Dual Carbon–Chlorine Stable Isotope Investigation ... - ACS Publications

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Dual Carbon−Chlorine Stable Isotope Investigation of Sources and Fate of Chlorinated Ethenes in Contaminated Groundwater Charline Wiegert,† Christoph Aeppli,†,∥ Tim Knowles,‡ Henry Holmstrand,† Richard Evershed,‡ Richard D. Pancost,‡ Jiřina Machácǩ ová,§ and Ö rjan Gustafsson*,† †

Department of Applied Environmental Science (ITM), Stockholm University, Svante Arrhenius väg 8c, SE-106 91 Stockholm, Sweden ‡ School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom § AECOM CZ s.r.o., Liberec 460 11, Czech Republic S Supporting Information *

ABSTRACT: Chlorinated ethenes (CEs) are ubiquitous groundwater contaminants, yet there remains a need for a method to efficiently monitor their in situ degradation. We report here the first field application of combined stable carbon and chlorine isotope analysis of tetrachloroethene (PCE) and trichloroethene (TCE) to investigate their biodegradation in a heavily contaminated aquifer. The twodimensional Compound Specific Isotope Analysis (2D-CSIA) approach was facilitated by a recently developed gas chromatography-quadrupole mass spectrometry (GCqMS) method for δ37Cl determination. Both C and Cl isotopes showed evidence of ongoing PCE transformation. Applying published C isotope enrichment factors (εC) enabled evaluation of the extent of in situ PCE degradation (11−78%). We interpreted C and Cl isotopes using a numerical reactive transport model along a 60-m flow path. It revealed that combined PCE and TCE mass load was dechlorinated by less than 10%, and that cis-dichloroethene was not further dechlorinated. Furthermore, the 2D-CSIA approach allowed estimation of Cl isotope enrichment factors εCl (−7.8 to −0.8‰) and characteristic εCl/εC values (0.42−1.12) for reductive PCE dechlorination at this field site. This investigation demonstrates the benefit of 2D-CSIA to assess in situ degradation of CEs and the applicability of Cl isotope fractionation to evaluate PCE and TCE dechlorination.

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INTRODUCTION Tetrachloroethene (PCE) and trichloroethene (TCE) were mainly used as dry cleaning and degreasing agents, respectively,1 and are among the most common chlorinated solvents. Improper handling and disposal led to leaks and spills, causing these solvents to become major environmental contaminants.2 Under anoxic conditions in the contaminated soil and groundwater, PCE and TCE can undergo reductive dechlorination, where they act as electron acceptors and are sequentially transformed to cis-dichloroethene (cDCE), vinyl chloride (VC), and ultimately to ethene.3 The toxicological potential of these compounds is widely recognized and stems largely from the high toxicity of the degradation product VC.4 The European Environment Agency (EEA) estimated that there are nearly three million sites in Europe with past and present potentially polluting activities,5 many of them involving PCE or TCE contaminated groundwater. The EEA urged for remediation of about 250,000 of these sites, which are heavily contaminated. Furthermore, it pointed out the lack of standardized investigation and data collection methods.6 Therefore a strong motivation exists for the development of monitoring and remediation methodologies of contaminated sites. Since 1999, Monitored Natural Attenuation (MNA), i.e. relying on in situ microbial or chemical degradation to reduce © 2012 American Chemical Society

contaminant mass load, has been accepted and applied as a remediation strategy in the United States and has gained interest in European countries, as it offers a cost-effective alternative to traditional cleanup methods.1,7 However MNA requires extensive, precise, and careful quantification of the effectiveness of biotransformation, which is often difficult to accomplish based on concentration assessment. Compound Specific Isotope Analysis (CSIA) is increasingly recognized as a useful tool for evaluating MNA,8,9 since it allows for direct and in situ identification and quantification of transformation processes.10 CSIA therefore sharpens the conclusions regarding the extent of in situ degradation. This has been demonstrated for stable carbon isotope measurements (δ13C) at numerous contaminated field sites.11 The use of other stable isotope systems, such as hydrogen (δ2H) or chlorine (δ37Cl), is also gaining interest for this purpose. Sturchio and co-workers were the first to demonstrate that δ37Cl was informative to evaluate ongoing reductive dechlorination of TCE.12 However, wider application of δ37Cl has been hampered by labor-intensive and offline measurement Received: Revised: Accepted: Published: 10918

