In-Situ Alcohol Flushing of a DNAPL Source Zone at a Dry Cleaner Site

former dry cleaner site located in Jacksonville, Florida. This study was .... Present address: School of Civil Engineering, Purdue University,. West L...
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Environ. Sci. Technol. 2000, 34, 3722-3729

In-Situ Alcohol Flushing of a DNAPL Source Zone at a Dry Cleaner Site J A M E S W . J A W I T Z , †,‡ R A N D A L L K . S I L L A N , §,# M I C H A E L D . A N N A B L E , * ,† P . S U R E S H C . R A O , ‡,§ A N D KEVIN WARNER# Interdisciplinary Program in Hydrologic Sciences, Department of Environmental Engineering Sciences, Soil and Water Science Department, and University of Florida, Gainesville, Florida 32611, LFR Levine-Fricke, Inc., Tallahassee, Florida 32308

A pilot-scale field test of in-situ alcohol flushing for enhanced solubilization and extraction of a dense nonaqueous phase liquid (DNAPL) source zone was conducted at a former dry cleaner site located in Jacksonville, Florida. This study was conducted to evaluate the feasibility of in-situ flushing for remediation of DNAPL sites in Florida. Groundwater at this site was contaminated with tetrachloroethylene (PCE) that had migrated below the water table, located at 3 m below ground surface (bgs), and collected at high saturations in thin, discontinuous layers in the 7.9 m to 9.4 m bgs depth interval. An oblong source zone (7.3 m × 2.7 m) was delineated using direct-push technologies and further characterized using soil coring and partitioning tracer techniques. Tracer tests and in-situ alcohol flushing were conducted using three injection wells that approximately bisected the source zone and six recovery wells located on the outer perimeter of the source zone. Overextraction through the recovery wells ensured hydraulic containment within the test zone. A partitioning tracer test conducted before the alcohol flood provided an estimate of about 68 L of PCE within the zone swept by the wells. The test zone was flushed with 34 kL (equivalent to 2 pore volumes) of a 95% ethanol/5% water mixture over a period of 3 days. Packers were used in the injection wells to focus the flushing solution delivery to regions of the swept zone that showed larger initial NAPL saturations. Alcohol flushing removed approximately 43 L of PCE from the test zone (62% removal effectiveness). These results were in agreement with soil core data that indicated approximately 65% removal and a postflushing partitioning tracer test that indicated approximately 26 L of PCE remaining (63% removal). Postflushing groundwater concentrations of PCE were an average of 92% lower than preflushing values at 21 of 35 multilevel sampling locations within the test zone, but the combined effects of residual ethanol and incomplete flushing resulted in elevated postflushing PCE concentrations at the other 14 locations. Alcohol flushing successfully removed a substantial volume of DNAPL; however, evidence indicated that continued alcohol flushing would have resulted in a greater NAPL removal effectiveness.

