Effects of Volatilization on Carbon and Hydrogen Isotope Ratios of MTBE

Feb 12, 2009 - Contaminant attenuation studies utilizing CSIA (compound- specific isotope analysis) routinely assume that isotope effects. (IEs) resul...
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Environ. Sci. Technol. 2009, 43, 1763–1768

Effects of Volatilization on Carbon and Hydrogen Isotope Ratios of MTBE TOMASZ KUDER,* PAUL PHILP, AND JON ALLEN School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, SEC 810, Norman, Oklahoma 73019

Received October 6, 2008. Revised manuscript received January 6, 2009. Accepted January 7, 2009.

Contaminant attenuation studies utilizing CSIA (compoundspecific isotope analysis) routinely assume that isotope effects (IEs) result only from degradation. Experimental results on MTBE behavior in diffusive volatilization and dynamic vapor extraction show measurable changes in the isotope ratios of the MTBE remaining in the aqueous or nonaqueous phase liquid (NAPL) matrix.AconceptualmodelforinterpretationofthoseIEsisproposed, based on the physics of liquid-air partitioning. Normal or inverse IEs were observed for different volatilization scenarios. The range of carbon enrichment factors (ε) was from +0.7‰ (gasoline vapor extraction) to -1‰ (diffusive volatilization of MTBE from gasoline), the range of hydrogen ε was from +7‰ (gasoline vapor extraction) to -12‰ (air sparging of aqueous MTBE). The observed IEs are lower than those associated with MTBE degradation. However, under a realistic scenario for MTBE vapor removal, their magnitude is within the detection limits of CSIA. The potential for interference of those IEs is primarily in confusing the interpretation of samples with a small extent of fractionation and where only carbon CSIA data are available. The IEs resulting from volatilization and biodegradation, respectively, can be separated by combined carbon and hydrogen 2D-CSIA.

Introduction A growing number of contaminant degradation studies, in particular those associated with volatile organic compounds (VOCs), such as chlorinated ethenes, MTBE, and benzene, utilize compound-specific isotope analysis (CSIA) (1-3). The principle of those applications is that degradation results in enrichment of the heavy isotope species in the remaining contaminant (4), with a necessary premise that no other processes cause interfering isotope effects. Stable isotope effects resulting from sorption, dissolution, dispersion, and volatilization are generally considered insignificant as far as interpretation of field data is considered. For volatilization, the studies cited in this context test simple physical scenarios of phase equilibrium or progressive evaporation of organic liquid (5-7). While the physical conditions in those studies did not approximate the conditions of sediment where the processes of contaminant volatilization and dissolution occur, some of the measured isotope effects were significant. Wang and Huang (8, 9), for example, reported strong fractionation of hydrogen isotopes in progressive volatilization of neat VOC liquids and neat ethanol (Supporting Information Table S3). More recently, in a controlled release of a gasoline-like * Corresponding author e-mail: [email protected]. 10.1021/es802834p CCC: $40.75

Published on Web 02/12/2009

 2009 American Chemical Society

mixture into an aquifer and in a companion sediment column study, Bouchard et al. (10, 11) observed preferential diffusion of 12C VOCs isotopomers through unsaturated sediment. By inference, a corresponding enrichment of 13C isotopomers in source gasoline could be expected in the aquifer. The source enrichment was directly observed in the latter study. The objective of the present study is an experimental determination of MTBE volatilization-related isotope effects (IEs) in sediment columns to test realistic physical scenarios that mimic aquifer/vadose zone conditions. Physical attenuation of VOCs is a complex process that involves partitioning between residual nonaqueous phase liquid (NAPL), dissolved aqueous phase, soil solids (sorption), and pore space vapor. Sorption of MTBE is a minor factor in subsurface mass distribution and no MTBE sorption-related isotope phenomena could be detected in earlier work (12). While natural volatilization of MTBE from aqueous phase is not considered a major pathway of attenuation at most sites (13), it can be significant locally (e.g., in warm climates and in high permeability sediments). Natural volatilization of MTBE from nonaqueous phase liquid (NAPL) can be more significant (13). In a more general sense, MTBE can be a model for other VOCs that are more susceptible to natural volatilization. IEs in advective vapor removal-based attenuation (e.g., soil vapor extraction) are potentially more important, since significant mass attenuation of MTBE and VOCs in general occurs under such conditions. The results have been evaluated to assess whether volatilization of MTBE may cause isotope fractionation interfering with the application of CSIA to detect and quantify in situ biodegradation. The present study is a demonstration of IEs in degradationfree environment, so that the contributions of IEs from volatilization alone, if any, can be assessed. In the field, IEs from biodegradation may, and probably will, dominate the overall isotope signatures in dissolved phase (14) and in vadose zone vapor (10, 11).

