Research Communications Field Study of the Partitioning Tracer Method for Detection of Dense Nonaqueous Phase Liquid in a TrichloroetheneContaminated Aquifer NICOLE T. NELSON† AND M A R K L . B R U S S E A U * ,†,‡ Soil, Water, and Environmental Science Department and Hydrology and Water Resources Department, University of Arizona, Tucson, Arizona 85721
Introduction Dense nonaqueous phase liquids (DNAPLs) occur in the subsurface at numerous contaminated sites and can act as long-term sources of both vapor-phase and groundwater contamination. Successful remediation and effective risk assessment of DNAPL-contaminated sites is limited by DNAPL behavior and current site characterization techniques. DNAPL sources are difficult to identify because they are denser than water, can migrate below the water table, and move along pathways that may be distinct from those of water flow. Furthermore, DNAPL is often present as residual saturation and is difficult to observe in soil or water samples. A major weakness of current characterization methods, such as soil-gas analysis, core sampling, and well sampling, is that they provide data at discrete points, such that the probability of sampling a zone of localized DNAPL saturation is quite small. In addition, given the heterogeneity typical of field sites, constructing an accurate, comprehensive map of DNAPL distribution is usually cost prohibitive using such point values. A new method of site characterization for NAPL involves the use of partitioning tracers. Partitioning tracers have been employed since the 1970s in the petroleum industry to determine residual oil saturations (1-3). Recently, partitioning tracers have been used successfully to measure NAPL saturations in environmental systems. In column experiments with Ottawa sand, Jin et al. (4) demonstrated that partitioning tracer tests using alcohols provided good estimates of the known amount of liquid tetrachloroethene in the columns. In column experiments with aquifer sand, Wilson and Mackay (5) used sulfur hexafluoride to successfully predict the known amounts of trichloroethene residual saturation in the columns. Pilot-scale field experiments involving several investigators are being conducted at an Air Force base in Utah to test the use of several alcohol tracers for measuring the amount of petroleum-based NAPL * Corresponding author telephone: 520-621-1646; fax: 520-6211647; e-mail address:
[email protected]. † Hydrology and Water Resources Department. ‡ Soil, Water, and Environmental Science Department.
S0013-936X(96)00148-4 CCC: $12.00
1996 American Chemical Society
FIGURE 1. Illustration of the impact of NAPL residual saturation on the transport and retardation of a partitioning tracer. Figure from Brusseau (8).
contained within 5 m by 3 m fully enclosed cells (6). Results to date indicate that transport of some of the alcohols is retarded with respect to that of the nonreactive tracers (7). All of the research referenced above involved the use of partitioning tracers to measure quantities of known NAPL contamination. However, it is feasible that partitioning tracers may also be useful as “detectors” of NAPL saturation. The purpose of this work is to test the use of partitioning tracers as detectors of potential DNAPL saturation in a trichloroethene-contaminated aquifer at a Superfund site.
Theory The experimental and theoretical basis for the retention of dissolved solutes by immobile, immiscible liquid phases and the resultant impact on solute (tracer) transport have been described previously (4, 8). Organic fluid phases reversibly retain the partitioning tracer, which retards the tracer’s transport with respect to that of conservative tracers. To illustrate, the influence of three quantities of residual saturation on the transport of a partitioning tracer is shown in Figure 1. These breakthrough curves were produced using a model developed by Brusseau (8). The procedure for estimating Sn, NAPL saturation, involves calculation of a retardation factor, R, for the partitioning tracer, which is done by a comparative moment analysis with the conservative tracer. The retardation factor is defined as the mean travel time of the partitioning tracer divided by the mean travel time of the conservative tracer. With knowledge of the NAPL-water partition coefficient (Knw), soil-water partition coefficient (Kd), bulk density of porous media (pb), and volumetric water content (θw), Sn can be calculated by use of
R ) 1 + (pb/θw)Kd + [(Sn)/(1 - Sn)]Knw
(1)
When there is negligible sorption of the tracer to the soil, Kd ) 0 and
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R ) 1 + [(Sn)/(1 - Sn)]Kn
(2)
TABLE 1
Properties of Sulfur Hexafluoridea The Sn values obtained from an analysis such as that presented above are “global” values, representing an averaging across the measured domain. The magnitude of the observed retardation, and thus of Sn, is a function of the area of influence of the tracer test (swept volume) versus the mass of NAPL in that swept volume. For monitoring wells, the swept volume is relatively small, so even a small, localized mass of NAPL can be easily “measured”. However, the swept volume of an extraction well can be enormous in comparison. Thus, a small, localized mass of NAPL could have a very small impact on retardation as measured at an extraction well, which could easily be lost in the normal uncertainty associated with field data. In addition, because of factors such as bypass flow (water flows around a NAPL zone due to reduced relative permeability), rate-limited mass transfer, and mass loss, the measured Sn values may often be underestimates of the true values. Thus, NAPL saturation measurements obtained with the partitioning tracer method must, at least initially, be considered as underestimates of actual values.