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Introduction Experience at a number of sites has led to the recognition that even small nonaqueous phase liquid (NAPL) source zones can generate extensive dissolved plumes and that traditional pump-and-treat techniques designed to provide hydraulic containment of plumes often require long-term operation (1-3). These findings have prompted the development of alternative technologies to provide enhanced remediation of NAPL source zones (2, 3). Such innovative technologies require extensive field testing in diverse hydrogeologic and contaminant settings in order to establish credibility and determine their applicability, transferability, and eventual commercialization (3). Several innovative remediation technologies were recently field-tested in a sideby-side comparison at a single site at Hill Air Force Base (AFB), UT (4). Many of the technologies tested at Hill AFB relied on in-situ flushing with agents such as alcohols, surfactants, or cyclodextrins to either solubilize or mobilize the NAPL. Published results from several of these studies have established the feasibility of in-situ flushing techniques for NAPL source zone remediation (5-8). In addition, it should be noted that many of these in-situ flushing technologies have been adapted from petroleum industry techniques for enhanced oil recovery (e.g., ref 9). Each of the Hill AFB experiments cited above was conducted within a hydraulically isolated test cell constructed of interlocking sheet-pile walls (e.g., ref 10). At the Hill AFB site, the NAPL was lighter than water (LNAPL) and was located in a 2-m thick zone as a result of water table fluctuations (5, 6). Conversely, sites contaminated with denser-than-water NAPLs (DNAPLs) are often characterized by sparsely distributed, thin, disconnected layers of NAPL accumulations above zones of subtle changes in hydraulic conductivity or geologic formation structure (e.g., ref 11). The discontinuous nature of the spatial extent and vertical distribution of residually trapped or pooled DNAPLs complicates site characterization and the effective design of aggressive remediation measures. This paper reports a pilot-scale test of in-situ alcohol flushing for enhanced remediation of a DNAPL source zone where isolation of the test zone was achieved entirely by hydraulic containment (i.e., no sheet-pile walls). The field study was conducted at the site of a former dry cleaning operation in Jacksonville, FL where the shallow, unconfined, sandy aquifer had become contaminated with tetrachloroethylene (PCE), commonly used as a solvent in dry cleaning operations. The DNAPL source-zone was delineated using soil cores, groundwater samples, partitioning tracer tests, and direct-push cone penetrometer methods. Enhanced DNAPL solubilization was achieved by in-situ flushing with 34 kL (equivalent to 2 pore volumes, PV) of a 95% ethanol/ 5% water mixture that were delivered over a 3-day period. These experiments were conducted between June and September 1998. The pilot test described here is the first field-scale demonstration of in-situ alcohol flushing for enhanced remediation of a DNAPL source zone. This field test was * Corresponding author phone: (352)392-3294; fax: (352)392-3076; e-mail: [email protected]. † Interdisciplinary Program in Hydrologic Sciences, Department of Environmental Engineering Sciences, University of Florida. ‡ Present address: School of Civil Engineering, Purdue University, West Lafayette, IN 47907. § Interdisciplinary Program in Hydrologic Sciences, Soil and Water Science Department, University of Florida. # LFR Levine-Fricke, Inc. 10.1021/es9913737 CCC: $19.00

 2000 American Chemical Society Published on Web 07/26/2000

FIGURE 1. Site map of well, MLS, and soil core boring locations within the approximated DNAPL source zone (units are Florida State Plane Coordinate System). conducted to evaluate the feasibility of in-situ alcohol flushing at this and other dry cleaner sites managed by the Florida Department of Environmental Protection. As such, this pilot test was conducted under a more-limited budget than fullscale implementation would have required. Given the regulatory and financial constraints, the in-situ alcohol flushing strategy was developed based on the following criteria: efficiency of DNAPL extraction, potential for DNAPL mobilization or fugitive emissions, and waste disposal costs. A related objective was to monitor the source zone and the dissolved plume for an extended period (approximately one year) following in-situ flushing to examine changes in the geochemical processes and microbial reductive dechlorination of PCE. This information will be used in subsequent studies on linking abiotic remedial measures with biotic approaches to achieve site cleanup.

Field Methods Field Site Background. Details of the characteristics of the subsurface matrix and an initial contamination assessment can be found elsewhere (12); however, a summary is provided below. The water table was approximately 3 m below ground surface (bgs), with a natural hydraulic gradient of approximately 0.0025. Fine to very fine sand (i.e., grain diameters, φ, in the range 0.0625 mm-0.125 mm) occurred at the site to a depth of approximately 9 m bgs, below which a 1.7 m layer of very fine to silty sand (0.002 mm < φ < 0.0625 mm) was encountered. The average saturated hydraulic conductivity, estimated from slug test data, was about 6 m/day in the upper sand unit (0-9 m bgs) and about 3 m/day in the lower sand to silty sand zone (9-10.7 m bgs). A thin clay layer (0.15 to 0.3 m thick) was detected at approximately 10.7 m bgs in most soil borings; however, the clay layer was judged to be discontinuous. Very fine sand to silty fine sand was observed beneath the clay layer. The PCE release history at this site was unknown; however, the suspected release location was a former floor-drain sump open directly to the subsurface (Figure 1). Concentrations of PCE in groundwater samples collected from existing monitoring wells during the initial site assessment ranged from 70 to 150 mg/L. About 15 cm of free-phase PCE were detected at the bottom (9.8 m bgs) of a 2.5-cm diameter former supply well (Figure 1). The screened interval for this well was unknown. After the recovery of approximately 100 mL of PCE from this well, subsequent site visits over a period of