Experimental Section Volatilization Experiments. The following scenarios were simulated experimentally: (1) passive volatilization of aqueous solution; (2) passive volatilization from gasoline (nonaqueous phase liquid, NAPL); (3) air sparging (AS) of aqueous solution; and (4) soil vapor extraction (SVE) of NAPL. Passive volatilization of MTBE was simulated in unsaturated columns packed with sand or glass beads. AS conditions were simulated in a watersaturated sediment column packed with sand, with a constant stream of air injected at the bottom of the column. SVE was simulated in unsaturated columns packed with sand, under four different air flow rates. While SVE relies on vacuum-driven air circulation, for practical reasons we have used a positive pressure gradient instead. Additionally, IEs in water-air and NAPL-air partitioning and in diffusion of MTBE vapor, respectively, were also determined. Water medium in all column experiments contained 10 g/L of trisodium phosphate to increase pH of the medium and prevent biodegradation. Carbon and hydrogen isotope ratios of MTBE from the volatilization and equilibrium experiments were determined by purge and trap-GC-IRMS (14) and headspace-GC-IRMS, respectively. Results of MTBE isotope ratio and concentration analysis are summarized in Figure 1. Details of experimental conditions, including the descriptions of sediment columns, column sampling procedures, and analytical procedures (concentrations and isotope ratios) are given in the Supporting Information. Isotope EffectssDefinitions. Isotope ratios are reported in ‰ units (δ notation), representing deviation of the measured isotope ratio vs international standard, where for VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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carbon isotopes, R is 13C/12C and the standard is VPDB. For hydrogen, R is 2H/1H and the standard is VSMOW. δ13C ) (Rsample⁄Rstandard- 1)103[‰]

(1)

Reaction rates of isotopomers (molecules of the same compound with different isotope composition) are often different in chemical and physical reactions. Resulting changes of isotope ratios of the substrate over the progress of reaction, where v’/v ) R (for a given reaction, v and v’ are rates for the light and heavy isotopomers, R is a constant, known as fractionation factor), can be described in terms of Rayleigh fractionation model (eq 2). C and C0 are substrate concentrations; R and R0 are isotope ratios at the time of measurement and initial, respectively. The constant ε, enrichment factor, is derived as ε ) (R - 1) 103. ln(R ⁄ R0) ) 10-3ln(C ⁄ C0)ε

(2)

In phase equilibria, IEs correspond to differences in partitioning of different isotopomers and can be expressed in terms of relative changes of concentrations of the isotopomers or as a difference (∆) of δ values between vapor and the condensed phase (eq 3). ∆condensed-vapor ) δvapor - δcondensed

(3)

A so-called normal IE occurs when the original substrate becomes enriched in the heavy isotope over the progress of the reaction (negative values of ε), or when liquid-vapor partitioning results with preferential retention of the heavy isotope in the condensed phase (negative value of ∆). An opposite pattern of isotope enrichment is known as an inverse IE.