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Figure 1. (A) Map of the investigated site. The North Bohemian Carcass Disposal Plant (SAP) Mimoñ, Czech Republic is delimited by the dashed purple line. The concentrations of chloroethenes (CEs) in the Quaternary aquifer are indicated by the turquoise gradient (the darkest color corresponds to concentrations above 10 000 μg·L−1, and the lightest color corresponds to concentrations lying between 1000 and 3600 μg·L−1). The main groundwater (GW) flow is indicated by the blue arrow. The investigated wells are indicated by their numbers. The contamination plume originated from building 3 and is marked by the red well N48. The wells marked by an asterisk are shown projected in panel B. (B) Geological crosssection of the site. The dashed blue line indicates the groundwater table (GWT) of the Quaternary aquifer. The projected locations of the wells along the transects indicated in panel A with their screens as well as building 3 are shown. The main groundwater flow is indicated by the blue arrow, as going from building 3 toward the river.

techniques. The recent advent of online δ37Cl-CSIA methods using gas chromatography-quadrupole mass spectrometry (GCqMS) now allows rapid and accurate δ37Cl measurements at concentrations relevant for field samples, i.e. with a method detection limit in the pmol·L−1 range for organic extracts.13,14 The combination of two isotopic systems (2D-CSIA) can now be widely adopted for chlorinated compounds, likely providing improved constraints on degradation processes. Such a 2D-CSIA approach has already been demonstrated for benzene, toluene and methyl tert-butyl ether (MTBE), using δ13C and δ2H.15−17 However, applications for chlorinated compounds using δ13C and δ37Cl are scarce. In addition to the use of 2D-CSIA to distinguish contaminant sources,18 recent model-based work on the carbon−chlorine isotopic system established its applicability to assess reaction pathways and mechanisms at both laboratory and field scales.19 This approach has been evaluated for the degradation of cDCE and VC in a laboratory experiment,20 and, for the first time, successfully applied in one field study of these compounds.21 Further studies are needed, particularly involving the prioritized pollutants, PCE and TCE, to both fully explore the strength of this promising 2D-CSIA methodology and integrate it as a useful higher-tier tool for site assessment and remediation monitoring. The present study therefore aims to (i) apply the newly developed GCqMS based CSIA method to δ37Cl analysis of PCE and TCE from a contaminated groundwater site, in order to determine the extent of biodegradation, (ii) quantify chlorinated ethenes (CEs) mass removal along a transect using a comprehensive reactive transport model, and (iii) adopt the dual carbon−chlorine isotope approach to obtain a detailed assessment of PCE and TCE sources and fate. To this end, we investigated a heavily contaminated aquifer at an industrial site in Czech Republic, and used both δ13C and δ37Cl to assess the degradation of PCE and TCE. To the best of our knowledge, this is the first study to report on 2D-CSIA of these two prioritized and ubiquitous contaminants at a field site.

located in the Bohemian Cretaceous Basin, an important drinking water source, and along a small river, Ploučnice. PCE was used at the site for fat extraction from processed material (e.g., animal carcasses, slaughter houses waste, food production waste) for 25 years, from 1963 to 1988, with an estimated total consumption of 4250 tons. The factory then switched from PCE to a thermo-mechanical based treatment technology. Recurrent operational leakages and improper waste management led to a large groundwater contamination plume (Figure 1A). The amount of spilled PCE was estimated to range from 149 to 246 tons, and it was found that the contamination migrated up to 40 m below the surface. Multiple overlaying aquifers were affected by the contaminants (Figure 1B). Cleanup activities focusing on the Cretaceous aquifer started in 1997, involving pump-and-treat along with air sparging and venting. A more detailed description of the geological structure of the site is available in the Supporting Information (SI). Field investigations from 2006 to 200922 identified a highly contaminated Quaternary aquifer (Figure 1B), not yet affected by any cleanup activity. This research work focuses on this previously uncharacterized part of the plume (Figure 1A). The groundwater samples were exclusively taken in the southeastern part of the Quaternary aquifer, which exhibits the highest occurrence of daughter product (cDCE). The contaminated plume in this area originated from solvent storage tanks (building 3 in Figure 1). The groundwater at the site has a pH in the range 6−8. The groundwater table level of the 2−6 m thick aquifer ranges from 2 to 4 m below surface. The average transmissivity was determined by hydrodynamic testing to be 8.66 × 10−4 m2·s−1, the hydraulic conductivity was 4.12 × 10−4 m·s−1, and the effective porosity was 15−20%. We estimated the general groundwater velocity from the Darcy’s Law at 75 m·day−1. However, the small river Ploučnice drains the entire area, and influences the groundwater flow. During seasonal wet periods, river water infiltrates the Quaternary aquifer, whereas the river drains the groundwater during dryer periods (SI Figure S1). The river meanders the site, so that the natural groundwater flow in the Quaternary aquifer generally follows the southwestern direction during flooding phases and goes toward the river during drainage periods, e.g. it follows the eastern direction in the investigated part of the site. These seasonal groundwater flow patterns can also be observed from the

