Environ. Sci. Techno/. 1995, 29, 1255-1258
Direct Detection of Residual Nonaqueous Phase Liquid in the Saturated Zone Using SFg as a Partitioning Tracer RYAN D. WILSON* AND DOUGLAS M. MACKAY Waterloo Centre for Groundwater Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3Gl
~
~~~~
Sulfur hexafluoride (SF6) was investigated as a potential detector of residual dense nonaqueous phase liquids (DNAPL) below the water table. In NAPLwater batch partitioning tests, SF6 had a partitioning coefficient (KNW= concentration in NAPL t concentration in water) of 32 f 1.33 to TCE, 24 f 1.27 to DCM, and 45 f 1.89 to o-DCB. Tracer tests with SF6 and bromide in packed sand columns containing residual DNAPL show that SF6 breakthrough is retarded relative to bromide. The SF6 retardation factor was 2.2 relative to bromide in a column containing 3.69% residual TCE and 2.6 in a column with o-DCB at 3.52% residual saturation. A relatively simple relationship based on equilibrium partitioning and retardation theory was developed that allows very accurate estimates of DNAPL residual content encountered by SF6 during transport. Using observed retardation factors from the column tests and the experimentally determined KO, values, this simple calculation was found to predict DNAPL residual saturations to within 2.5% of that actually emplaced.
Introduction Downward migration of dense nonaqueous phase liquids (DNAPLs) results in the formation of residual zones and pools below the water table (1). The low solubility of such compounds means that they may represent very long term sources of dissolved contaminants (2). Any remediation scheme must therefore involve targeting these residual zones to be effective at complete aquifer restoration. However, even slight aquifer heterogeneities can result in complicated and widespread distributions of residual DNAPL (3,4).A quick and effective method to detect and delineate such zones is needed. A promising method of detecting DNAPL below the water table is with groundwater tracers with different magnitudes of organic phase partitioning (5). A wide range of partitioning tracers have been employed for a number of years in the oil exploration industry to detect hydrocarbons using chromatographic separation theory (6-10). Alcohols, surfactants, fluorinated organic compounds, and gases have been tested or proposed as hydrocarbon detectors. Many of these tracers are themselves organic compounds, potentially viewed as pollutants, and are subject to confounding transport phenomena such as sorption and transformation. The ideal partitioning tracer would be one that is nontoxic and behaves conservativelyduring transport in the absence of NAPL, but whose breakthrough would be delayed relative to a nonpartitioning tracer if NAPL was encountered. Sulfur hexafluoride (SFd possesses many of the properties of an ideal conservative tracer (11). SF6 is nontoxic and has been shown to behave as a conservative, nonreactive tracer in typical saturated media (11) and in media with a high percentage of organic carbon, showing no sorptive retardation (12). Since SFs has a low but significant octanol-water partitioning coefficient (1.21,SF6 may also partition sigmficantlyto other organic liquids from water. If so, comparative forced-gradient tracer tests with SF6 and a relatively nonpartitioning compound such as bromide could be conducted to identify flow paths where SF6 encounters DNAPLs and thus is retarded relative to bromide. This study had two general goals. The first was to establishwhetherand to what degree SF6partitions to dense organic liquids. The second was to determine if SF6 would partition to NAPL duringtransport. Various DNAPL-water partitioning coefficientswere determined by batch testing. Column experiments were conducted to test whether SF6 could be used to detect the presence of residual amounts of DNAPL during transport. Bromide was chosen as the nonpartitioning tracer in the column tests.
Materials and Methods DNAPL-Water PartitioningCoefficientExperiments. For the partitioning experiments, trichloroethylene (TCE), dichloromethane (DCM),and o-dichlorobenzene (0-DCB) were chosen as NAPLs representative of common environmental contaminants from two distinct chemical families: aliphatics and aromatics. For each compound, five ~~
~~
* Corresponding author; Fax: (519) 746-5644; e-mail address:
[email protected].