Results and Discussion Isotope effects in VolatilizationsGeneral Considerations. Net IE expressed on pool of the reactant reflects kinetic and equilibrium contributions up to the first rate-limiting or nonreversible step. In the example of a reaction A T B f C T D T etc., A is the initial reactant, and B, C, D represent intermediates of a reaction. The IE expressed for the reactant A is a combination of an equilibrium (A T B) and a kinetic effects (B f C). Isotope fractionation on the right site of the irreversible step B f C does not affect the left side of the sequence. A well understood complex reaction with a sequence of equilibria and kinetic elements is biological reduction of sulfate (15). A classic model of IEs in evaporation and condensation of surface water is the Craig-Gordon Model or CGM (16). CGM uses a similar concept of a composite IE. Water evaporation rate is controlled by the exchange between the air-side phase boundary layer and the open atmosphere. Strong predominance of the air-side resistance over the water-side resistance facilitates the vapor equilibrium layer. The net IE in water evaporation results from adding two elements: water-air equilibrium IE and kinetic vapor diffusion IE. The kinetic IE is modified by atmospheric humidity h (maximum IE at h ) 0) and aerodynamic properties at the boundary (restricted under turbulent conditions) (17). Equation 4 shows the cumulative IE (IE ) δvapor - δwater) at h ) 0 (adopted for notation shown in the Isotope EffectssDefinitions section). The diffusive element, ε’, is the enrichment factor ε modified to account for the effects of aerodynamic conditions, ε′ ) ε in static air. δvapor ≈ δwater + ∆water-vapor + ε′vapor diffusion

(4)

Conceptual models proposed herein for interpretation of IEs in subsurface volatilization of VOCs (Figure 2) follow the CGM approach (Supporting Information Figure S7). Depending on the physical setting, the net IE includes contributions from all or some of the following processes: (1) 1764

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liquid diffusion (rate-limiting liquid-side resistance); (2) molecular exchange at the phase boundary (phase equilibrium); and (3) gas diffusion (rate-limiting air-side resistance). For poorly mixed liquid (e.g., in porous medium with restricted advective mixing), sampling of stratified liquid can additionally mask isotope fractionation. While surface liquid can be fractionated, the deeper layers may show less effect. A sample drawn from the deeper, stagnant liquid may not show significant IEs. The boundary layer resistance can be described in terms of air-side (v′ia) and liquid-side (vil) transfer velocities, where the subscript i indicates the specific volatile compound (Supporting Information Table S2) (18). For water and also for miscellaneous polar compounds in aqueous solution v′ia , vil (air-side transfer resistance), and their IEs follow the pattern described above for water. On the other hand, volatilization rates of most VOCs in aqueous solution are mainly restricted at the liquid-side boundary (v′ia . vil). IEs for those compounds will be constrained by the phenomena in the liquid boundary layer. MTBE characteristics suggest that contributions from both liquid- and air-side transfer are likely (v′ia ≈ vil). Equation 5 shows contributions to the cumulative IE for volatilization of a VOC (εdiff. is a kinetic IE, ∆ is an equilibrium IE). For a given physical setup, some of the elements are effectively reduced or nullified, as discussed later for the representative scenarios of MTBE volatilization. IE ∼ ∆liquid-air + εdiff. air or IE ∼ εdiff. liquid(v ia for . vil) (5) In a recent study, Bouchard et al. (11) used Fick’s law to obtain mathematical solution for IE in steady-state diffusive volatilization of NAPL. Their solution was identical to that of eq 5 where v’ia , vil. At this point, it is worthwhile to observe that while IEs in gas, and probably in liquid diffusion, are normal, phase equilibria of various VOCs may cause normal or inverse IEs. In the latter case, the equilibrium and diffusive IEs would subtract from each other. If the inverse equilibrium IE is larger than the diffusive IE, the net IE is inverse, as reported previously (9) and in the present study. Finally, in VOCs remediation techniques, such as soil vapor extraction (SVE) and air sparging (AS), advection is the predominant mechanism of vapor removal. Under such conditions, diffusive transport of vapor is suppressed and the phase equilibrium at the air-liquid interface cannot be taken for granted. Isotope Effects in Phase Equilibria. Equilibrium IEs for VOCs are summarized in Supporting Information Table S3. Small inverse carbon IEs, +0.3 ( 0.1‰ and +0.2 ( 0.1‰, respectively, were determined in the present study in NAPL-air and water-air partitioning. Stronger hydrogen IEs were determined, an inverse +9 ( 1‰ in NAPL-air and a normal -9 ( 1‰ in water-air partitioning. Additionally, water-air hydrogen IE of benzene (-11 ( 1‰) and NAPL-air hydrogen IE of toluene (+4 ( 1‰) were determined. The direction and range of the IEs in this and other studies are comparable (Supporting Information Table S3) and agree with the classical theory of condensed phase IEs (19). The presence of a heavy isotope in the molecule reduces its vibrational energy (zeropoint energy, ZPE, effect) with an effect of decreasing its vapor pressure. ZPE effect is particularly significant for the isotopomers differing by 2 daltons, such as 35Cl vs 37Cl. For isotope species differing by 1 dalton, ZPE effect may be overshadowed by intermolecular forces (van der Waals forces, VDW). For a number of compounds, inverse IEs have been observed, explained by a reduction of the so-called dispersive VDW force by heavy isotope substitution. On the other hand, the normal strong hydrogen IEs and weak inverse carbon IEs for MTBE and benzene in aqueous solution-air equilibrium appear to reflect preferential binding of water to 2H-MTBE and 2H-benzene. MTBE and benzene form hydrogen bonds with water (both compounds are hydrogen acceptors). While