several months revealed that additional PCE (typically 10 to 20 mL) had entered the well. This indicated that, at some depth above the bottom of the well, DNAPL existed at high enough saturations to form a continuous, flowing phase. Flow Configuration and Design. Preliminary site characterization work, conducted using direct-push cone penetrometer (CPT) technologies, identified an oblong PCE source area of approximately 7.3 m × 2.7 m in the depth range of 7.9 m to 9.4 m bgs (12). Numerical simulations were conducted by Sillan (12) to select a well configuration pattern based on how effectively the source zone would be swept by the flushing fluids. The simulation domain was homogeneous with a uniform thickness of 1.5 m and a saturated hydraulic conductivity of 6 m/day (equal to that measured in the upper sand unit). A well configuration of six recovery wells (RWs) encircling three injection wells (IWs), forming two four-spot well patterns on either side of a five-spot pattern (Figure 1), was selected in preference to a double four-spot pattern based on improved sweep of the source zone (12). Simulated streamlines for both of these well configurations are included as Supporting Information. The locations of the six recovery wells were selected to be just outside the perimeter of the initial estimated horizontal extent of the PCE source zone (Figure 1). All wells were constructed of 10-cm diameter poly(vinyl chloride) and were installed with a 25-cm diameter hollow stem auger. A sand pack was used in the screen interval, and a bentonite plug was placed immediately above the screen zone. The three IWs were screened from 7.6 to 9.9 m bgs, while the six RWs were screened from 7.9 to 9.6 m bgs. This design was selected to promote upward flow of the injected fluids. A total flow of 15.1 L/min was distributed equally to the three IWs. The interior recovery wells (RWs 3, 4, 6, and 7) had extraction rates of 5.9 L/min, and the outer two wells (RWs 2 and 5) were assigned extraction rates of 3.4 L/min, resulting in a 2:1 extraction-to-injection ratio. During all experiments, steady-state flow was maintained such that the water table position (located approximately 5 m above the flushed zone) remained constant. Seven multilevel samplers (MLSs) were installed within the NAPL source zone (Figure 1), with five sampling depths (8.1, 9.7, 9.1, 9.4, and 9.9 m bgs) at each MLS. Source Zone Characterization. The NAPL source zone was further characterized both before and after alcohol flushing using soil cores, groundwater samples, and interwell partitioning tracer tests (13, 14). Detailed descriptions of the field methods and laboratory analytical protocols can be found in Sillan (12). Soil Cores. Soil cores were collected using hollow-stem auger and split-spoon barrel methods during installation of the wells and using CPT methods during installation of the MLSs. Soil cores were collected every 60 cm and subsampled at 3 cm intervals for methylene chloride extraction. For each subsample, approximately 10 g of soil were placed in 40 mL vials containing 5 mL each of methylene chloride and water. The samples were transported to the University of Florida for PCE analysis by gas chromatography. A total of 251 subsamples was collected from 10 borings (3 IWs and 7 MLSs) within the source zone prior to alcohol flooding, and 61 subsamples were collected from 6 CPT borings following alcohol flushing (see Figure 1 for postflushing soil core locations). Grain size distributions were also measured from soil cores; however, these measurements were not made at the same resolution as the PCE concentrations. Note that the depth to which the wells were installed, and thus the depth to which preflushing soil cores were collected, was limited to 9.9 m by regulatory constraints because of concerns about puncturing the relatively thin clay layer at 10.7 m. After the removal of a substantial amount of PCE VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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during the alcohol flood, this constraint was relaxed for postflushing coring. Groundwater Samples. Groundwater samples were collected from all 35 MLS locations both before and after cosolvent flushing for PCE analysis by gas chromatography. The postflushing samples were also analyzed for ethanol. Partitioning Tracers. Partitioning tracer tests were conducted before and after alcohol flushing using methods similar to those described by Annable et al. (14). In the preflushing tracer test, methanol (C0 ) 2200 mg/L) was used as a nonpartitioning tracer and n-hexanol (C0 ) 820 mg/L), 2,4-dimethyl-3-pentanol (DMP, C0 ) 440 mg/L), and 2-ethyl1-hexanol (e-HEX, C0 ) 490 mg/L) were used as partitioning tracers. For both the pre- and postflushing tracer tests, a tracer pulse of approximately 0.20 pore volumes (3.8 h) was delivered to the IWs during steady water flow using the flow distribution described above. Throughout the tracer injection and displacement periods, samples were collected from the RWs, IWs, and MLSs at frequent intervals (ranging from 1 to 8 h). Data from analyses of these samples yielded tracer breakthrough curves (BTCs). Incomplete BTCs were exponentially extrapolated (e.g., ref 15). Temporal moment analysis was applied and the average PCE saturation, SN, within the swept volume of each recovery well was determined using the following equation (adapted from 13)