0013-936~95/0929-1255$09.00/0
0 1995
American Chemical Society
VOL. 29, NO. 5, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
1255
batch tests containing roughly equal fractions of organic and water (withwater-only controls) were conducted with various aqueous SFs concentrations. The system used to produce aqueous SF6 solutions is described in a previous study (11). At room temperature (22 "C),the chargingvessel used required approximately 36-48 h to attain an aqueous SF6 concentration near 30 mglL. The batches were filled at periodic intervals during the charging time, resulting in different average SF6 concentrations for each batch. After each batch preparation, sample bottles were shaken on an orbital mixer for 20 min and then allowed to stand unmixed for 1 h. This technique is similar to the "shake flask method for the determination of octanol-water partitioning coefficients (13). It was shown that shaking for as little as 5 min was sufficient to reach equilibrium as long as emulsions were not formed (13). There is little likelihood of emulsion formation with the highly hydrophobic compounds used in this study, and indeed none was observed. A headspace was drawn, and SF6 concentrations were determined by direct headspace analysisusing the equipment and method described elsewhere (11). The detection limit for SF6 in water was approximately 5 pg/L. SFs concentrations in NAPL and water were calculated based on mass balance relationships. Column Experiments. The system used to prepare an aqueous solution of SF6and bromide for the column tests was similar to that used in the partitioning experiments. Three column experiments were conducted to examine the effect of the presence of NAPL at residual amounts on SF6 transport and breakthrough. One each contained o-DCB and TCE at approximately the same residual percentage, and the third was residual-free for experimental control. All three columns were run concurrently using a stainless steel manifold injection system. The experiments were run at lab temperature (22 "0. The acrylic columns (2.5 cm diameter by 40 cm long) were wet packed in 2-4 cm lifts, tamping and mixing the sand surface before each lift to prevent layering. Sand from the surlicial aquifer at C. F. B. Borden, Ontario, Canada, was used because it represents a typical silica sand, and its properties are well studied (14-17). The bulk sample was riffle-split into three equal subsamples. In the columns containing NAPL at residual, each of the pure organic phases was mixed into one of the sand subsamples to an approximate residual content of 3-4% of estimated porosity, which was calculated as the difference between wet and dry column weight. Gases that may have been trapped in the columns during packing were removed in the manner described by Wilson and Mackay (12). The effluent during this preconditioning step was collected in beakers to allow calibration of flow rates and to permit calculation of the mass of organic removed before the actual tracer experiments begun. Flow of tracer-spiked water was introduced to the columns for 12 h at approximately 18 mL/h, with samples taken every 2 h. After the 12-h tracer pulse, the charging vessel was switched off the injection line, and tracer-free water was allowed to flow at the same rate until 3-4 pore volumes had passed through the columns. Headspace-free samples were collected from each column every 3 h into 14.3 f 0.2 mL preweighed glass bottles. The bottles were weighed, a headspace was drawn by syringe, the bottles were reweighed, and the headspace was analyzed for SF6. SFe water concentrations were calculated by mass balance. Bromide analysis was per1256 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5, 1995
loo 4
J
0
P
rn = Kw = 32
~
80
R2= 0.9997
I
2 Water Coneneation (mg/l)
0 0
o-DCB
A
DCM
TCE
4
3
FIGURE 1. Concentration of SFS in NAPL vs that in water for the batch partitioning tests. The slope of each line is the respective NAPL-water partitioning coefficient (KNw). TABLE 1
Parameters fmm Column Transport Experiments column 1
NAPL
packed column pore vol (mL) packed column porosity I%) residual NAPL saturation (% of porosity) water flow (mLimin) av f SD
av linear velocity ( c m/d a y ) tracer injection VOI (mL) sF6 retardation factors
2
3
none 0-DCB (control)
TCE
135
142
137
44.7 0.0
47.0 3.52
45.4 3.69
0.30i 0.007 116.4
0.23f 0.26f 0.02 0.009 84.9 99.4
216
168 2.6
1.o
186 2.2
a Defermined by difference of bromide and SFe mean arrival time calculated by method of moments.
formedwith a Corning Model 476128 ion-specificelectrode and a Corning Model 476067 double-junction reference electrode. The lower detection limit for bromide with this method is approximately 1.0 mglL.