FIGURE 1. Rayleigh-type fractionation plots for the different scenarios of MTBE volatilization. (a) NAPL, passive volatilization (all data pooled). Note the anomalous point in the hydrogen isotope data set; (b) aqueous solution, passive volatilization (all data pooled); (c) air sparging (all data pooled); (d) SVE (all data pooled); (e) SVE, carbon isotope ratios, split by air flow rates; (f) SVE, hydrogen isotope ratios, split by air flow rates. Measured concentrations and isotope ratios shown after normalization to the initial concentrations and isotope ratios (C/C0 and R/R0, respectively), cf. eq 2. 2

H preference has been reported for hydrogen-bonded donor species (e.g., water) no information is available on the role of vicinal 2H substitution. Isotope Effects in Diffusion. In ideal gas there is an inverse square root relationship between molecule mass (m) and velocity (v). If the medium of diffusion is gas with molecular mass of mm, the velocity ratio for molecules with masses m and m’ (the superscript indicates the heavy molecule) is estimated from eq 6 (20): v′ ⁄ v ) R)[(mm′ + mmm) ⁄ (m′m + m′mm)]1⁄2

(6)

In diffusion, the v′/v is equivalent to the Rayleigh fractionation factor (R). For MTBE (m ) 88, m′ ) 89) in air (mm ) 28.8), R ) 0.9986, which corresponds to an enrichment factor ε ) -1.4‰. In the present study, an identical experimental carbon ε ) -1.4 ( 0.1‰ was obtained. Since adding 13C or 2 H results with identical increases of molecular mass, the values of carbon and hydrogen ε are assumed to be identical (-1.4‰). In liquid diffusion the mass effect can be modified by intermolecular forces affecting viscosity (20). In most cases the published of liquid diffusive IEs are lower than the values obtained from eq 6, with an exception of deuterated or tritiated water where the IEs are augmented by stronger hydrogen bonding of heavy molecules. It can be expected that hydrogen bonding in general will result with an augmentation of a normal IE, while dispersive force would tend to reduce a normal IE. Even so, hydrogen IEs in aqueous diffusion of two studied organic compounds were 2 and 4 times lower, respectively, than the values predicted by Equation 6 (21). Carbon IEs of

