Stable Isotope (C, Cl, and H) Fractionation during Vaporization of

Stable isotope fractionation during vaporization of trichloroethylene has been measured, with possible application as a technique to investigate subsu...
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Environ. Sci. Technol. 1999, 33, 3689-3694

Stable Isotope (C, Cl, and H) Fractionation during Vaporization of Trichloroethylene SIMON R. POULSON* Department of Geological Sciences, MS-172, University of Nevada-Reno, Reno, Nevada, 89557-0138 JAMES I. DREVER Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, 82071-3006

Stable isotope fractionation during vaporization of trichloroethylene has been measured, with possible application as a technique to investigate subsurface behavior. The equilibrium value of ∆13Cvapor-liquid has been measured between 5 and 35 °C, and ∆13Cvapor-liquid, ∆37Clvapor-liquid, and ∆Dvapor-liquid have been measured during progressive evaporation of liquid trichloroethylene at 22 ( 2 °C. Equilibrium values of ∆13Cvapor-liquid show a total range of 0.07-0.82‰, with a trend of decreasing ∆13Cvapor-liquid with increasing temperature, from approximately +0.7‰ at 5-15 °C to approximately +0.1‰ at 35 °C. Progressive evaporation experiments yield values of ∆13Cvapor-liquid ) +0.35‰ and +0.24‰, ∆37Clvapor-liquid ) -1.64‰, and ∆Dvapor-liquid ) +8.9‰. The positive values for carbon and hydrogen isotope fractionation, while unexpected, are consistent with available quantitative and qualititative data for trichloroethylene and other contaminant hydrocarbons, but a satisfactory explanation for these observations, particularly in combination with the negative value for chlorine, remains elusive. Vapor-liquid fractionation factors have application to the investigation of the behavior of trichloroethylene at contaminated sites, particularly sites undergoing remediation by techniques such as soil vapor extraction and soil bioventing.

Introduction Stable isotope analysis has long been realized to be a valuable technique to investigate the sources and the subsurface behavior of inorganic contaminants, such as nitrate (1-3). Stable isotope analysis also has great potential as a technique to investigate the subsurface behavior of organic contaminants since, by definition, all organic contaminants contain carbon. Moreover, virtually all organic contaminants of environmental concern also contain hydrogen, while many may also contain elements such as chlorine (e.g., chlorinated solvents), oxygen (e.g., the gasoline additive methyl tert-butyl ether, MTBE), nitrogen (e.g., atrazine), and/or sulfur (e.g., various pesticides). Hence, the potential exists to use multiple stable isotope analyses of a single individual contaminant that can provide additional discriminants to be used to investigate the sources and the subsurface behavior of an organic contaminant. * Corresponding author phone: (775)784-1104; fax: (775)784-1833; e-mail: [email protected]. 10.1021/es990406f CCC: $18.00 Published on Web 09/11/1999