SN )

R-1 KN,2 - 1 -R(KN,1 - 1)

(1)

where the retardation factor, R, is the ratio of the pulsecorrected, mean arrival times of two tracers, and KN is the tracer PCE-water partitioning coefficient. The measured values of KNsdetermined using batch equilibration methods with PCE collected from the sitesfor methanol, n-hexanol, DMP, and e-HEX were 0, 6, 20, and 81, respectively. Jin (16) showed that errors in the SN estimate increase rapidly for R values of less than 1.2. In these experiments, R was greater than 1.2 only for e-HEX. Thus, for the preflushing partitioning tracer test, SN was determined from the methanol and e-HEX data. In the postflushing tracer test, potassium iodide and tert-butyl alcohol were used as nonpartitioning tracers instead of methanol because of analytical interferences with the in-situ flushing agent (ethanol). However, analytical interference problems also precluded quantification of the iodide and tert-butyl alcohol. Therefore, in the postflushing tracer test, SN was determined using the n-hexanol and e-HEX data. The total PCE volume, VN, within the swept volume of each well was determined by multiplying the average SN and the swept volume, VS, determined for each well. Because a 2:1 extraction-to-injection ratio was required for hydraulic containment, the RWs drew in large amounts of groundwater from outside the zone swept by the tracers. Thus, the nonpartitioning tracer mean arrival times were not strictly representative of the swept volumes for the wells. Therefore, the swept volumes of the RWs were determined by scaling the nonpartitioning tracer mean residence times (i.e., pulsecorrected, first normalized temporal moments, tn) to the fractional recovery of the injected tracer mass at each RW

VS ) fnQitn

(2)

where Qi is the total flow rate at the IWs and fn is the mass of nonpartitioning tracer recovered at the RW divided by the total mass of nonpartitioning tracer that was injected. In Situ Alcohol Flushing. A total of 34 kL (2 PV) of 95% ethanol was delivered to the three IWs during a period of about 3 days. A neoprene-rubber well packer was placed in each injection well in order to focus delivery of the alcohol 3724

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to zones of high SN. Alcohol was injected below the packer, while potable water was injected above the packer to limit migration of the alcohol into the aquifer above the packer. The combined flow rate of water and alcohol (Qi) was held constant at 4.2 L/min for IW-1 and IW-3 and at 6.8 L/min for IW-2. The rate of alcohol delivery (Qc) to each IW was determined from the height (h) of the packer above the bottom of the well screen and the total flow rate per well screen length

Qc ) h

Qi ls

(3)