methane and ethane measured in another study (22) were lower than predicted by an order of magnitude. Isotope Effects in Volatilization of MTBE from Porous Media. For natural volatilization, the mass flux of vapor in the unsaturated zone occurs primarily via molecular diffusion. The sediment columns used in the present study simulated two scenarios: (1) partitioning of MTBE from freephase NAPL with diffusive venting of vapor; (2) partitioning of MTBE from aqueous solution with diffusive venting of vapor. A number of VOC remediation techniques, e.g., soil vapor extraction (SVE) and air sparging (AS), rely on advection for removal of vapor, limiting the role of molecular diffusion. Two scenarios tested in this study were (1) SVE-like attenuation of MTBE in NAPL in unsaturated sediment; and (2) AS-like attenuation of aqueous MTBE in saturated sediment. Passive volatilization of MTBE from dilute solutions is governed by the liquid-air partitioning coefficients (Henry’s Law). The calculated values of v’ia and vil suggest a combined transfer resistance at liquid- and air-side boundary layers, so that the composite IEs will be rather complex and involve contributions from liquid and vapor diffusion and liquid-air equilibrium (eq 5, Figure 2a). In the case of a spill of fresh gasoline with MTBE, concentration of MTBE vapor follows Raoult’s Law and MTBE volatilization rate would be restricted at the air-side boundary, with the net IE reflecting the NAPL-air equilibrium plus the air diffusion IE (Figure 2a assumes that liquid mixing is fast enough to compensate for volatilization at the surface, but in reality, some restriction in the liquid mixing rate may occur even at high MTBE content). Experimental carbon IEs were ε ) -1.0 ( 0.1‰ and ε ) -0.8 ( 0.1‰, for VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Conceptual model of isotope effects in MTBE volatilization scenarios: fluxes of MTBE and corresponding changes of MTBE concentrations and isotope ratios. IE and concentration scales are qualitative. Horizontal arrows indicate the direction and magnitude of change of isotope ratios of MTBE. ∆: equilibrium IE, ε: kinetic IE (cf. eq 5). (a) MTBE in NAPL, carbon and hydrogen IEs in passive volatilization; (b) MTBE in NAPL, hydrogen IEs in vapor extraction (SVE), extraction under slow (left) and fast (right) advective air flow1. In the model proposed for SVE under fast advective flow, ∆NAPL-air does not contribute to the net IE because of the volatilization rate restriction at the liquid-diffusive boundary layer. Additional scenarios are illustrated in Supporting Information Figure S9. NAPL and aqueous solution volatilization, respectively. Hydrogen IEs determined experimentally were ε ) -5 ( 1‰ for aqueous solution and an inverse ε ) +4 ( 1‰ for NAPL. One data point, representing most advanced NAPL volatilization (a decrease of MTBE load from 200 mg to ∼0.2 mg) showed a reduced hydrogen IE (ε ) +2‰, based on the initial and the end data points). All data fit the Rayleigh fractionation model (Figure 1), with R2 of 0.8-0.9. No apparent changes of the magnitude and direction of IEs were observed, except for the one mentioned δD measurement, suggesting that steady state was established quickly and maintained throughout the experiment. Experimental IEs agree with the proposed conceptual model of volatilization. The net carbon IEs are dominated by their vapor diffusion elements, whereas the net hydrogen IEs are dominated by their large equilibrium elements. However, the net IEs were lower than expected from adding the equilibrium and vapor diffusion IEs, with this being most apparent for hydrogen. Surface layer fractionation in stagnant pore liquid is a likely explanation of the reduced hydrogen IEs. In advective volatilization, the mechanisms controlling IEs are primarily those related to liquid-air interface transport, whereas vapor diffusion is not significant. Unlike in the passive volatilization, isotopic phase equilibrium cannot be taken for granted. Because of the diffusive bottleneck at the liquid-air interface, the velocity of molecular exchange at the liquid-air boundary is much larger than the overall air-water exchange velocity and it is normally assumed that molecular exchange is never rate-limiting (18). However, equilibrium isotope effects are minute in comparison with the overall mass flux, and small changes in the relative significance of ZPE and VDW contribu1766