 1999 American Chemical Society

Carbon isotope analysis of CO2 and/or dissolved inorganic carbon has been used successfully to investigate contaminant behavior (4-9). However, modern biological production of CO2 from organic matter can interfere with the interpretation of these results, although analysis of 14C concentrations can help correct for this interference (8). The ability to measure the carbon isotopic composition of the contaminant itself has been facilitated by gas chromatography/isotope ratio mass spectrometry (GC/irMS; 10, 11), which allows for the measurement of the carbon isotopic composition of individual compounds within a complex mixture. Samples can readily be prepared for GC/irMS analysis by a number of techniques including direct injection of gas (12), pentane extraction from aqueous solution (12, 13), or rapid extraction from either gas or aqueous solution by solid-phase microextraction (14) and can permit isotope analysis of contaminants with sub-ppm concentrations (15) with a typical analytical uncertainty of (0.5‰. These techniques can variously be used to measure the isotopic composition of many types of contaminants, and GC/irMS has been applied (or can potentially be applied) to study the sources and behavior of subsurface contaminants as diverse as monoaromatic hydrocarbons (15), polycyclic aromatic hydrocarbons (16, 17), chlorinated solvents (18, 19), polychlorinated biphenyls (20), and crude oils and other refined hydrocarbon products (21). The ability to measure the isotopic composition of individual compounds may allow for the determination of whether some contaminants are being affected by certain subsurface processes, whereas other contaminants are not. In comparison, monitoring the isotopic composition of CO2 can only provide information concerning the processes affecting the contaminant mixture as a whole rather than individual compounds. Furthermore, GC/irMS can be used to measure the nitrogen isotopic composition of individual compounds (22), and GC/irMS systems that allow for the measurement of the hydrogen isotopic composition of individual contaminant compounds have recently become commercially available. The ability to monitor more than one isotope composition will greatly improve the ability to identify the processes controlling contaminant behavior in the subsurface. Although GC/irMS systems that can measure the chlorine isotopic composition of individual chlorinated hydrocarbons are currently unavailable, it is clear that chlorine isotope analysis is also a useful technique to consider for study (19, 23, 24). To date, stable isotope analysis has been used more frequently to determine the sources of organic contamination rather than being used to understand the processes affecting organic contaminant concentrations. However, the lack of studies that have used stable isotope analysis to investigate subsurface processes may be related to the paucity of data available concerning the isotope fractionation factors associated with the various biological (e.g., microbial degradation) and abiological processes (e.g., vaporization, dissolution of a liquid phase, adsorption, inorganic degradation) that can affect contaminant concentrations in the subsurface. While vapor pressure data of isotopically labeled compounds can provide qualitative measurements of isotope fractionation during vaporization (25), quantitative measurements are only available for benzene, toluene, ethylbenzene (carbon only; 12, 26), trichloroethylene (carbon and chlorine; 12, 27), dichloromethane (carbon and chlorine; 27), tetrachloroethylene, 1,1,1-trichloroethane, carbon tetrachloride, and chloroform (carbon and chlorine; 28). Of these studies, only one has performed measurements over a range of temperatures (26). Other inorganic processes are even more poorly VOL. 33, NO. 20, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Stable Isotope Compositions (C, Cl, and H) of Residual Trichloroethylene vs Extent of Evaporation for Progressive Evaporation Experiments at 22 ( 2 °Ca (exp 1)

(exp 2)

F

δ13CVPDB (‰)

F

δ13CVPDB (‰)

F

δ37ClSMOC (‰)

F

δDVSMOW (‰)

1 0.502 0.331 0.227 0.159 0.0992 0.0819 0.0689 0.0569 0.0485 0.0339

-29.81 -30.08 -30.08 -30.24 -30.36 -30.52 -30.54 -30.66 -30.74 -30.74 -31.04

1 0.446 0.312 0.232 0.154 0.0985

-30.46 -30.53 -30.64 -30.56 -30.90 -30.97

1 0.446 0.312 0.232 0.154 0.0985 0.0692

-2.37 -1.68 -0.98 -0.43 +0.18 +0.96 +2.02

1 0.438 0.309 0.234 0.161 0.132

+528 +550 +516 +530 +516 +501

a

F ) fraction of original amount of liquid trichloroethylene remaining.

documented. Isotope fractionation between the vapor phase and the dissolved aqueous phase has been studied only for toluene and trichloroethylene (carbon only; 12, 29). Fractionation associated with adsorption has been quantified only for toluene in regard to sample extraction using a poly(dimethylsiloxane)-coated solid-phase microextraction fiber (14) and qualified for benzene, toluene, and ethylbenzene based on high-pressure liquid chromatography analyses of isotopically labeled and unlabeled compounds (carbon and hydrogen; 26, 30). Isotope fractionation associated with the reductive dechlorination of chlorinated ethylenes by zerovalent iron and zinc has been studied (carbon and chlorine; 29, 31, 32), while isotope fractionation associated with natural or microbial degradation has been studied for dichloromethane (carbon and chlorine: 33), trichloroethylene (chlorine only; 19, carbon only; 29), tetrachloroethylene (chlorine only; 34), and toluene (carbon only; 35). This objective of this study is to extend our knowledge concerning isotope fractionation during vaporization of trichloroethylene, the most frequently detected groundwater contaminant at hazardous waste sites (36) by measuring equilibrium carbon isotope vapor-liquid fractionation between 5 and 35 °C and by measuring the carbon, chlorine, and hydrogen isotope fractionation during progressive evaporation of trichloroethylene at room temperature.

