated formation that contains DNAPL and serves as a persistent source for an aqueous plume is typically termed a “source zone”. It is now widely acknowledged that conventional pump-and-treat remediation of DNAPL source zones has been ineffective in reducing contaminant concentrations to acceptable regulatory levels (4, 5). The performance of pump-and-treat systems is adversely affected by rate-limited dissolution, spatial variability in subsurface characteristics, and nonuniformity of NAPL distributions (6-9), all of which contribute to increased remediation times and costs. The addition of surfactants, to enhance DNAPL recovery through micellar solubilization and/or interfacial tension reduction and consequent free phase mobilization, offers a promising alternative to traditional pump-and-treat remediation. Surfactants have been demonstrated to be highly effective for DNAPL recovery in a number of laboratory column and sand box experiments (10-15). However, surfactant enhanced aquifer remediation (SEAR) field demonstrations have produced mixed results. In field applications, DNAPL source removal can be adversely influenced by such factors as formation textural variations, fluid density contrast, surfactant losses (16), unfavorable emulsion formation (e.g., ref 10), rate-limited mass transfer (17, 18), changes in solution viscosity, and irregular contaminant distributions. Reviews of SEAR technology field trials for LNAPL- and DNAPL- contaminated sites (19-23) report organic mass 10.1021/es0495819 CCC: $30.25
2005 American Chemical Society Published on Web 02/12/2005
recovery ranging from very little to 75-99%. The most completely documented of the field trials that have targeted chlorinated solvents include those at Canadian Forces Base Borden, Ontario (18, 21, 24), Hill AFB Operating Unit 2 (Hill OU2) (25, 26), and Marine Corps Base Camp Lejeune in North Carolina (27, 28). The Borden site pilot-scale test involved a PCE release and subsequent SEAR treatment using approximately 14 pore vol of a 2% (1:1) (wt) solution of nonylphenol ethoxylate (Alkasurf NP10) and phosphated nonylphenol ethoxylate (Rexophos 25/97). Excavation and a high density of in situ measurements at this site yielded detailed information on the DNAPL migration pathways and facilitated analysis of pre- and post-treatment distributions of DNAPL. Approximately 80% of the PCE was recovered (18); however, general conclusions regarding the effectiveness of SEAR are limited by the test conditions, which were not representative of typical DNAPL source zones. This demonstration employed physical isolation (sheet piling) and there was exact knowledge of the spill volume and an absence of contaminant aging. The two more recent SEAR demonstrations (Hill OU2 and Camp Lejuene) were conducted at previously contaminated sites and used only hydraulic control for test isolation. In both cases the presence of a lower confining clay layer minimized the risk of mass loss from the swept zone due to DNAPL mobilization. At Hill OU2, very high recovery (98.5%) of a chlorinated solvent DNAPL (TCE, PCE, and 1,1,1trichloroethane) was reported, as estimated from pre- and post-treatment partitioning interwell tracer tests (PITT). This demonstration employed approximately 2.4 pore vol of a solution of 8% sodium dihexyl sulfosuccinate (Aerosol MA) and 4% 2-propanol with 7 g/L NaCl (25-26), a high solubilizing formulation that substantially lowered interfacial tensions with the Hill-DNAPL (from 7-4 dyn/cm to 0.2-0.02 dyn/cm) (29). Although such solutions have been shown to cause DNAPL mobilization in the laboratory (10, 12, 30, 31), no free product was collected in the effluent at Hill OU2. Post-treatment sampling and PITT results suggest that any mobilized DNAPL was solubilized before it exited the swept zone (26). At Camp Lejeune, the presence of a continuous lower confining layer led to the selection of a low interfacial tension (IFT) surfactant formulation (a 4% (wt) Alfoterra 145-4PO sulfate surfactant with 16% (wt) 2-propanol and 0.16-0.19% (wt) CaCl2) (27) for PCE recovery. Here approximately 5 pore vol of solution were injected, and 20% of the recovered PCE was collected as mobilized free product. Geologic stratification at the site resulted in an incomplete sweep of the treatment zone, with the surfactant solution failing to penetrate the finer-grained layers (with hydraulic conductivities on the order of 1 × 10-4 cm/s). Subsurface heterogeneity also made it difficult to interpret PITT results, and initial DNAPL mass and mass recovery estimates were judged unreliable (27). A review of SEAR case studies (19-23) revealed that SEAR trials have often failed at sites where subsurface characterization of the site was not performed or where laboratory evaluation of site contaminants and soils was omitted. Generally, the most successful SEAR demonstrations have been at sites that are relatively homogeneous and underlain by a continuous low-permeability (e.g., clay) layer. Although substantial mass removal has been reported, none of the field trials has resulted in the attainment of site closure criteria. The persistence of some DNAPL following every documented SEAR field test suggests that subsurface heterogeneity will inevitably result in some residual contamination. Aquifer heterogeneity contributes to highly irregular DNAPL distributions and diminished effectiveness of SEAR by impeding surfactant delivery to the source zone region (7, 18, 32-34). Additionally, mass transfer limitations during solubilization and variations in permeability result in in-