solubility vapor pressure Henry’s law constant at 25 °C Knw to TCE at 22 °C log Kow
35.2 mg/L 23.9 atm 169.7 (concn/concn)
ref 10 ref 10 calculated
32 (1.33 (concn/concn) 1.14(0.03 (concn/concn)
ref 5 ref 11
a Henry’s law constant calculated as (1 atm ÷ solubility) ÷ RT; R is the ideal gas constant, and T is temperature (K).
Materials and Methods Sulfur hexafluoride (SF6), selected as the partitioning tracer, is a colorless, odorless, noncombustible gas at room temperature (relevant properties are listed in Table 1). SF6 has many characteristics desirable for an ideal partitioning tracer. It (1) is nontoxic even at high concentrations (9); (2) behaves conservatively in typical saturated sandy media and in media with high percentages of organic carbon (10, 11); (3) has a low but significant octanol-water partition coefficient, Kow (11); and (4) has been shown to partition into liquid-phase trichloroethene (5). Previous tests have shown SF6 to be resistant to chemical degradation in the presence of NH3, F2, Cl2, Br2, I2, HCl, and elemental carbon (12) and resistant to microbial degradation (13). Thus, SF6 should behave conservatively with respect to mass loss due to transformation reactions. Bromide (CaBr2) was used as the nonsorbing, nonpartitioning tracer. The tracer experiment was conducted within a portion of the Tucson International Airport Area Superfund Site, in Tucson, AZ. A large, composite plume of trichloroethene exists in the upper portion of the regional aquifer, which is the sole source of potable water for the city. Contaminants entered the subsurface by seepage from pits and ponds used to dispose of organic solvents, including trichloroethene, during the 1960s and 1970s. The experiment was conducted in an area coincident with the former location of a large unlined disposal pit (18.3 m × 3.7 m deep × 6.1 m wide), which is considered a contaminant source zone. In 1976 the pit was closed, and a percolation pond was built on top of the pit and used for several years. Various remediation programs have been in operation at different parts of the Superfund site for several years. A large-scale pump-and-treat system has been in operation in the vicinity of the tracer-test location for about 9 years. To conduct the tracer experiment, an injection/extraction well couplet, 7.5 m in length, was used to generate steady flow through a 6 m thick semiconfined aquifer 42 m below ground surface. The aquifer grades from a clayeysand to a gravel with cobbles up to 20 cm in nominal diameter and contains observed aqueous trichloroethene concentrations ranging from 100 to 10 000 µg/L. Such
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FIGURE 2. Map showing location of wells sampled during experiment.