Experimental Methods All experiments used ACS grade trichloroethylene (minimum assay 99.5% C2HCl3; Spectrum Quality Products Inc., Lot No. LD0093). Briefly, equilibrium vapor-liquid carbon isotope fractionation experiments vs temperature were conducted by equilibrating liquid trichloroethylene with trichloroethylene vapor in an otherwise empty glass reaction vessel, equipped with a valved aliquotter. The reaction vessel was placed in a temperature-controlled incubator overnight at the temperature of interest. Trichloroethylene vapor was then collected (at experimental temperature) by means of the valved aliquotter. The apparatus and technique used have been described in more detail previously (26). Samples for carbon isotope analysis were combusted in quartz tubes with CuO and Cu to form CO2 (37). The quantity of CO2 formed was measured during cryogenic cleanup and compared to the amount of CO2 expected for the experimental temperature, volume of the sample aliquotter, and vapor pressure of trichloroethylene (38) calculated using the ideal gas equation. Values of ∆13Cvapor-liquid have been corrected for the quantity and isotopic composition of sample removed by sampling and for the vapor-liquid isotope mass balance in the reaction vessel by using the known mass of trichloroethylene introduced, the size and isotopic composition of samples withdrawn, the initial isotopic composition of the 3690

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trichloroethylene, the volume of the reaction vessel, the ideal gas equation, and the vapor pressure of trichloroethylene (38). The error associated with this correction is approximately (0.02‰. The progressive evaporation experiments were conducted at room temperature (22 ( 2 °C) in a manner similar to previous experiments (27), except that a glass cover and forced air were not used. Approximately 10 mL of trichloroethylene were introduced into a glass 12-mL centrifuge vial and simply allowed to evaporate in a fume hood. Periodically, samples were capped (with a PTFE-lined cap) and weighed in order to measure the quantity of trichloroethylene remaining, and samples were taken for isotope analysis using a syringe. The vial was then recapped and reweighed in order to measure the total amount of trichloroethylene removed during sampling. The quantity of trichloroethylene remaining has been corrected to reflect the amount of trichloroethylene removed by sampling. For carbon isotope analyses, trichloroethylene was converted to CO2 as described above. For hydrogen isotope analyses, trichloroethylene was converted to H2O by combustion in Pyrex tubes loaded with CuO and Ag foil at 550 °C. H2O was then converted to H2 by reaction with Zn at 500 °C (39). For chlorine isotope analyses, trichloroethylene was combusted with CuO to form CuCl, followed by reaction with excess CH3I to form CH3Cl (40). Experimental results are reported in the usual δ notation vs VPDB for carbon, vs VSMOW for hydrogen, and vs SMOC for chlorine. The equilibrium vapor-liquid samples and carbon isotope evaporation experiment 1 samples were converted to CO2 at the University of Wyoming and then analyzed for isotopic composition by Mountain Mass Spectrometry (Evergreen, CO). Replicate measurements indicate an analytical uncertainty (1σ) of 0.04‰ for each δ13C analysis. The samples from the evaporation experiments for hydrogen, chlorine, and carbon (experiment 2) were processed and analyzed by Global Geochemistry Corporation (Canoga Park, CA). Replicate measurements indicate analytical uncertainties (1σ) of 0.08‰ for carbon, 0.25‰ for chlorine, and 15‰ for hydrogen. The large analytical uncertainty associated with the hydrogen isotope experiments is a function of the small sample sizes analyzed, the low concentration of hydrogen in trichloroethylene, and the lack of internationally recognized hydrogen isotope standards similar to the extremely heavy isotopic composition of the trichloroethylene used in the experiments (see Results). Different δ13C values for the initial trichloroethylene were reported by the two analytical laboratories (-29.81‰ vs -30.46‰; Table 1). The critical measurements in these experiments concerned changes in isotopic composition over the course of the experiments, and each individual experiment had all relevant analyses performed by a single laboratory. Hence, it has been assumed