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MATERIALS AND METHODS Site Description. The field study site is the North Bohemian Carcass Disposal Plant (SAP) Mimoñ, located near Liberec, Czech Republic, and represents one of the most CEs-contaminated site in the country (Figure 1). The factory is 10919

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samples’ concentrations within a 20% interval. Average standard deviation (SD) in δ37Cl was ±0.6‰ vs SMOC. Stable Carbon Isotope Analysis. The isotopic standards PCE and TCE used for δ37Cl analysis were measured for δ13C using an elemental analyzer coupled to a Delta V Advantage isotope ratio mass spectrometer (EA-IRMS, Thermo Scientific) at the Stable Isotope Laboratory (SIL), SU as follows: δ13CPCE = −27.11 ± 0.15‰ vs VPDB (n = 2) and δ13CTCE = −29.53 ± 0.15‰ vs VPDB (n = 2). The samples’ aliquots sent for δ13C analysis were extracted at UB, following the same procedure as for δ37Cl analysis. The δ13C values of the cyclopentane extracts were determined on a Thermo DeltaPlusXL isotope ratio mass spectrometer coupled to a HP 6890 GC with split/splitless injector via a GC/C−III interface. The extracts were injected onto the GC column in splitless mode before separation on a 30-m BP-624 column (0.25 mm i.d.; 1.4 μm film thickness). Helium was used as the carrier gas at 1.2 mL·min−1 constant flow rate. The temperature program was 54 min at 40 °C, ramped to 150 at 20 °C·min−1 and held for a further 10.5 min. The oxidation reactors contained Cu/Ni/Pt wires, kept at 980 °C. A reduction reactor containing Cu wires (650 °C) was employed in the interface in order to scrub HCl gas from the analyte stream. Average SD in δ13C was ±0.5‰ vs VPDB. Reactive Transport Modeling. A generic finite-element 1D reactive transport model, set up in MATLAB and incorporating advection, diffusion, and transformation, was used to assess different isotope scenarios along a selected transect of the field site (using the wells marked with an asterisk in Figure 1). Measured CEs concentrations and isotopic signatures as well as the estimated groundwater velocity were used. Degradation rates and isotopic enrichment factors were chosen based on literature values and fit to field observations. The sequential degradation of CEs was separately modeled for carbon and chlorine isotopomers, following an approach described in the literature.19 See SI for the MATLAB code.