where ls is the screened length of the IWs (2.3 m). The water flow rate was the difference between the total flow rate and the alcohol flow rate at each IW. Initially, h was set at 0.3 m in order to deliver alcohol to the zone immediately below the deepest detected free-phase PCE. The flooding of this region with alcohol before flushing the PCE-contaminated zone was intended as a precaution to mitigate adverse impacts of potential PCE mobilization. If downward mobilization of free-phase PCE occurred at the alcohol injection front in the contaminated region, a layer of alcohol solution would be available below to solubilize fugitive PCE. Complete dissolution of mobilized PCE would depend on the ratio of the rates of PCE downward migration and solubilization in ethanol. The ethanol concentration in the injection fluid was ramped from 0 to 95% over 10 h (0.5 PV) using a continuously stirred tank reactor (CSTR) that had a tank volume of 340 L. The gradient injection of alcohol was implemented as a means of reducing potential flow instability effects (5, 17) caused by differences in fluid density, F, between the resident groundwater (F ) 1.0 g/cm3) and the 95% ethanol/5% water cosolvent solution (F ) 0.8 g/cm3). The 95% ethanol/5% water mixture was injected below the initial packer height for 6 h (0.3 PV), and the packers were then raised at a rate of approximately 15 cm/hr to a maximum height of 1.7 m at IW-1 and IW-2 and 0.9 m at IW-3. The maximum packer heights were chosen based on the locations of PCE detected in soil cores. After 70 h of flushing (3.7 PV), the packers were lowered at the same rate to a final height of 30 cm. This scheme was employed at the latter stages of the ethanol flood to facilitate the subsequent removal of the ethanol mixture from the swept zone by minimizing the potential for gravitational segregation of the ethanol mixture and the displacing water (e.g., ref 18). Note that because of the efforts to minimize the potential for flow instabilities and uncontrolled NAPL migration, approximately 76 kL (4.5 PV) of water and ethanol were flushed through the NAPL source zone in order to deliver 34 kL of ethanol (2.0 PV). Water flooding was initiated after 3.5 days (4.5 PV) to remove the remaining ethanol and was continued for 4.5 days (i.e., an additional 5.8 PV). Waste Management. To minimize costs associated with the disposal of waste containing high levels of PCE, a macroporous polymer extraction (MPPE) system, developed by Akzo Nobel, was used to separate PCE from the effluent solution that contained ethanol, water, and dissolved PCE. This technology uses a porous polyolefin material with a proprietary extraction fluid immobilized within its structure (19). During treatment, the waste stream was passed through a column containing MPPE material into which the PCE preferentially partitioned. After loading with PCE, the columns were regenerated with low-pressure steam stripping. The steam extracted the PCE from the MPPE material and was then condensed and separated into free-phase PCE for disposal. Waste fluids containing greater than 1% ethanol were transported for off-site disposal, while those containing less than 1% ethanol were processed through an air stripper and discharged on site.

FIGURE 2. Preflushing PCE soil concentration distribution at all borings with concentrations greater than 100 mg PCE/kg soil.

Results and Discussion Preflushing DNAPL Distribution Characterization. Soil Cores. High PCE concentrations were detected in soil cores collected at all three IWs and MLSs 1 and 4 (Figure 2). The samples collected at these locations were all within the vertical interval of 7.9 to 9.6 m bgs, with an average concentration of 2800 µg PCE/g soil and a standard deviation of 11,000 µg PCE/g soil (n ) 146 samples). These values resulted in a coefficient of variation (CV, defined as the standard deviation divided by the mean) of 4.0. Note that several gaps exist in the data because of poor soil recovery during coring. The average concentration measured in the 105 soil samples collected from the other 5 MLSs (2, 3, 5, 6, and 7) was only 10 µg PCE/g soil. Thus, it was concluded that these locations were more likely to be outside of the PCE source zone determined by the cores collected from the IWs and MLSs 1 and 4. A complete table of the soil core data has been included as Supporting Information. The high-frequency sub-sampling technique delineated thin (5 to 8 cm thick) layers of PCE that were not necessarily horizontally continuous over the extent of the source zone (Figure 2). An important consequence of this distribution was that a large fraction of the nearly uncontaminated aquifer had to be flushed in order to contact the PCE present in a few thin layers. Thus, large volumes of tracer and alcohol solutions were likely to completely bypass the PCE, making detection difficult and in-situ flushing remediation an inefficient process. Groundwater Samples. The depth-averaged aqueous PCE concentrations at the seven MLSs were 77, 65, 76, 76, 0.1, 38,