concentrations are less than the rule-of-thumb value (1% of aqueous solubility) that is used as an indicator of the probable presence of NAPL. However, it is not proof of the absence of NAPL given the large degree of heterogeneity associated with the site, the relatively low density of sampling points, and the large screened intervals associated with most of the monitoring wells. Both the bromide and SF6 tracers were injected for 49.5 h, and then tracer-free water was injected for an additional 1400 h using equal injection and extraction rates of 136 L/min. SF6 was metered into the injection line through a stainless steel porous cup to yield a final concentration of 3.9 mg/L (9% of solubility). A total of 1.6 kg of SF6 was injected. Breakthrough curves for SF6 and bromide were obtained for the centerline monitoring well (M72), the extraction well (SVE7), and at three surrounding monitoring wells (P6, P8, and E14) (Figure 2). Additional experiment parameters and aquifer properties are reported in Table 2. Special care was taken to reduce losses of SF6 during sampling. A four-port gas-driven sampling device was used to collect water samples from M72. The head of the multilevel sampler and the discharge line on the extraction well were equipped with two-way valves ending in luerlock fittings. The samples were extracted with a 20-mL glass syringe fitted with an on/off valve and luer-lock and then injected into 80-mL aluminum aerosol canisters through a stainless steel fitting. The surrounding wells P6, P10, and E14 were sampled by bailing. Twenty milliliter glass vials with Teflon septums were filled approximately half-way with water from the bailer. The vials were
TABLE 2
Experiment Parameters and Aquifer Propertiesa sampling zones in m72 A C D B pre-experiment groundwater gradient (m/m) pre-experiment groundwater velocity (m/day) pre-experiment groundwater flow direction experiment groundwater velocity (m/day) experiment max groundwater gradient (m/m) av hydraulic conductivity (cm/s) av depth to groundwater (m) depth lower confining unit (m) bromide injection concn (mg/L) SF6 injection concentration (%) bromide mass recovery at SVE7 (%) SF6 mass recovery (SVE7) relative to bromide (%) total pumpage (m3) grain size analysis (cuttings from M72: 44.2-45.7 m) gravel (%) sand (%) silt (%) clay (%) fraction of organic carbon (%) a
42.4-43.0 43.7-44.5 44.5-45.3 45.3-50.0 ∼0.01 ∼0.26 west 0.9-11.6 0.32 10-2 37 48.4 56 3.9 73 73 11863 26.4 65.9 3.0 4.7 0.03
Data from ref 14.
immediately capped and turned upside down. The SF6 partitions from the water to the air phase and the layer of water hinders diffusion of SF6 through the septum. All samples were refrigerated at 10 °C until analyzed. Analysis of SF6 was performed at Tracer Research Corp., Tucson, AZ, using a gas chromatograph with an electron capture detector (Model 3300, Varian Associates, Sugar Land, TX). The GC was equipped with a 1.85 m long, 0.32 cm o.d. stainless steel column with liquid phase SB-1000 1% and solid phase Carbopack B 60/80 mesh. The ECD signal was acquired and integrated using a Spectra Physics 4400 integrator. The detection limit for SF6 is approximately 0.05 µg/L using this method. The low detection limit allows for 5 orders of magnitude resolution when SF6 is injected into the aquifer at a few milligrams per liter.
Results and Discussion Tracer Transport. Breakthrough curves measured for bromide and SF6 at the centerline monitoring well (M72-B) and at a perimeter monitoring well (E14) are shown in Figure 3. The positions and general shapes of the SF6 breakthrough curves are very similar to those of bromide at these locations. This indicates that SF6 experienced no measurable sorption by the solid phase and also that its dispersive behavior was similar to that of bromide. These results are consistent with the results of laboratory experiments (10, 11). The non-retardation of SF6 also indicates the absence of trapped gas phases within the aquifer that could serve as a source of retention. The data for bromide and SF6 were analyzed by calculating the zeroth and first temporal moments to quantify mass recovery and retardation (e.g., ref 4), which are reported in Table 3. Breakthrough curves for bromide and SF6 measured at two other perimeter monitoring wells (P6 and P8) are shown in Figure 4. The arrival of the SF6 breakthrough curves is clearly delayed in comparison to bromide at these two wells. The primary question to resolve is the cause of this retardation. Given that sorption by the solid phase and
FIGURE 3. Breakthrough curves for SF6 and bromide at selected monitoring wells: (A) centerline monitoring well M72-B and (B) monitoring well E14.
retention by trapped gas phases are not occurring, as discussed above, partitioning into immiscible liquid phases appears to be the only other likely process that could cause retardation of SF6. There are four additional sets of data that indicate the possible presence of liquid-phase trichloroethene within the aquifer. First, samples from the extraction well showed a rapid, 50% reduction in trichloroethene concentration at the start of the tracer experiment. However, the concentration did not continue to decline, even though trichloroethene-free water was flushed through the aquifer for 60 days (14). This is equivalent to 71 pore volumes given the flow rate and zone of influence associated with the well. With measured retardation factors of less than 2 for trichloroethene, this extensive tailing is not likely a result of sorption/desorption processes alone. Second, concentrations of trichloroethene at M72 returned to approximately the initial starting concentrations after the extraction well was turned off at the end of the tracer experiment (14). Third, during a pilot-scale soilvapor extraction operation conducted for several months (∼2 months operating time), about 1000 kg of liquid chlorinated solvent were removed from the vadose zone directly above the saturated zone wherein our tracer experiment was conducted (15). Fourth, the location of the tracer experiment is coincident with an identified source zone, as discussed previously. These factors are additional evidence of the possible presence of liquid-phase trichlo-
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TABLE 3
Mass Recoveries, Retardation Factors, and Residual Saturations
M-72-B M-72-C M-72-D P6 P8 E14 SVE7 b
mass recovery (%)a
retardationb
4.5 10.8 12.2 53.3 1.8 60.3 73.5
1.0 1.2 1.9 1.8 2.7 1.0 1.0
Snc 0.006 0.027 0.024 0.050
a Mass recovery of SF normalized by bromide mass recovery. 6 Retardation factor calculated for SF6. c Sn is residual saturation.