creased flushing volume requirements (11, 17, 35). Elevated dissolved-phase concentrations resulting from remaining DNAPL following SEAR field tests suggests the need for continued site monitoring and the application of follow-on treatment technologies (21, 26). This two-part paper presents a comprehensive description of the design, implementation, and assessment of a pilotscale SEAR demonstration for DNAPL (PCE) source zone mass recovery at the site of a former dry cleaning facility in Oscoda, Michigan. The small site area, the unknown source release history, the absence of evidence of significant DNAPL pooling, and the need to accommodate ongoing activities in occupied commercial and residential structures are factors that make this site both representative of small DNAPL source zones across the United States and unique among those documented in previous SEAR trials. The favorable hydrogeology (relatively homogeneous, shallow, sandy formation underlain by clay) and low suspected DNAPL saturations made this site an ideal candidate for the application of SEAR using micellar solubilization as the recovery mechanism. Efforts to accommodate physical constraints and to minimize injected surfactant solution volume resulted in a unique SEAR design involving a cross-gradient flushing scheme to provide hydraulic control. Part one of this two-part series presents the integration of site characterization efforts, laboratory experiments, and mathematical modeling for the development of the SEAR pilot-test design. This systematic approach may serve as a template for demonstrations of innovative technologies at other sites. Part two details design implementation, test performance, model/data comparisons, and test evaluation.
Site Background The Bachman Road site is located in northeastern Michigan along the coast of Lake Huron in the town of Oscoda. The Michigan Department of Public Health first detected volatile organic compounds in water samples from private supply wells along Bachman Road in late 1979. Fourteen monitoring wells were subsequently installed and sampled between 1985 and 1989 during a remedial investigation initiated by the Michigan Department of Natural Resources, now the Michigan Department of Environmental Quality (MDEQ). Data from the monitoring wells were used to delineate four distinct plumes of groundwater contamination in the area between US Route 23 and Lake Huron (Figure 1) (36). The southern chlorinated-ethene plume (plume B, Figure 1) was relatively narrow (170 m at its widest point for the 5 µg/L contour) and appeared to emanate from the vicinity of a former dry cleaning facility, discharging into Lake Huron approximately 230 m down gradient (36). Locally, the contaminated formation is composed of glacial outwash sand deposits, with a confining clay layer at approximately 7.6 m below ground surface. Depth to groundwater varies seasonally between 2.4 and 3.0 m, with groundwater flow generally west to east at an estimated rate of 0.13 m/d (36). Between 1992 and 1994, in preparation of a remedial plan for plume B, additional sampling was conducted around the former dry cleaning facility. Free-phase PCE was not found, but aqueous PCE concentrations in excess of 80 mg/L and soil core concentrations in excess of 100 mg/kg were observed at locations downgradient of the former dry cleaning facility. These high levels of contamination suggested the presence of a DNAPL source zone in front (i.e., east) of the building (36). Throughout these investigations, data collected from soil and water samples indicated an absence of substantial concentrations of PCE transformation products (e.g., trichloroethene, cis-dichloroethene, vinyl chloride). In 1997, the MDEQ funded a research team, under the auspices of the US EPA Great Lakes and Mid-Atlantic Hazardous Substance VOL. 39, NO. 6, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Estimated plume boundaries (5 µg/L contour) as established in 1994 impacting residential wells along Bachman Road in Oscoda, MI (data from ref 36). Description of the bioreactive barrier approaches employed at the “halorespiration site” (plume A) may be found in Lendvay et al. (37). Research Center, to conduct a feasibility study for a SEAR pilot-scale demonstration for treatment of the suspected plume B source zone. The feasibility study involved extensive site characterization, laboratory testing of potential surfactant formulations, economic analysis, numerical modeling, and pilot-scale system design.