and 40 mg/L, respectively (a complete table of the groundwater data has been included as Supporting Information). The average concentration from all 35 sampling locations was 53 mg/Lsapproximately one-third of the PCE aqueous solubility limit of 150 mg/L. Note that the hydraulic gradient was generally in the direction from MLS 5 to MLS 1 (Figure 1) and that the very low average concentration at MLS 5 suggests that this location was most likely upgradient from the PCE source zone. Partitioning Tracers. Tracer BTCs measured at RWs 3, 6, and 7 are shown in Figure 3 along with exponential extrapolations. The NAPL volume, VN, estimates at these locations were significantly higher than at the other three recovery wells (Table 1). Note that at all three wells shown in Figure 3, there was a lack of separation in the BTC peaks for the different tracers and that all of the measured retardation occurred in the BTC tails (similar behavior was also observed at RWs 2, 4, and 5). These data suggest that only a small fraction of the zone swept by the tracers contained NAPL, while most of the streamlines were in uncontaminated zones (20). This result is consistent with the soil core data. The fractional recoveries of the total injected mass of methanol, the methanol mean residence times, and the corresponding well swept volumes (determined from eq 2) are presented in Table 1 for all six RWs. Note that 90% of the injected methanol mass was recovered at the RWs. The VN estimates determined from the e-HEX data are also presented in Table 1 for each RW. The largest quantities of PCE (>50% of the total) were found in the swept volumes of two wells VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Preflushing partitioning tracer BTCs measured at RWs 3, 6, and 7; methanol (0) was the nonpartitioning tracer, and e-HEX (3) was the partitioning tracer. Dashed lines are exponential extrapolations.

TABLE 1. Fractional Recoveries, Swept Volumes, and NAPL Volumes for Each Recovery Well VN RW

f (methanol)

tn (h)

VS (kL)

f (ethanol)

preflushinga (L)

extracted (L)

postflushinga (L)

2 3 4 5 6 7 Total

0.074 0.243 0.232 0.031 0.153 0.171 0.904

22.4 14.5 14.1 39.1 33.5 23.0 23.7b

1.52 3.25 3.02 1.12 4.72 3.63 17.3

0.059 0.252 0.177 0.023 0.183 0.228 0.922

4.9 (0.0032) 18 (0.0055) 7.1 (0.0023) 6.4 (0.0057) 11 (0.0023) 21 (0.0058) 68 (0.0039)

1.0 11 3.7 0.02 12 15 43

2.8 (0.0018) 4.1 (0.0013) 4.9 (0.0016) 3.6 (0.0032) 4.6 (0.0010) 5.9 (0.0016) 26 (0.0015)

a

SN values listed parenthetically, where SN ) VN/(VS + VN). b Swept volume weighted average arrival time.

(RWs 3 and 7). These two wells were on opposite sides of the sump that was the suspected point of PCE entry into the subsurface. The total estimated volume of PCE within the swept zone of the wells was approximately 68 L, equivalent to an overall average SN of 0.004. In-Situ Alcohol Flushing. The initial flow rates from the RWs were similar to those maintained during the preflushing partitioning tracer test. However, flow rates were varied during flushing in an effort to optimize the volume of fluid applied to a given swept zone based on the estimated volume of PCE initially present. A detailed description of these efforts can be found in ref 12. The BTCs for ethanol and PCE at the six RWs are shown in Figure 4. With the exception of RW 5, the PCE concentrations substantially increased with the arrival of ethanol at each well. At RW 5, the maximum ethanol concentration was approximately 4.5%, and the PCE concentration did not increase beyond 2 mg/L, indicating that very little of the injected ethanol flowed to this well. At RWs 3 and 7, the PCE concentrations began to decrease prior to the start of water 3726

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flooding, indicating that most of the PCE within these swept volumes had been removed prior to the initiation of water flooding. At RWs 2, 4 and 6, the PCE concentrations decreased with the ethanol fraction, indicating that continued ethanol flushing would have resulted in a larger mass of PCE recovered from these swept volumes. The highest PCE concentration (1300 mg/L) was observed in RW 7, which also had the largest initial volume of PCE in its swept zone, as estimated from the tracer test. Note that dilution resulting from 2:1 over-extraction at the RWs was manifested as reduced measured flux-averaged concentrations of ethanol and PCE compared to those in fluids arriving at the RWs. For example, the peak ethanol concentrations measured for all depths at MLS 4 were 90 to 95% (data not shown here, see ref 12), nearly equal to the maximum injected concentration and approximately double the maximum values measured at RW 4 (45%), the RW closest to this MLS. The peak concentrations of PCE measured at the MLSs were also significantly higher than those observed at the RWs, due to both the over-extraction at the RWs and the sparse

