Sulfur Hex e as a Gas Tracer in Soil Ventkg Operations ANDRE OLSCHEWSKI,~ ULRICH FISCHER,+ MARKUS H O F E R , * A N D R A I N E R SCHULIN*mt ETH-Institute of Terrestrial Ecology, Grabenstrasse 311 1a, CH-8952 Schlieren, Switzerland, and Swiss Federal Institute for Environmental Science and Technology (HWAG), CH-8600 Diibendorf; Switzerland
Introduction
Soil venting is an effective technique for cleaning up spills of volatile organic compounds (VOCs) in the unsaturated zone. It is particularly suited in cases where low biodegradability or unfavorable site conditions prevent bioremediation and where due to inaccessibility, costs, or other reasons, excavation and ex situ treatment are not practicable. The basic setup of a soil venting operation consists of a vapor extraction well connected to a vacuum pump and a vapor treatment unit (Figure 1). The problem of soil venting operations is to direct the air flow as much as possible through the contaminated zone and to prevent short-circuiting through bypasses. Possibilities to exert control of the air flow pattern include the positioning of injection and extraction wells, the regulation of pressure, the screening ofwells, and the sealing of the soil surface, in particular around the extraction wells to avoid short-circuiting. The design and operation of soil venting systems is discussed in detail in refs 1 and 2. As with all in situ techniques, a principle drawback of soil venting is the limited knowledge about the site-specific subsurface characteristics (geology and hydrogeology) and the extension of the contaminated zone. Relevant knowledge about flow paths and travel times of the extracted air in the heterogeneous underground can be gained using gas tracer techniques, These results can be used to optimize the configuration and operation of the venting system. In this paper, we describe a procedure employing sulfur hexafluoride (SF6) as a tracer of air flow in soil venting operations. SF6 has been widely used as gas tracer in atmospheric studies (3-6) and also to trace radon entry into buildings from below ground and in the underground migration of volatile contaminants from landfills to residences (7, 8). SFs is an almost ideal tracer for the gas phase. of soils. It is purely anthropogenic; thus, the natural background is essentially zero contrary to other gases used for tracer tests such as helium (9). It is chemically stable and nontoxic. The detection limit is very low using electron capture detection. Because of its highHenry coefficient (H, = 132), * Author to whom correspondence should be addressed. ETH-Institute of Terrestrial Ecology.
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TABLE 1
Physicochemical Properties of Sulfur Hexafluoride molecular weight triple point (bar)
("C) density at 1 atm and 0 "C (kg m-3) Bunsen water solubility at 15 "C (g m-3 atm-') Henry coefficient at 15 "C (C$CJ
146.05 2.26 -50.8 6.5
47 132
partitioning into soil water is neghgible. It can be assumed that sorption is of importance only in subsurfaces with high organic carbon content or when large amounts of an organic pollutant is present as a separate liquid phase. The latter point has to be taken into account when a tracer test with SFs is carried out during the initial stage of a soilventing operation. Relevant physicochemical properties of SF6 are listed in Table 1. The tracer method described here was tested in several field experiments at an industrial site, which was vented over two years prior to the test to clean up a spill of chlorinated hydrocarbons in the unsaturated zone. The subsurface zone of the site was highly stratified, consisting of alluvial sediments with varying texture. The contaminants found were primarily tetrachlorethylene and, to a minor degree, trichloroethylene. As these compounds interfere with the method previously used for the chemical analysis of SF6as described in ref 10,the analytical method had to be adapted. In particular, an additional dilution procedure was included.
Experimental Section The tracer technique described here is illustrated using the example of two experimental runs that were performed at the field test site. Different operation schemes of the air wells were employed for these two runs. In the first experiment, one well (A) was used for tracer injection and one well (B) was left in operation for extraction. The other wells were left open so that air entry was possible during the tracer test. In the second experiment, air was extracted simultaneously from two wells (B and C) while another well (D)was opened for air entry and tracer injection. The pumps at the other wells were shut down, but as in the first experiment the wells were not capped. Before applying the tracer pulse, air samples of all wells were taken for the determination of SF6 background concentration. Also the gas flow rate was measured at all wells. In the first experiment, a pulse of 4.5 mL of pure SF6 was injected over a time period of 5 s at the air injection well (A). For the second experiment, 8.5 mL of SFs was injected at well D within 1 s. Breakthrough curves of the tracer were determined by taking 20-mL samples of the effluent air of the respective extraction wells at intervals of a few seconds. Samples were taken using 50-mL all-glasssyringes with luer-lock adapter and three-way valve. A gas-tight seal of the syringes was achieved by inserting the glass barrel into destilled water
0013-936x/95/0929-0264%09.00/0
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1 Extraction Well 2 Pump 3 Vapour Treatment Unit 4 Surface Sealing 5 Injection Well
Contaminant Flume FIGURE 1. Basic setup of a soil venting operation.
before inserting it into the syringe body. The sampleswere immediately transferred to the laboratory for analysis. The samples were analyzed by gas chromatography at ambient temperature. The apparatus used was equipped withadryingagentcartridge[f~ledwithMg(CIOJ21,al-mL sampling loop, a 1.6 m x 2 mm stainless steel column packed with molecular sieve 5 8, (washed, 80/100 mesh), an electron capture detector (temperature, 200 “C),and a Carlo Erba control unit (Model 251). Nitrogen was used as carrier gas at a flow rate of 70 mL min-I. Because of the molecular characteristics of SFs and the contaminants in the soil (trichloroethylene and tetrachloroethylene), these substances could not be separated by the analytical procedure. Therefore the samples were diluted 1000 times in the laboratory to decrease the background of the chlorinated compounds below the detection limits. Forthis purpose, a50-mLaJl-glasssyringe with luer-lock adapter and three-way valve was filled with 10 mL of nitrogen. To this volume, 50pL from the effluent air sample was given using the three-way valves. The diluted sample was then filled up to 50 mL with nitrogen whereby a complete mixing of the sample was achieved. As a result of this dilution procedure. no tri- and tetrachloroethylene could be detected any more in background air samples, which were taken from the extraction wells before the application of the tracer. Furthermore, the SF6concentration was in the optimum measurement range of 80-66 000 pg L-l.