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tions cannot be excluded when vapor advection reduces the vapor-liquid contact time. In the initial phase of the SVE experiment (during the removal of at least 90-99% of MTBE) the residual MTBE became depleted in 2H and 13C. There was an apparent Rayleigh-type correlation between the isotope ratios and the MTBE concentration (Figure 1e). The inverse carbon ε ranged from +0.4‰ at the highest air flow rate to +0.7‰ at the lowest air flow rates, exceeding the +0.3‰, the largest value possible if it is assumed that isotope equilibrium was reached. The approximate value of hydrogen ε ) +7‰ was within bounds of the equilibrium IE. The pattern of isotope fractionation changed as MTBE mass was further reduced. At low air flow rates (3 and 15 mL/min), the inverse carbon isotope fractionation continued but at higher flow rates (40 and 100 mL/min) the carbon IE shifted to normal, with ε ) -0.9‰. The shift to normal hydrogen IE (ε ) -5‰) was also observed in the advanced stage of volatilization (except for the 3 mL/min experiment) but instead of a single IE direction reversal, there was an interval of fluctuating δD values (Figure 1f). Inverse IEs observed initially both for carbon and hydrogen in SVE scenario suggest the predominance of molecular exchange (equilibrium or “near equilibrium”) at that stage. The reversal of IEs direction can be rationalized by a switch to a process controlled by supply of MTBE toward the phase interface. For a dilute solution, the air-side concentration being reduced to ∼zero, liquid diffusion eventually becomes unidirectional and rate-limiting, and thus it overwhelms the net isotope effect (net IE ∼ IE of the liquid diffusion). The fluctuations of the fractionation patterns apparent for some of the SVE data may suggest fluctuations from liquid diffusion-controlled process to phase molecular exchangecontrolled process. The barrier for MTBE supply (liquid diffusion?) apparently did not develop at lower efficiency of vapor removal and the inverse IE persisted. While the carbon IE in the late stage of the SVE experiment was about the value predicted by eq 6, the large hydrogen IE is difficult to explain by liquid diffusion in NAPL. In the AS experiment, the residual MTBE became progressively enriched in 2H while MTBE concentration was reduced, with larger effect for the more dilute solution. The early and the late sections of the trend could be approximated by the Rayleigh-type linear regression (initially, ε ) -4‰, R2 ) 0.88, then increasing to -12, R2 ) 0.99). A problematic (not exceeding analytical precision) small decrease of δ13C by 0.1-0.3‰, was observed in the samples collected over the time span of the AS experiment, with no correlation to mass attenuation (Figure 1c). The observed hydrogen IE can be rationalized both by liquid diffusion analogous to the latestage SVE (and in this case, the contribution from VDW forces is likely to boost the mass effect, cf. Figure 2b), and by water-air equilibrium, analogous to the passive volatilization (cf. Figure 2a). A contribution from liquid diffusion is probable, since the inverse carbon IE, expected for water-air equilibrium, is not expressed and the late-stage hydrogen IE is larger than predicted by equilibrium alone (Figures 1d, 1e, and 1f). More robust interpretation of SVE and AS data will be possible once IE in NAPL and aqueous diffusion of MTBE are determined. Discrimination of Volatilization and Biodegradation by 2D-CSIA. Two-dimensional carbon and hydrogen CSIA (2DCSIA) can discriminate between aerobic and anaerobic biodegradation of MTBE, owing to the difference in the involved biodegradation mechanisms (14, 23). A slope of a linear regression of δ13C and δD data collected over the progress of a reaction is identical to εH/εC in that reaction, and often diagnostic differences in that parameter permit discrimination between different reaction pathways. The same principle can be applied to distinguish degradation mechanisms of other compounds, e.g., benzene (24). IEs in the diffusive volatilization and AS followed the Rayleigh-type fractionation, so that 2DCSIA could be applied in the same manner as in the degradation studies. In SVE, there was no single ε characteristic of the pathway. Instead of a regression line, a cluster of data points