FIGURE 4. Ethanol (0) and PCE (4) BTCs measured at all six RWs. distribution of PCE within the flow domain. For example, the PCE concentrations measured at MLS 4 were approximately 40 to 112 times the maximum PCE concentration observed at RW 4 (400 mg/L). The total amount of PCE extracted was approximately 43 L (69 kg), determined from numerical integration of the BTCs shown in Figure 4 (Table 1). Substantially more PCE was extracted from RWs 3, 6, and 7 than from RWs 2, 4, and 5, which is in agreement with the preflushing partitioning tracer estimates of the amount of PCE within the swept volume of each well (Table 1). Numerical integration of the ethanol BTCs shown in Figure 4 indicated that approximately 92% of the injected alcohol had been recovered by the end of the 5.8 PV water flood. Waste Management. Considerable costs and management were required for the treatment and disposal of the extracted groundwater containing ethanol, PCE, and tracers. Approximately 35 L of PCE were separated from the waste stream by the MPPE system, representing a recovery of 83% (the effluent contained approximately 43 L of PCE). The treated

fluids had high ethanol contents but could not be re-injected because of state regulatory constraints. Approximately 640 kL of waste liquids were transported off-site to an industrial wastewater treatment facility. PCE Removal Effectiveness. The PCE removal effectiveness, defined as the fraction reduction in the amount of PCE initially present, was determined from soil cores, groundwater samples, and partitioning tracer data. Soil Cores. The locations of the six postflushing soil borings were selected to be within the PCE source zone defined by the preflushing soil cores collected from the three IWs and MLSs 1 and 4, within the physical limitations imposed by the presence of existing instrumentation and prior boreholes (Figure 1). High PCE concentrations were detected in soil cores collected from three of these locations (Figure 5), while low concentrations (less than 100 µg PCE/g soil) were found in cores collected from the other three locations. Note that a layer of PCE was detected in the previously uncharacterized depth interval of 9.6-10.7 m bgs in postflushing core (PFC) 2. VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Postflushing PCE soil concentration distribution at all borings with concentrations greater than 100 mg PCE/kg soil.

FIGURE 6. Postflushing n-hexanol (O) and e-HEX (3) BTCs measured at RWs 3, 6, and 7. The average concentration of PCE in the 41 soil samples collected from the 6 postflushing borings, and located in the vertical interval of 7.9 to 9.6 m bgs, was 980 µg PCE/g soil, with a standard deviation of 3200 µg PCE/g soil (CV ) 3.3). The PCE removal effectiveness was calculated to be 0.65 using the pre- and postflushing average PCE concentrations in the soil. Also, note that the CV for these data was in agreement with the preflushing value, indicating that the degree of variability in the NAPL source zone was maintained after alcohol flushing. Partitioning Tracers. The PCE removal effectiveness was calculated at each RW from the partitioning tracer data using two methods. In the first method, the volume of PCE removed was determined from the pre- and postflushing VN estimates. In the second method, the volume of PCE removed was determined from the integration of the PCE BTCs measured at the RWs. In both methods, the initial amount of PCE present was determined from the preflushing partitioning tracer test. The postflushing n-hexanol and e-HEX BTCs measured at RWs 3, 6 and 7 are shown in Figure 6; these data are 3728

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representative of the behavior observed at the other RWs. Note that the preflushing n-hexanol and methanol BTCs coincided almost exactly (not shown), because of the low n-hexanol KN value, and, thus, the postflushing n-hexanol behavior was believed to be very similar to that of a nonpartitioning tracer. As in the preflushing tracer test, partitioning tracer retardation was observed only in the BTC tails. The total volume of PCE remaining in the swept volume was estimated to be approximately 26 L (Table 2), equivalent to an average SN of 0.0014. Note that the R values measured at the RWs ranged from 1.1 to 1.2, and the postflushing VN results are therefore subject to greater uncertainty than the preflushing results (1.2 < R < 1.5). The average PCE removal effectiveness values determined from both partitioning tracer methods were very similar (Table 2), but the specific values at each RW varied between the two methods. This was likely caused by the changes in the flow distribution during the alcohol flood, resulting in a redistribution of the RW swept volumes. For example, the flow rate at RW 6 was increased from 5.9 L/min in the tracer test to 9.1 L/min during the alcohol flood (12), which would