hydrogeochemical parameters concentrations (SI Figures S2 and S3). The wells located near the river showed a higher variability in oxygen concentration over the sampling period. Groundwater Sampling. From September to October 2009, during the river to groundwater discharge season, 14 existing wells, drilled during 2006−2008, were sampled according to the USEPA guidelines.23 Groundwater samples were collected from small-diameter direct push wells (32 mm diameter, 4−6 m length, screened throughout the lower 2 m), at the bottom of the Quaternary aquifer. Prior to the collection of groundwater samples, the groundwater table level (GWT), pH, temperature, dissolved oxygen concentration, and conductivity were measured by field instruments (WTW, Weilheim, Germany). Groundwater samples were collected with a peristaltic pump, after stabilization of pH and temperature. They were filled without headspace into glass bottles with polytetrafluoroethylene (PTFE) sealed lids (250 mL) for CEs concentration analyses, into low-density polyethylene (LDPE) bottles (1000 mL) for inorganic parameters determination (chloride, nitrate, sulfate, and soluble iron and manganese), and into amber glass bottles (1000 mL) for stable isotopes analyses. See SI for the analytical methods for CEs and inorganic parameters quantification. Samples for isotope analyses were shipped to ITM, Stockholm University (SU). Upon arrival, the samples were spiked with 3 mL of concentrated hydrochloric acid (37%, 12 M) to reach pH 1, in order to stop any bacterial activity. They were then split into 250-mL amber glass bottles sealed with PTFE-lined screw caps, without headspace. An aliquot from each well was sent to the University of Bristol (UB), for δ13C analysis. The samples were stored in the dark at 4 °C until isotopic analysis. Stable Chlorine Isotope Analysis. PCE and TCE from Sigma-Aldrich were used as isotopic standards for the δ37Cl analysis. They were measured on the international SMOC (Standard Mean Ocean Chlorine) scale by thermal ionization mass spectrometry (TIMS), following an established method24 as follows: δ37ClPCE = −0.27 ± 0.31‰ vs SMOC (n = 5)13 and δ37ClTCE = −2.90 ± 0.38‰ vs SMOC (n = 3), with n being the number of successful TIMS measurements. Cyclopentane solutions of PCE and TCE standards used for δ37Cl measurements were prepared using volumetric glass flasks and gastight glass syringes. Standard stock solutions of PCE (6.5 g·L−1) and TCE (5.9 g·L−1) in cyclopentane were freshly prepared every month. Working solutions were prepared weekly by subsequent dilutions to 400 and 1.6 μmol·L−1 for PCE and 500 and 1.8 μmol·L−1 for TCE. Based on concentration data, volumes of 4−100 mL of groundwater samples were extracted with 2−6 mL cyclopentane in 10- or 40-mL glass vials or 100-mL volumetric flasks, in order to achieve consistent PCE and TCE concentrations of at least 1 μmol·L−1 in the solvent. The extracts were then shaken for 1 min on a vortex shaker and dried over sodium sulfate. No significant isotopic fractionation was observed for this liquid−liquid extraction procedure. The samples were measured within 10 days following extraction for δ37Cl using the GCqMS instrumentation (GC 8000 coupled to a MD 800 benchtop quadruple MS; Fisons, Manchester, UK), previously described in Aeppli et al.13 Following this procedure, the sample was injected 5−10 times in the GCqMS system, for each δ37Cl measurement, bracketed with its authentic isotopic standard. The concentrations of the isotopic standards solutions were adjusted to match the

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RESULTS AND DISCUSSION Hydrogeochemical Setting. The investigated aquifer exhibited mixed redox conditions. Concentrations of redoxsensitive species are given in Figures S2 and S3. Most of the wells were located in the anaerobic part of the aquifer, where the dissolved oxygen concentration was below 1 mg·L−1.25 Due to seasonal river infiltration, oxygen concentrations above 1 mg·L−1 were detected at wells located near the river (N37, N38, N54, N55; Figure 1A). Concentrations of NO3− ranged between 2 and 9 mg·L−1 at all wells and reached 39.1 mg·L−1 at well N17, indicating stronger oxic conditions in this zone. NO 2 − was always below 0.1 mg·L −1 . SO 4 2− showed concentrations between 5 and 19 mg·L−1 in the southern part of the investigated aquifer, and between 19 and 55 mg·L−1 in the western part and around the source zone, indicating partial sulfate reduction. Evidence of reducing conditions was also indicated around the source area (N48, N47, N53, AT34), with Fe2+ concentrations between 0.005 and 0.01 mg·L−1 and Mn2+ concentrations between 0.2 and 0.6 mg·L−1, compared to background values below 0.002 mg·L−1 for Fe2+ and 0.01 for Mn2+. Overall, these parameters describe a system with mixed redox conditions, as typical for numerous CEs’ contaminated field sites.25−28 In such aquifers, anoxic pockets with sulfate reducing conditions can allow PCE and TCE reductive dechlorination. 10920