Site Characterization The site characterization component of the SEAR feasibility study was designed to provide a more detailed understanding of site hydrogeology, to confirm the location of the PCE source zone, and to select an appropriate location for the SEAR pilot-scale test. Characterization of the aquifer material, including grain size distribution (GSD) and organic carbon content, was conducted with sample cores (38 mm diameter) collected using Geoprobe push coring equipment. Initial coring took place immediately in front (east) of the building 1780
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(samples 7-9 in Figure 2), and angled cores 1-6 were collected subsequently, further west beneath the former dry cleaning building. Inspection and analysis of the collected cores indicated that the aquifer was composed of relatively homogeneous, medium- to fine-grained, glacial outwash sands, having total carbon (TC) content of approximately 0.21% (wt). The total organic carbon (TOC) content was determined to be 0.02% (wt), indicating the presence of a large fraction of inorganic carbon (e.g., carbonates). Results from X-ray diffraction analysis indicated that the underlying clay layer consisted primarily of illite and smectite. The contact between the aquifer and the underlying confining unit was found to be gradational, with a sand-silt-clay transition zone (∼0.5 m thickness) present immediately above the lower confining layer. An interval of finer textured material was also observed in some cores in a zone approximately 4 m above the clay confining layer.
FIGURE 2. Overview of sampling locations used for characterization of the Bachman Road site. Grain size distributions were determined by sieve analysis for 167 homogenized core subsamples taken at 0.15-0.30 m intervals (consistent with visual observation of textural variation) from fourteen aquifer cores (cores 1-14 in Figure 2, see ref 38). To remove any bias in mean grain size estimates created by a multimodal gravel fraction present in approximately 10% of the samples, the GSDs were renormalized after excluding the solid fraction having diameters greater than the coarsest sieve size (850 µm). Saturated hydraulic conductivity, K [L/T], was then estimated using the CarmanKozeny relationship (39) for each sample:
K)
(
Ffg dm2 Ffg φ3 k) µ µ 180 (1 - φ)2
)
(1)
where Ff is the fluid (water) density [M/L3], g is acceleration due to gravity [L/T2], µ is the fluid dynamic viscosity [M/LT], k is the intrinsic permeability [L2], φ is porosity [L3/L3], and dm is a representative grain size [L]. Constant head permeameter tests on repacked samples indicated that best fit empirical K estimates were obtained when d10 (the diameter at which 10% of the particles by weight have smaller diameters) was used for the representative grain size diameter in the Carman-Kozeny equation. Estimated K values ranged from 1 to 48 m/d, with a geometric mean value of 16.8 m/d (40). A histogram of estimated K values is presented in Figure SI-1 in the Supporting Information. The second aspect of site characterization studies focused on defining the extent of the PCE source zone. Previous monitoring well and drive point sample analyses suggested that a suspected staging area in front of the former dry cleaning facility was a likely source of PCE contamination. Further characterization, and drive point sampling was undertaken at 14 locations (points A-N, Figure 2). Groundwater samples collected approximately 36 m upgradient (location N, Figure 2) from the building did not contain detectable levels of PCE (i.e.,