FIGURE 5. Breakthrough curves for SF6 and bromide at extraction well (SVE7).
FIGURE 4. Breakthrough curves for SF6 and bromide at selected monitoring wells: (A) P6 and (B) P8.
roethene in the aquifer and support the results of the partitioning tracer experiment. Assuming that partitioning into immiscible liquid phases is the only mechanism causing retention of SF6, apparent residual trichloroethene saturations can be calculated with eq 2 for those locations where the transport of SF6 was retarded. The values of residual saturation so obtained range from 0.6% to 5.0%. These values are at the lower range of laboratory measurements of Sn for trichloroethene in sandy media (16) and match the “large-scale average” values of about 1% reported by Poulsen and Kueper (17) for measurements made of a controlled release of tetrachloroethene into a sandy vadose zone.
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The breakthrough curves for bromide and SF6 measured at the extraction well (SVE7) are shown in Figure 5. The curves are quite similar in shape, which again indicates similar advective and dispersive behavior. The measured retardation factor for SF6 was 1.0. As discussed previously, the partitioning tracer method most likely provides underestimates of retardation and residual saturation. In addition, retardation and saturation values measured at an extraction well can be much smaller than those measured at a monitoring well because of the great differences in swept volumes. These factors are most likely the reason for the apparent lack of retardation measured for SF6 at the extraction well. Tracer Mass Recovery. The moment analyses indicate lower mass recovery of SF6 compared to bromide at all locations (see Table 3). Several factors may cause reduced recovery, including: sampling methods (multilevel sampler vs bailing), sample containers (glass vials vs canisters), miscalculation of initial concentration, degassing during injection, and transport behavior of SF6 in the aquifer. Losses of SF6 during sampling with the multilevel sampler (M72) could occur due to diffusion through the Teflon tubing and across the air-water interface in both the sample chamber and the tubing. Rates of mass loss due to diffusion are a function of concentration gradient. Samples with higher concentration would therefore have higher losses compared to samples with lower concentrations for a given time under nonequilibrium conditions. Since M72-B had significantly lower SF6 concentrations than M72-D, lower losses should have been observed for M72-B. However M72-B had significantly higher losses than M72-D, indicating that the sampling method is not the main mechanism of reduced recovery for M72. Preliminary laboratory experiments conducted with trichloroethene indicate 92% recovery during operation of the multi-level sampler (14). Two of the wells sampled by bailing showed fairly good recovery of SF6 (E14 and P6), whereas low recovery was obtained at P8. This wide range of recoveries was also observed for samples obtained by use of the gas-driven method, which indicates that bailing is not the main mechanism of loss. This is supported by the results of laboratory experiments, which indicate less than 2% loss is associated with bailing (14). Mass loss experiments were also conducted for both the glass vials and the aerosol canisters. These experiments indicate less than 2% loss for a 1-week holding time for both container types (14).