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Concentration of CEs. The groundwater concentrations of PCE, TCE, and cDCE were up to 35 μmol·L−1 (Figure 2A;

conditions such as sulfate reducing or methanogenic conditions.26,30 Qualitative Interpretations of δ13C and δ37Cl. For wells located along the investigated groundwater flow path in the anaerobic part of the aquifer, isotopic enrichment of both δ13C and δ37Cl points to ongoing PCE and TCE reductive dechlorination, which is consistent with the concentration data (Figure 2; Table S1). Observed isotopic enrichments in PCE relative to the source well (N48) values were up to 3‰ for δ13C and 1.5‰ for δ37Cl. TCE isotopic shifts showed the same trend with δ13C and δ37Cl enrichments up to 3.5 and 3.4‰, respectively. The isotopic values of wells located in a zone with seasonally changing redox conditions, close to the river (Figure S1) can reflect transformation as well as pure transport and dilution. An oxygen level >1 mg·L−1 is, e.g., observed at N54, because this well is influenced by river infiltration, and undergoes seasonal transition from oxic to anoxic conditions as exposed in the field site description. A similar trend is observed at N47. This well exhibits lower CEs concentrations, but a high cDCE proportion and isotopic values in the range of the source values, which is reasonable given its location near the source zone. N55 showed a relatively high PCE concentration and isotopic values similar to the source well. Although this well is located in an oxic part of the plume, the minor isotopic shifts reveal an example of contaminant transport with little degradation. N33 exhibits slight isotopic enrichment. Because this well presents oxic conditions, PCE and TCE degradation leading to isotopic enrichment likely occurred during their transport through anoxic pockets of the aquifer. Similarly, N17, with its strongly oxic conditions but low CEs concentrations and an enriched isotopic composition, could document contaminant degradation in an anoxic zone followed by transport to oxic parts of the aquifer. Note, however, that this well lies upstream of the source zone compared to the general groundwater flow. Therefore the present CEs might also originate from a different, so-far unidentified, source. The wells in the oxic part and with the lowest CEs concentrations (N57, N37, N38) showed a slight δ13C enrichment of PCE, in the range 0.5−0.7‰ compared to the source values, which is not significant, given the average SD of ±0.5‰. Similarly, the δ37Cl values were depleted by not more than 1‰. These data typically illustrate contaminant transport with little or no degradation. 1D-CSIA (δ13C) Based Quantification of In-Field PCE Degradation. The extent of PCE degradation can be calculated from the determined δ13C values, using the modified Rayleigh equation, as demonstrated and applied at field sites:8−10,31

Figure 2. (A) Concentrations of CEs at the monitored wells, where the gray dots represent the sum of the CEs’ concentrations (right y axis) and colored bars indicate distributions (left y axis). (B) Stable carbon isotope (δ13C) signatures for PCE (blue diamonds) and TCE (red squares). The error bars show an average standard deviation (SD) of ±0.5‰ for PCE and ±0.7‰ for TCE. (C) Stable chlorine isotope (δ37Cl) signatures. Average SD was ±0.6‰ for PCE and ±0.7‰ for TCE. The isotopic ranges for the putative source area (N48) are indicated in blue and red for PCE and TCE, respectively. These graphs indicate isotopic enrichment along the investigated transects (wells marked with an asterisk) respective to the source area. The other wells showing less enrichment are located in parts of the plume influenced by river infiltration, leading to seasonal oxic conditions unfavorable for hydrogenolysis of the present contaminants.

⎡ 1000 + δ13C ⎤1000/ εC ⎥ B=1−⎢ ⎣ 1000 + δ13C0 ⎦

(1)

where B represents the degraded fraction, δ13C and δ13C0 are the 13C isotopic signatures of the contaminant at a specific point and at the source, respectively, and εC is the carbon isotopic enrichment factor for the degradation. Typical literature values of εC for PCE microbial degradation by enriched mixed cultures from contaminated aquifers range from −2‰32 to −7‰.33 Note that a larger εC range has been determined for pure cultures.34−36 However, based on results from mixed consortia from contaminated aquifers and field measurements, there is currently no evidence that such isotopic