TABLE 2. PCE Removal Effectiveness Determined at the RWs from Both Partitioning Tracers and PCE BTCs Measured during In-Situ Alcohol Flushing RW

tracer (VN-pre - VN-post)/VN-pre

tracer and PCE BTC VN-removed/VN-pre

2 3 4 5 6 7 total

0.43 0.77 0.31 0.44 0.58 0.72 0.63

0.20 0.61 0.52 0.003 1.1 0.71 0.62

have increased the swept volume of this well, resulting in the extraction of PCE from regions that were not in the original swept volume determined from the tracer test. Hence, the removal effectiveness was greater than one at RW6. Groundwater Samples. The average postflushing aqueous PCE concentration from all 35 MLS locations was 44 mg/L. The PCE removal effectiveness determined from the preand postflushing average groundwater concentrations was therefore only 0.15. This value is significanlty lower than those determined from the soil core and partitioning tracer data, both of which indicated 62 to 65% removal of the PCE. However, at 21 of the 35 MLS locations, postflushing aqueous PCE concentrations were an average of 92% lower than preflushing values (a complete table of the groundwater data has been included as Supporting Information). The average ethanol content at these 21 locations was 0.1%. At the remaining 14 locations, the postflushing aqueous PCE concentrations were higher than the preflushing values, with an average ethanol content of 7%. The presence of residual ethanol increased the aqueous PCE solubility limit, likely causing the postflushing PCE concentrations to be artificially high at these locations. PCE Mobilization. A determination of whether PCE mobilization occurred during ethanol flushing could only be inferred from three types of indirect evidence gathered during the flood. First, free-phase PCE was present in only 2 of the more than 3000 solution samples collected from the 35 MLS locations. Second, aqueous PCE concentrations were substantially higher following the flood only at locations where the residual ethanol content was high. Third, the average concentration of PCE in the 37 postflushing soil samples collected from locations below the flushed zone (i.e., at depths greater than 9.6 m bgs) was 510 µg PCE/g soil. This value is lower than the average PCE concentrations (2800 µg PCE/g preflushing and 980 µg PCE/g postflushing) in soil cores collected from within the flushed zone. While these three observations collectively suggest that significant downward mobilization of PCE did not occur, logistical and regulatory constraints precluded a more definitive assessment. The goal of this study was the removal of a significant amount of PCE from the DNAPL source zone and not necessarily the reduction of aqueous PCE concentrations in the NAPL source zone to below regulatory limits. However, the evaluation of the impact of partial DNAPL mass removal on groundwater quality is an important issue for future study. Long-term monitoring will be continued at this site to monitor for decreases in PCE concentrations resulting from reductive dechlorination, possibly enhanced by the presence of residual ethanol.

Acknowledgments The following agencies funded, in part, the field study: the Florida Department of Environmental Protection through a contract with LFR Levine-Fricke, Inc.; the Technology Innovation Office of the U.S. Environmental Protection Agency;

and the Florida Center for Solid and Hazardous Waste Management. Carl Enfield and Lynn Wood, with the USEPA, provided site characterization, instrumentation, and sample analysis support. The authors thank those who helped with the field work and laboratory analyses: Irene Poyer, Meifang Zhou, John Rayner, Heonki Kim, Lane Evans, Clayton Clark II, Michael Brooks, Gloria Sillan, Jaehyun Cho and Claire Shukla, all with the University of Florida; and Robert Cowdery and Ben Witmeier, both with LFR.

Supporting Information Available Complete tables of the pre- and postflushing core samples and groundwater data along with numerical simulation streamlines of proposed flushing patterns. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review December 14, 1999. Revised manuscript received June 6, 2000. Accepted June 13, 2000. ES9913737 VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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