FIGURE 1. Equilibrium values of ∆13Cvapor-liquid vs temperature for trichloroethylene. Error bars represent cumulative uncertainties associated with the mass balance correction (see Experimental Methods) and two isotope analyses (total of (0.06‰). that all analyses performed by the same laboratory are internally consistent and allow for calculation of the relevant fractionation factors. The reason for the analytical discrepancy between the two laboratories is undetermined.

Results Isotopic Composition of Trichloroethylene. The initial isotopic composition of the trichloroethylene used in these experiments is δ13C ) -30.1‰ (average), δ37Cl ) -2.37‰, and δD ) +528‰. These values fall within the range of isotope compositions measured previously for trichloroethylene (δ13C ) -48.0 to -27.8‰, δ37Cl ) -2.54 to +4.08‰, δD ) -30 to +530‰; 23, 41). Equilibrium Carbon Isotope Vapor-Liquid Fractionation. Results of the equilibrium carbon isotope vapor-liquid fractionation experiments are plotted vs temperature in Figure 1. Experimental values of ∆13Cvapor-liquid show a total range between +0.07 and +0.82‰ and show a general trend of decreasing with increasing temperature, from approximately +0.7‰ at 5 to 15 °C to approximately +0.1‰ at 35 °C. The only previous studies of the equilibrium carbon isotope vapor-liquid fractionation factor for trichloroethylene (12, 29) measured a value of ∆13Cvapor-liquid ≈ 0‰, within the experimental uncertainty of (0.5‰ typically associated with GC/irMS analysis. However, the measured values of ∆13Cvapor-liquid are consistently within the range of 0.3-0.8‰ (12), which is in good agreement with the value of ∆13Cvapor-liquid measured in this study. Fractionation Associated with Progressive Evaporation. Results of the progressive evaporation experiments are presented in Table 1. As a first approximation, it has been assumed that isotope fractionation associated with evaporation in these experiments follows a Rayleigh distillation trend:

δ ) (δi + 1000)F(R - 1) - 1000

(1)

where δ is the the isotopic composition of trichloroethylene for a particular value of F, δi is the initial isotopic composition of trichloroethylene, F is the fraction of liquid trichloroethylene remaining, and R is the vapor-liquid fractionation factor. Rearrangement of eq 1 gives

ln[(δ + 1000)/(δi + 1000)] ) (R - 1) ln F

(2)