The initial concentration of SF6 was calculated by conducting a mass balance for the SF6 gas cylinder. The difference in mass of the cylinder before and after the experiment was considered to be the mass injected. This mass was then divided by the amount of water injected to yield an input concentration of 3.9 mg/L. Assuming that all the gas that left the cylinder entered the aquifer, this is an acceptable technique for calculating initial concentration. However, if some of the SF6 did not enter the aquifer (e.g., due to degassing in the well), this method would give erroneously low values for relative concentration and lead to erroneously low mass recovery values. However, given a maximum observed mass recovery of 73% (SVE7), no more than 27% of the reduced recovery can be due to potential degassing and the resultant miscalculation of initial concentration. Another possible cause of the reduced recovery is the transport behavior of SF6 in the aquifer. The liquid-liquid partitioning of the tracer is a reversible process at the microscopic scale. However, there are several processes that can cause the partitioning to appear to be nonreversible at the field scale. Consider, for example, a NAPL phase that is relatively thick in the dimension normal to water flow. When a partitioning tracer first contacts the NAPL phase, there is a concentration gradient driving the tracer into the NAPL. The concentration gradient reverses when the tracer pulse is followed by tracer-free water, which causes the tracer to transfer back to the advecting water. However, if the length of the pulse is short enough such that the tracer has not fully saturated the NAPL phase prior to the elution step, an inward concentration gradient will still exist in the interior of the NAPL phase. This could significantly delay the return of some of the tracer mass to the water. Depending on the time scale of the experiment and the nature of the NAPL, this hysteretic behavior could cause reduced mass recoveries and, thus, apparent nonreversible partitioning. Given the elimination of other possible sources of mass loss, as discussed above, the transport behavior of SF6 in the aquifer appears to be the most likely primary cause of the reduced mass recoveries. In summary, SF6 appears to have been used successfully as a partitioning tracer for detection of potential DNAPL saturation in a trichloroethene-contaminated aquifer. Moment analysis of breakthrough curves allowed mass recovery, retardation, and apparent DNAPL saturation to be calculated. Assuming that partitioning to DNAPL is the only mechanism for retention of SF6, the results indicate the possible presence of DNAPL saturation in some portions of the aquifer. Future experiments will be conducted to
substantiate these results and to examine the effect of ratelimited mass transfer and porous media heterogeneities on the behavior of partitioning tracers.
Acknowledgments This research is supported by the United States Air Force/ ASC/EMR Project F33657-81-E-2096. Several people and organizations were instrumental in the gathering of these data and deserve recognition: Tina Decker and HMSC personnel who helped in the implementation of this project; Bill Taylor of Taylor Controls for his help in constructing the injection apparatus; Glenn Thompson and Tracer Research Corp. for helpful discussions regarding sample collection and for use of analytical instrumentation; Jon Rohrer, Denise Putz, and other University of Arizona researchers who took time out of busy schedules to help in this project.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
(15) (16) (17)
Tang, J. S. SPE Form. Eval. 1995, 10, 33. Tang, J. S.; Harker, B. J. Can. Pet. Technol. 1991, 30 (3), 76. Tang, J. S.; Harker, B. J. Can. Pet. Technol. 1991, 30 (4), 34. Jin, M.; Delshad, M.; Dwarakanath, V.; McKinney, D. C.; Pope, G. A.; Sepehrnoori, K.; Tilburg, C. E. Water Resour. Res. 1995, 31, 1201-1211. Wilson, R. D.; Mackay, D. M. Environ. Sci. Technol. 1995, 29, 1255-1258. Enfield, C. G.; Annable, M. D.; Brusseau, M. L.; Falta, R.; Gierke, J.; Rao, P. S. C.; Sabatini, D. A.; Wood, A. L. Unpublished data, 1996. Annable, M. D.; Rao, P. S. C.; Hatfield, K. H.; Graham, W. D.; Wood, A. L.; Enfield, C. G. J. Environ. Eng., in press. Brusseau, M. L. Water Resour. Res. 1992, 28 (1), 33-45. Lester, D.; Greenberg L. A. Arch. Ind. Hyg. Occup. Med. 1950, 2, 348-349. Wilson, R. D.; Mackay, D. M. Groundwater 1993, 31 (5), 719724. Wilson, R. D.; Mackay, D. M. Groundwater 1996, 34 (2), 241249. Mellor, J. W. A Comprehensive Treatise on Inorganic and Theoretical Chemistry; Longmans, Green, and Co. Ltd.: London, 1930; Vol. X, pp 630-631. Watson, A. J.; Ledwell, J. R.; Sutherland, S. C. J. Geophys. Res. 1991, 96 (C5), 8719-8725. Brusseau, M. L. Advanced Characterization Study to Improve the Efficiency of Pump and Treat Operations at a Superfund Site: An Integrated Field, Laboratory, and Modeling Approach. Interim progress report, Feb 1996. Decker, T. Personal communication, 1995. Cohen, R. M.; Mercer J. W. In DNAPL Site Evaluation; Smoley, C. K., Ed.; CRC Press: Boca Raton, FL, 1993. Poulsen, M. M.; Kueper, B. H. Environ. Sci. Technol. 1992, 26 (5), 889-895.
Received for review February 16, 1996. Revised manuscript received May 24, 1996. Accepted June 6, 1996. ES960148B
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