10
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200
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Time
L
800 1000 1200 1, 00 (0)
!4GURE 2 Breakthrough CUNO of SFe at well B for experiment 1.
tbroughthesoilsurfaceandairentryat inactivesoilventing extraction wells. The smooth form of the breakthrough curve is another indication of the reproducibility ofthe analytical procedure. The curve shows a steep increase. a tall peak, and a long tail. Considering SF, as an ideal gas tracer, the breakthrough curve of spike pulse can be interpreted as a travel time distribution of the injected air between the injection well (A) and the extraction well (B).The maximum elution rate was reached 150 s after the injection ofthe pulse. After 600 Results and Discussion s, 85%of the injected tracer mass was recovered (Figure31. The procedure described here proved to be very practical The long tail of the breakthrough curve indicates that the and reliable. All samples that were diluted and analyzed transport domain was not uniform and that a significant twoormoretimesdifferedbylessthan5Wfromeachother. fraction of rather slow flow pathways was limiting the efficiencyof gas extraction from the soil. About 15%of the The breakthrough curve that was obtained in the first tracer masses had travel times more than four times as field experiment at well B is shown in Figure 2. The airflow high as the peak travel time. After 1200 s, mass recovery rates at the injection well (A) and the extraction well (B) was 96% and, thus, almost complete (Figure 31. were3.4 and 14.8Ls-’, respectively. Thus, the air entering through the injection well accounted for one-fourth of the The breakthroughcurves recorded at the extraction wells extracted air. the remainder being due to diffuse entry (B and C) for the second experiment are shown in Figure VOL. 29. NO. 1.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY.
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evaluation of an efficient operational scheme for the injection of hot air to enhance the extraction of contaminants.
Conclusions
0
200
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800 1000 1200 1400
Time (s) FIGURE 3. Cumulative mass outflow of SFg at well B for first experiment.
-.
35
~
The high mass recoveries achieved in the tracer tests together with the analytical precision show the reliability of the tracer technique described in this paper. This technique can be used in particular to determine the travel time distributions between injection and extraction wells for different operation schemes. Such tracer tests can provide useful informationfor making decisions concerning the installation of new wells, the closure of existing wells, the operation of the venting system (e.g., flow rates), and the construction of impermeable surface seals. The technique is easy to apply, not very time-consuming, cost-effective, and reliable. It can easily be combined with air pumping tests to obtain simultaneously information about undergound air permeabilities and to calibrate transport models. As the success of soil venting operations can be seriously limited by uncontrolled air entry and by unidentified short-circuits of undergound airflowpathways, such tests can help to avoid wasting money and time by inefficient operation schemes and, thus, should be further developed in the future.
Literature Cited (1) Johnson, P. C.; Stanley, C. C.; Kemblowski, M. W.; Byers, D. L.; Colthart, J. D. Ground Water Monit. Rev. 1990, 10, 159-178. (2) Johnson,P. C.; Kemblowski, M. W.; Colthart, J. D. Ground Water 1990, 28 (31, 413-429. ( 3 ) Turk, A.; Edmonds, S. M.; Mark, H. L. Environ. Sci. Technol. 1968, 2 (11, 44-48. (4) Clemons, C. A.; Coleman, A. I.; Saltzman, B. E. Environ. Sci. Technol. 1968, 2 (71, 551-556. (5) Dietz, R. N.; Cote, E. A. Environ. Sci. Technol. 1973, 7 (4), 338-
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Time (s) FIGURE 4. Breakthrough curves of SFg at well B and C for second experiment.
4. The peak maxima at the extraction wells (B and C) represent different travel times (40 and 120 s, respectively) and differ in concentration (30 and 19,ugL-l). But the total mass fractions recovered at these wells within 900 s were similar (45 and 52% of the injected mass; thus, the total tracer recovery in this experiment was 97%). Because the injected air was evenly distributed between two airstreams from well D to well B and from well D to well C, respectively, this configurationwas tested in the course of a preliminary
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342. (6) Ludwig, F. L.; Liston, E. M.; Salas, L. J. Amos. Environ. 1983, 17 (111, 2167-2172. (7) Nazaroff, W. W.; Lewis, S. R.; Doyle, S. M.; Moed, B. A.; Nero, A. V. Environ. Sci. Technol. 1987, 21 (5), 459-466. (8) Hodgson, A. T.; Garbesi, K.; Sextro, R. G.; Daisey, J. M. J . Air Waste Manage. Assoc. 1992, 42, 277-283. (9) Sabadell, G. P.; Gustafson; J. B.; Johnson,P. C.; Cruz, E. R.; Dicks, L. W. R.; Wang, C. C. Proceedings ofJoint CSCE-ASCE National Conference on Environmental Engineering; Montreal, Canada; CSCE-ASCE: 1993; pp 1435-1446. (10) Wanninkhof, R.; Ledwell, J. R.; Broecker, W. S. Science 1985,227, 1224-1226.
Received for review January 4, 1994. Revised manuscript received April 30, 1994. Accepted October 6, 1994.
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