Hence, if evaporation does follow a Rayleigh distillation trend, a plot of ln[(δ + 1000)/(δi + 1000)] vs ln F should give a straight line with a slope of (R - 1). ln[(δ + 1000)/(δi + 1000)] has been plotted vs ln F in Figure 2. Figure 2A demonstrates a good linear least-squares fit between ln[(δ13C + 1000)/(δ13Ci + 1000)] and ln F (values of R ) 0.99 and 0.90 for experiments 1 and 2, respectively), which strongly suggests that evaporation does indeed follow a Rayleigh distillation trend. Slopes of the regression gives values of RCvapor-liquid ) 1.00035 and 1.00024, corresponding to values of ∆13Cvapor-liquid ) +0.35‰ ((0.02‰) and +0.24‰ ((0.06‰) for experiments 1 and 2, respectively. Similar previous experiments (27) measured a value of ∆13Cvapor-liquid ) +0.31‰ ((0.04‰) for trichloroethylene, and ∆13Cvapor-liquid ) +0.65‰ ((0.02‰) for dichloromethane. Figure 2B also demonstrates a good linear least-squares fit between ln[(δ37Cl + 1000)/(δ37Cli + 1000)] and ln F (R ) 0.99), again suggesting that evaporation follows a Rayleigh distillation trend. The slope of the regression gives a value of RClvapor-liquid ) 0.99836, corresponding to a value of ∆37Clvapor-liquid ) -1.64‰ ((0.13‰). Similar previous experiments (27) measured a value of ∆37Clvapor-liquid ) -1.81‰ ((0.22‰) for trichloroethylene, and ∆37Clvapor-liquid ) -1.48‰ ((0.06‰) for dichloromethane. Figure 2C demonstrates a linear least-squares fit between ln[(δD + 1000)/(δDi + 1000)] and ln F with considerably more scatter than the regression lines for carbon and chlorine (R ) 0.60). While it is possible that during evaporation trichloroethylene would undergo Rayleigh distillation for carbon and chlorine but not hydrogen, it seems more likely that hydrogen isotope fractionation also follows a Rayleigh distillation trend. The lower value of R for the hydrogen isotope regression as compared to the carbon and chlorine isotope regressions is almost certainly attributable to the large analytical uncertainty associated with the hydrogen isotope analyses (see Experimental Methods). The slope of the regression gives a value of RHvapor-liquid ) 1.0089, corresponding to a value of ∆Dvapor-liquid ) +8.9‰ ((5.9‰). No similar measurements are available in the literature. VOL. 33, NO. 20, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Discussion Isotopic Composition of Trichloroethylene. The extremely heavy hydrogen isotope composition of trichloroethylene is probably a function of isotope fractionation associated with the various chemical synthesis reactions used to produce trichloroethylene. Comprehensive discussions of the industrial synthesis of trichloroethylene and other halogenated hydrocarbons are available (42, 43). As noted previously (41), elimination of hydrogen chloride is a common reaction during industrial synthesis of chlorinated hydrocarbons. For example, one process used to produce trichloroethylene is the chlorination of acetylene, followed by dehydrochlorination:

C2H2 + 2Cl2 f CHCl2CHCl2

(3)

CHCl2CHCl2 f C2HCl3 + HCl

(4)

Reaction 4 is interesting in that it produces HCl, which is extremely isotopically light as compared to virtually all other common H-bearing species (44-46). For example, RCH4-HCl ) 1.294 at 300 °C (46; using the harmonic approximation for the CH4 calculations). Hence, chemical reactions producing isotopically light HCl will result in other chemical products being isotopically enriched, since yields for these reactions are designed to be high (e.g., reaction 4 has a typical yield of 90%; 42). Dehydrochlorination reactions are common reactions during the industrial production of many chlorinated hydrocarbons, so it seems likely that other compounds of environmental concern will also have isotopically enriched δD signatures. Dehydrogenation and dehydrobromination reactions may also produce isotopically enriched compounds, since H2 (in particular) and HBr are also isotopically depleted vs other H-bearing compounds (44), whereas dehydrofluorination is unlikely to produce isotopically enriched compounds, as HF shows relatively small fractionation effects vs other H-bearing compounds (46). Moreover, various halogenated compounds are likely to show a very wide range in isotopic signatures due to the various chemical reactions used to produce the compounds (some of which may not involve dehydrochlorination or dehydrogenation reactions) and the various operating conditions used by different manufacturers (e.g., temperature, catalyst used, engineering design, etc.), which will result in a wide range of isotope fractionation effects associated with manufacture. This wide range in δD signatures is likely to be extremely useful in forensic geochemistry applications when trying to assign chlorinated hydrocarbon contamination to various possible sources, as it suggests that isotope differentiation between various sources is very likely as compared to the relatively narrow range of δ13C and δ37Cl values measured for various contaminants (13, 15-17, 20, 23, 26). Equilibrium Carbon Isotope Vapor-Liquid Fractionation. There are no studies of the equilibrium carbon isotope vapor-liquid fractionation factor for halogenated hydrocarbons other than trichloroethylene (this study, 12, 29) available in the literature. However, data are available for benzene, toluene, and ethylbenzene over the temperature range of 5-70 °C (26). Benzene, toluene, and ethylbenzene show similar behavior to trichloroethylene in that all values of ∆13Cvapor-liquid are positive and relatively small (≈ +0.2‰). However, benzene, toluene, and ethylbenzene show contrasting behavior to trichloroethylene in that values of ∆13Cvapor-liquid are independent of temperature. Positive values of ∆13Cvapor-liquid are counter-intuitive, as the expected result is that the vapor should be isotopically depleted with respect to the liquid during evaporation. However, there is convincing evidence that values of ∆13Cvapor-liquid are positive for many contaminant hydrocarbons of environmental concern. Lines of evidence include 3692