Observations and Discussion NUL-Water PartitioningExperiments. The NAPL-water partitioning coefficient (Kw) is defined as the ratio of SFs concentration in NAPL to the SF6 concentration in water. Figure 1 shows the batch average SF6 concentrations in water and NAPL for each organic compound. These data result in the following Kw estimates and standard deviations: KTcE.w= 32 f 1.33;Ko.~ca.w= 45 =t1.89; KDCM-W = 24 f 1.27. The slope ofthe best fit line throught the averaged data in each case (Figure 1) shows a 99.9% correlation to the values calculated from the analytical data. Column Experiments. Table 1 shows for each column the flow rates, the injection volumes, total pore volumes, column porosities, and estimated initial residual NAPL content. The average and standard deviation of injection concentrations was 23.4 f 1.0 mg/L for SF6, and 199.1 f 5.1 mg/L for bromide. Figures 2-4 show the concentration histories Of SF6 and bromide in the control (no residual), TCE residual, and
1.2
TCE COLUMN
CONTROL COLUMN M residual organics c o bromide = 199.1 mgiz c a SF6 = 23.4 mg/L
0
3.6Ph resrdual TCE C,bromide = 199 1 m a C, SF6 = 234 m@
Y
3
0.8
Eu 5 w
E
0.4
U
t3&
-4-
Bromide
1
00 00
OS 0.0
4.0 6.0 PORE VOLUMES
2.0
8.0
10.0
FIGURE 2. Concentration histories of SF6 and bromide in the control column (no residual NAPL). Tracers are transported with no detectable separation. IZ-
DICHLOROBENZENECOLUMN 3.52%residual *DCB
Y
U
z
5E c
08'
Y 8
20
40
60
EO
PORE VOLUMES
FIGURE4. Concentration histories of SF6 and bromide in the column containing TCE at 3.69% residual saturation. Separation of tracers is caused by partitioning of SF6 to NAPL.
equilibrium is fast compared to the other processes of transport affecting concentration (advection and dispersion). The average linear velocities in the sand columns were approximately 110 cmlday, which is higher than typical of field velocities. It can therefore be assumed that if the LEA was valid in these column tests, it will be valid at field velocities given the same or similar residual distribution. Calculation of Residual NAPL Saturation. The equilibrium concentration expression for SFs NAPL-water partitioning is
W
2c
04.
KNW
U
= cNAF'L/cH20
(1)
-1
9
where C = SFs concentration in the two phases. For flow through a porous medium containing residual NAPL, a retardation factor may easily be derived (analogousto that resulting from sorption) as
00 00
20
4.0
60
80
PORE VOLUMES
FIGURE3. Concentration histories of SF6 and bromide in the column containing 0-DCB at3.9% residual saturation. Separation of tracers is caused by partitioning of sF6 to NAPL.
o-DCB residual columns, respectively. All three columns were preconditioned in the same manner, and the experiments were conducted at the same time using the manifold system. Thus, it was assumed that the sole difference between the columns was the presence of residual NAPL saturations in columns 2 and 3. In the absence of residual NAPL, SFGand bromide both break through sharply at the same time and elute at the same time (Figure 2). In the column containing residual TCE (Figure 31, the mean breakthrough arrival of SFs is retarded relative to bromide by approximately 1.2 pore volumes ( R = 2.2). SFs mean breakthrough arrival is retarded by 1.6 pore volumes ( R = 2.6) relative to bromide in the column containing residual o-DCB (Figure 4). Retardation factors were determined from the difference between the mean arrival time of SF6 and bromide. Mean arrival times were calculated by the method of moments for instantaneous readings (18). The similarity of the shape of the SFs and bromide breakthrough curves in either column containing NAPL suggests that the local equilibrium assumption (LEA)is valid. This means that the rate of approach to partitioning
R=l+
KNWVNAPL
(2)
H 'O ,
The saturation of NAPL residual relative to total pore volume can be determined by rewriting (eq 2) in terms of the NAPL/ H20volume ratio (3)
In the column containingTCE, the observed SF6retardation factor was 2.2, and the measured KNWwas 32. From eq 3, the calculated TCE residual saturation relative to the pore volume was 3.75%,which compares favorably to the actual emplaced residual saturation of 3.69%. Similarly, the retardation of SFs in the column containing o-DCB was 2.6, and the measured KNWwas 45. The o-DCB residual saturation calculated by eq 3 was 3.55%,which is very close to the actual emplaced residual of 3.52%. While it is recognized that the calculation of residual NAPL presented here is most applicable to a onedimensional column, other workers (5) are developing methods to interpret partitioning tracer data from twodimensionalfield tests. Futureworkwill examine the effects of different flow rates and other NAPL residual distributions VOL. 29, NO. 5, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
* 1257
on the relative behavior of SF6 and bromide transport as well as the magnitude and controlling mechanisms of SF6 partitioning to other NAPLs at both laboratory and field scale.