FIGURE 3. Patterns of 2D-CSIA for different scenarios of MTBE volatilization and biodegradation. Data points are measurements of carbon and hydrogen isotope ratios in samples collected from sediment columns throughout volatilization experiments. Data are normalized to the initial δ of 0. The pattern for anaerobic degradation drawn after ref 14; the patterns for aerobic degradation represent the end members in ref 26. Shaded contours represent the approximate limit of IEs at 95% of MTBE mass attenuation in a given volatilization pathway. Data points plotted outside of the contours were collected after more than 95% of MTBE was volatilized. The labeled markers on the 2D biodegradation trends show the % of mass attenuation correlated to the observed isotope ratios. was obtained. As apparent in Figure 3, the four different volatilization pathways are different from each other and from MTBE degradation. Considering analytical precision of CSIA, clear differentiation of the trends is not possible for early stages of the processes, where analytical error boxes overlap between different pathways. Volatilization IEs and Assessing in Situ Biodegradation of VOCs. Based on the sediment column experiments, it is apparent that volatilization from the aqueous plume or from NAPL at the plume source area can change MTBE isotope ratios. In the latter case, a change of isotopic composition of the source NAPL would lead to a corresponding change of isotopic composition in the aqueous phase in equilibrium with the NAPL. Figure 3 shows the range of IE for 95% of mass attenuation for the studied volatilization pathways, and for reference, mass attenuation is indicated on the 2D trends from aerobic and anaerobic biodegradation. The magnitudes of those effects are proportional to the ε values of the involved attenuation pathways. The largest carbon IE in volatilization is approximately half of the values observed for most aerobic MTBE biodegradation experiments and less than one tenth of the values in anaerobic MTBE biodegradation (14, 25). The largest hydrogen IE is approximately 1/3 of the value measured for aerobic and anaerobic MTBE biodegradation (14, 26). The volatilized mass of MTBE must be proportionally larger than the mass lost to biodegradation to overwhelm the isotopic signal from biodegradation. Fractionation of isotope ratios over time would have the same effect on the accuracy of CSIA-based biodegradation assessment as heterogeneity of the contaminant source (e.g., due to multiple pulses of spilled gasoline with different isotopic compositions) in reducing the quality of Rayleigh-type (cf. Figure 1) or 2DCSIA plots (cf. Figure 3). On the other hand, CSIA evidence of volatilization, if present, can be useful in contaminated site remediation, by discrimination between MTBE mass reductions by degradation, and by physical attenuation. Regarding the potential of volatilization to mimic isotope signatures of biodegradation, relatively low magnitude of the volatilization IEs limits the possibility of false detection