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FIGURE 2. Isotope fractionation, presented as ln[(δ + 1000)/(δi + 1000)], vs extent of evaporation, presented as ln F, during progressive evaporation of trichloroethylene for carbon, chlorine, and hydrogen. See text for discussion. Lines represent linear least-squares fits. Error bars represent cumulative uncertainties associated with two isotope analyses (C 1: (0.00006, C 2: (0.0011, Cl: (0.00035, H: (0.014). the following: direct measurement of equilibrium vaporliquid fractionation factors for trichloroethylene (this study), benzene, toluene, and ethylbenzene (26); progressive evaporation experiments for trichloroethylene (this study, 27) and dichloromethane (27); distillation experiments for benzene, toluene, anethole, chloroform, and carbon tetrachloride (47, 48); vapor pressure measurements for 13C- and 12Csubstituted benzene, chloroform, and carbon tetrachloride (47, 49; for other organic compounds, 25); and the observation of 13C enrichment in the earlier eluting portion during GC/ irMS analysis of toluene and other, noncontaminant hydrocarbons (14). Fractionation Associated with Progressive Evaporation. There is a good agreement between values of ∆13Cvapor-liquid and ∆37Clvapor-liquid measured in this study and those measured previously (27), even though the experimental designs are

slightly different (carbon: +0.35‰ and +0.24‰ vs +0.27‰, chlorine: -1.64‰ vs -1.81‰). In contrast to the positive value of ∆13Cvapor-liquid, the negative value of ∆37Clvapor-liquid is the expected result for vaporization and is consistent with the negative value of ∆37Clvapor-liquid measured during evaporation of dichloromethane (27) and with vapor pressure measurements for 37Cl- and 35Cl-substituted compounds such as chloromethane, chloroform, and carbon tetrachloride (47, 49, 50). The positive value of ∆Dvapor-liquid (+8.9‰), like the positive value of ∆13Cvapor-liquid, is again an unexpected result for vaporization. While there is a large uncertainty associated with the value of ∆Dvapor-liquid, Figure 2C suggests that the value of ∆Dvapor-liquid is indeed positive, which is consistent with indirect evidence for other compounds (48, 51, 52). There is a small disagreement between the value of ∆13Cvapor-liquid measured by equilibrium vapor-liquid experiments (≈0.5-0.6‰ at 22 °C; Figure 1) and the value of ∆13Cvapor-liquid measured by the progressive evaporation experiments (+0.35‰ and +0.24‰ at 22 ( 2 °C, this study; +0.27‰ at 23 ( 2 °C, 27). Although the disagreement between the equilibrium and progressive evaporation experiments is small and further study of this difference is warranted, these data suggest that the value of the vapor-liquid fractionation factor measured by the progressive evaporation experiments does not provide an equilibrium ∆13Cvapor-liquid value. As discussed previously (27), there are two general mechanisms that control isotope fractionation, namely, kinetic isotope effects (typically fast, incomplete, and unidirectional processes) and equilibrium isotope effects (typically reversible processes, controlled by different free energy changes associated with reactions involving different isotopes). A value of ∆13Cvapor-liquid ) -3.74‰ has been calculated for the kinetic isotope effect associated with the evaporation of trichloroethylene (27). Hence, the progressive evaporation experiments conducted previously (27) and in this study appear to be dominated by vapor-liquid equilibrium effects, with a small contribution from kinetic effects producing a small negative shift in the value of ∆13Cvapor-liquid. Vapor-Liquid Isotope Fractionation. Equilibrium vaporliquid isotope fractionation factors are positive for carbon and hydrogen but negative for chlorine as compared to the negative values that might be expected to accompany vaporization for all elements. Although inverse isotope effects associated with vapor pressure (i.e., positive vapor-liquid fractionation factors) have long been known (47), a simple explanation for these observations remains elusive. It is particularly puzzling that positive fractionation factors for carbon and hydrogen together with a negative fractionation factor for chlorine are observed for the same molecule. Furthermore, a review of the literature indicates that virtually all investigations to date of vapor-liquid isotope fractionation factors for a comprehensive suite of organic compounds with very different compositions and physicochemical properties are positive for carbon and hydrogen but are negative for chlorine. Last, it appears that while some compounds have vapor-liquid isotope fractionation factors that are a function of temperature, other compounds have vapor-liquid isotope fractionation factors that are essentially independent of temperature. Consideration of a variety of physical, chemical, and thermodynamic properties of these compounds together with continued measurements of vapor-liquid fractionation factors for additional compounds of interest is currently in progress in an effort to provide a comprehensive explanation for all of the observations of this interesting phenomenon. Application to Contaminant Remediation. Stable isotope analysis has already been used to a significant extent as a technique to investigate the behavior of organic contaminants in the subsurface, particulary during remediation implementation. Vapor-liquid fractionation factors are particularly suited to investigation of remediation techniques such as