Conclusions Results of the batch partitioning tests suggest that in a NAPL-water system, SF6 will partition to NAPL from water. The NAPL-water partitioning coefficient of SF6 was measured to be 32 for TCE, 45 for o-DCB, and 24 for DCM. The column tests confirm SF6 partitioning to DNAPL, which was manifested as breakthrough retardation relative to a tracer with a lower partitioning coefficient. When SF6 and bromide were passed through columns containing 3-4% residual NAPL, the SF6 retardation relative to bromide allowed accurate estimates of residual TCE and o-DCB. The partitioning nature of SF6 should prove useful in directly detecting the presence of DNAPL below the water table. The otherwise conservative behavior of SFGmakes interpretation of tracer breakthrough curves less complicated than in the case of tracers that sorb or transform during transport. Careful design of comparative forcedgradient tracer tests with SF6 may allow more accurate mapping of the extent of NAPL residual distributions and permit the reduction in area and cost of residual zone remediation.
Acknowledgments Funding for this work was provided by the University Consortium Solvents-In-GroundwaterReasearch Program (contributions from The Boeing Company, Ciby-Giegy Corporation, Eastman Kodak Company, General Electric Company, The Mitre Corporation, Laidlaw Environmental Services Ltd., the Natural Sciences and Engineering Research Council of Canada, and the Ontario University
1258 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5,1995
Research Incentive Fund) and Dr. John Cherry, University of Waterloo, Waterloo, Ontario, through an NSERC operating grant.
Literature Cited (1) Schwille, F. Dense chlorinated solvents in porous and fractured
media-Model experiments; Pankow, J. F., Translator; Lewis Publishers Inc.: Chelsea, MI, 1988; 146 pp. (2) Mackay, D. M.; Cherry, J. A. Enuiron. Sci. Technol. 1989,23,630. (3) Keuper, B. H.; Abbot, W.; Farquhar, G. J. Contam. Hydrol. 1989, 5, 83. (4) Poulsen, M. M.; Keuper, B. H. Enuiron. Sci. Technol. 1992, 26, 889. (5) Jin, M.; Delshad, M.; McKinney, D. C.; Pope, G. A.; Sepehmoori,
K.; Tilburg, C.; Jackson,R. E. Proceedings ofrheA. I. H. Conference on Toxic Sustances and the Hydrological Sciences, Austin, TX, Am 11- 14,1994;American Institute of Hvdrolopv: ", MinneaDolis, MN, 1994; p 131. (6) Cooke, C. E., Jr. Method of determining fluid saturations in reservoirs. U.S.Patent 3590923, 1971. (7) Sheely, C. Q., Jr.; Baldwin, D. E., Jr. J. Pet. Technol. 1982, 34, 1887. (8) Tang, J. S.; Harker, B. J. Can. Pet. Technol. 1991, 30 (31, 76. (9) Tang, J. S.; Harker, B. J. Can. Pet. Technol. 1991, 30 (41, 34. (10) Tang, J. S. J. Can. Pet. Technol. 1992, 31 (81, 61. (11) Wilson, R. D.; Mackay, D. M. Ground Water 1993, 31, 719. (12) Wilson, R. D. M. Sc. Thesis, University of Waterloo, 1993. (13) Leo, A,; Hansch, C.; Elkins, D. Chem. Rev. 1971, 71, 525. (14) Ball, W. P.; Buehler, C. H.; Harmon,T. C.; Mackay, D. M.; Roberts, P. V. J. Contam. Hydrol. 1990, 5, 253. (15) Mackay, D. M.; Freyburg, D. L.; Roberts, P.V.; Cherry, J. A. Water Resour. Res. 1986, 22, 2017. 116) Mackay, D. M.; Ball W. P.; Durant, M. G. J. Contarn. Hydrol. 1986, 1 , 119. (17) Sudicky, E. A. Water Resour. Res. 1986, 22, 2069. (18) Levenspiel, 0. The Chemical Reactor Omnibook; O.S.U. Bookstores Inc.: Columbus, OH, 1979.
.,
Received f o r review July 27, 1994. Revised manuscript received January 30, 1995. Accepted January 31, 1995.@
ES940470R @Abstractpublished in Advance ACS Abstracts, March 15, 1995.