of biodegradation to the samples with small range of δ13C and/or δ2H fractionation. Following the study by Lahvis (13), feasible losses of MTBE from the aqueous phase are low, implying that the resulting isotope fractionation would be below CSIA resolution. In the same study, larger potential for MTBE volatilization from the source NAPL was shown. Corresponding changes of carbon and hydrogen isotope ratios are likely to be detectable once more than half of the original mass of MTBE present in spilled NAPL is volatilized (note that in the case of hydrogen isotopes, the IE is inverse, i.e., opposite to that of biodegradation). Assessment of MTBE attenuation based on carbon isotopes alone is the predominant practice in site remediation projects. Considering that small shifts of δ13C by 1-2‰ resulting from volatilization alone are practical, it is recommended that unless independent evidence eliminates the probability of advanced volatilization (from NAPL phase in particular), only large isotope shifts be considered as a positive identification of in situ degradation without the need for additional validation. Values of δ13C observed to date at multiple sites with confirmed anaerobic degradation of MTBE (2, 14) showed large δ13C enrichments exceeding +10‰ in some of the monitoring wells of each site. This range of fractionation by volatilization is not realistic because it would require a reduction of MTBE concentration in the NAPL or in water by 5 orders of magnitude or more. However, small shifts in δ13C should be validated by hydrogen CSIA. The 2D-CSIA trends, similar to that of anaerobic biodegradation pathway and dissimilar to those of the volatilization pathways, would be an indication that the observed effects are indeed resulting from degradation. Aerobic biodegradation can be more difficult to distinguish from the effects of diffusive or advective volatilization even if 2D-CSIA were used. While under field conditions the chance of widespread mass attenuation by diffusive volatilization from aqueous plume is negligible (13), some volatilization at plume extremities is probable, with a corresponding isotope ratio effect. The expected isotope fractionation, a small carbon IE, and a relatively stronger hydrogen IE could be misinterpreted for a result of aerobic biodegradation, particularly if only a few data points are collected. A more complex situation is anticipated for advective volatilization because techniques such as SVE and AS are commonly applied to plume source areas where both NAPL and aqueous solution are volatilized simultaneously. The volatilization-related IEs will be defined by the relative contribution of aqueous and NAPL volatilization to MTBE mass attenuation. Isotope data are potentially informative for assessment of physical attenuation progress in SVE and/ or AS. While exact quantitative assessment is not feasible, the relative significance of NAPL volatilization, aqueous solution volatilization, and biodegradation can be inferred. Opposite direction of IEs in advective volatilization from NAPL and from aqueous solution would counteract each other, with the net effect reflecting the predominant element of the total flux of MTBE vapor. Predominant inverse IEs would suggest predominance of NAPL attenuation, normal IEs would suggest predominance of aqueous phase attenuation or biodegradation. Standard conceptual model of AS involves both aerobic biodegradation stimulated by oxygen delivered with air purge and volatilization. While volatilization in this case results only with 2H/1H fractionation, aerobic biodegradation results with 2H/1H and 13C/12C fractionation. Combined contributions from aerobic biodegradation and from aqueous phase volatilization could be inferred from apparent excess of 2H/1H fractionation over the range expected in aerobic biodegradation (abnormally steep slope in 2D-CSIA). In any case, the presence of AS or SVE wells and timing of their operation should be taken in consideration when planning CSIA work. Monitoring wells intercepting the parts of MTBE plume that were affected by AS or SVE are VOL. 43, NO. 6, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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likely to show abnormal patterns of isotope fractionation, inverse trends if NAPL volatilization was involved and positive anomalies of 2H/1H for aqueous phase volatilization. Comparison of air and liquid transfer velocities in NAPL-air partitioning suggests that MTBE is a good model compound for BTEX and chlorinated ethenes and that the conceptual model of IEs occurring in volatilization of MTBE should be directly applicable there. Similar to MTBE, the NAPL-air equilibrium IEs for BTEX and chlorinated ethenes are inverse (Supporting Information Table S3). When available empirical equilibrium IEs and diffusive IEs (from eq 6) are applied within eq 5, the resulting composite carbon IEs are normal for BTEX (largest for benzene, smallest for xylene), and probably very low inverse or normal for chlorinated ethenes (accurate values of equilibrium IEs are not available, Supporting Information Table S3). BTEX hydrogen IEs in NAPL volatilization are likely to follow the same pattern as the IE of MTBE since their diffusive and equilibrium IEs are of similar scale (Supporting Information Table S3). This claim is supported by a recent study of carbon IEs of benzene and several other gasoline hydrocarbons in NAPL volatilization (11). For water-air partitioning of MTBE the air and liquid transfer velocities are of similar magnitude, but for BTEX and even more so for chlorinated ethenes, the air-side velocity is significantly larger than the water-side velocity (Supporting Information Table S2). For those compounds, volatilization rate will be controlled by liquid diffusion and isotope fractionation pattern similar to that of late stage SVE (IE ∼ εdiff. liquid, eq 5) is expected. No empirical values for carbon or hydrogen IEs are available for liquid diffusion of the compounds of interest. If the proposed interpretation of the SVE experiment results is correct, and the IEs observed at low concentrations of MTBE are indeed controlled at the liquid boundary layer, similar range of carbon IEs are to be expected for other VOCs of similar molecular mass. Hydrogen IEs are more difficult to predict due to differences in intermolecular forces for individual contaminants.

Acknowledgments Funding for this work was provided by a grant from Integrated Petroleum Environmental Consortium (EPA Project No. R830633-010).

Supporting Information Available Auxiliary information on experimental procedures. Summary of liquid-air transfer velocities of VOCs (Table S2). Summary of isotope effects in volatilization and diffusion of VOCs (Table S3). Auxiliary conceptual models of volatilization IEs not included in Figure 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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