soil vapor extraction (pumping air into the subsurface at a high rate to promote contaminant vaporization with subsequent entrainment and removal) and soil bioventing (stimulation of aerobic microbial degradation of contaminants by pumping air at a low rate into the subsurface, which also removes some contaminant as vapor). In the case of soil vapor extraction, if vaporization from a contaminant liquid is the dominant process taking place, monitoring stable isotope values vs time can provide useful qualitative and maybe quantitative information as to the efficacy of soil vapor extraction as a remediation technique. For the example of soil vapor extraction of trichloroethylene, it would be expected that the δ13C value of trichloroethylene vapor would decrease vs time, while the δ37Cl value would increase due to isotope fractionation of the residual trichloroethylene liquid as remediation proceeds. δD values would also be expected to decrease vs time, although the large analytical uncertainty presently associated with the measurement of trichloroethylene δD composition may make hydrogen isotope effects difficult to resolve, except for remediation with extreme vapor removal. However, it is anticipated that method development will allow for improved analytical uncertainty and, hence, facilitate δD analysis as a monitoring tool. In the case of soil bioventing, stable isotope analysis can provide an analysis of the relative importance of aerobic microbial degradation vs vapor extraction in the remediation effort. For example, aerobic microbial degradation of dichloromethane results in isotopic enrichment of residual dichloromethane in both carbon and chlorine (33), whereas vaporization of dichloromethane results in an isotopic enrichment of residual dichloromethane in chlorine but an isotopic depletion in carbon (27). Consideration of the evolution of dichloromethane δ13C and δ37Cl compositions (and δD compositions, once the relevant fractionation factors are known) as soil bioventing proceeds can provide not only the extent of total dichloromethane remediation but also the relative importance of aerobic degradation vs vapor extraction if the fractionation factors associated with aerobic degradation and vaporization are known. While vapor-liquid fractionation factors are small, they are measurable and significant, particularly in situations where extensive removal of vapor takes place, as might be expected during remediation by soil vapor extraction. Assuming vapor-liquid equilibrium for trichloroethylene, further assuming that Rayleigh distillation trends are observed (see eq 1) and using example values of ∆13Cvapor-liquid ) +0.7‰, ∆37Clvapor-liquid ) -1.6‰, and ∆Dvapor-liquid ) +9‰, removal of 90% of the initial contaminant will result in shifts of the isotopic composition of the residual liquid trichloroethylene of δ13C by -1.6‰, δ37Cl by +3.7‰, and δD by -21‰.

Acknowledgments This research has been supported by an award from NSFEnvironmental Geochemistry and Biogeochemistry (Grant EAR-9631735). This manuscript has benefitted from comments and suggestions by Greg Arehart.

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Received for review April 12, 1999. Revised manuscript received July 30, 1999. Accepted August 2, 1999. ES990406F