Table S1), which exceeds safe drinking water limits.29 A decrease in CEs’ concentrations was observed along the general groundwater flow path indicated in Figure 1. VC was detected only at two wells close to the source area (N48, N53; Figure 2A). The presence of cDCE and VC, which have never been used during the factory operation, suggests ongoing PCE and TCE microbial hydrogenolysis with cDCE accumulation. This is often observed at field sites lacking strong reducing 10921

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Figure 3. Dual stable carbon−chlorine isotopes plots for (A) PCE and (B) TCE at the two major transects indicated by the arrows and the wells with an asterisk in Figure 1. The whole dual isotopic data set, including wells from the aerobic part of the aquifer is illustrated in Figure S3. The isotopic enrichment factors ratios εCl/εC were estimated from the field data. They are characteristic of the occurring degradation process for PCE and the involved microbial consortium at this site.

Figure 4. Concentrations (A, B) and isotopic signatures (C, D) along the observed transect, as well as 2D isotope plot (E). The lines are calculated from the 1D reactive transport model and circles are the field data. Note that we used relative concentrations (B) rather than absolute concentrations (A) in order to correct for dilution effects. The calculations reproduce the trends in concentrations and isotopes. These outputs show that only less than 10% of PCE and TCE mass load is dechlorinated along the transect N48−C004 (wells marked by an asterisk in Figure 1), and demonstrate that TCE could already have been present as primary contaminant in the source zone.

enrichment factors are typical for contaminated field sites. Inserting the selected values in eq 1, and with the measured δ13C for PCE at this site, we obtained degradation estimates B varying from 11 to 78% in the anoxic part of the plume (wells marked with an asterisk in Figures 1 and 2), where evidence of degradation was the strongest. In contrast, the degradation estimates were 6 to 32% for the oxic parts of the plume. Since PCE dechlorination is not expected in oxic groundwater, these results suggest that the degraded contaminant has been transported from anoxic areas. Although there is a potential for PCE reduction in zones of the investigated aquifer with reducing conditions, degradation is generally hampered by infiltration of oxic water from the river.

Overall, the analysis of isotopic data together with hydrogeochemical parameters and CEs’ concentrations allow for a more thorough interpretation of in situ PCE field degradation, compared to the traditional concentration-based approach. Assessment of in Situ PCE Degradation using 2D-CSIA (δ13C and δ37Cl). The C and Cl isotopic systems were combined to further reduce the degradation estimates’ uncertainty, caused by the uncertainty in the choice of enrichment factors. The δ13C and δ37Cl ratios allowed the evaluation of εCl/εC values, which are characteristic of the occurring degradation process and the involved microbial consortium. Using data along two transects with ongoing degradation (Figure 1A) led to εCl/εC of 1.12 ± 0.17 and 0.42 10922

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remaining PCE and δ13C depletion in the instantaneously produced TCE. For δ37Cl, in contrast, TCE shows the same value as the original PCE.19 Interpretation of field data becomes more complex, since TCE can also be transformed. At this site, the TCE δ13C values are mostly depleted compared to PCE (Figure 2B). However, the δ37Cl signatures of TCE ranged from +3.5 to +9‰ (Figure 2C), which is well above the measured δ37Cl values for PCE (−0.8 to +2.1‰). Since the TCE isotopic trends are dependent on its relative degradation rate to PCE degradation,19,34,41 two different scenarios can be evaluated. First, for decreasing degradation rate constants with increasing dehalogenation, TCE could originate from PCE degradation as well as have been used as a primary solvent at the site. Although TCE was only mentioned as a production impurity of PCE in the factory’s reports, TCE might have been present in the original contamination. Given that other industries (e.g., dry cleaning facilities) often switched between PCE and TCE, depending on availability and price, this might have been the case for the operators of this site. Second, if TCE has a higher degradation rate constant than PCE, TCE can become enriched in δ37Cl relative to PCE. TCE would then be a pure degradation product of PCE. Although PCE is generally assumed to be degraded faster than TCE,26,42 the current lack of available measurements does not yet allow a distinction between these two cases. Assessing the NA Potential at the Field Site. The isotopic data set shows that this site is not suitable for MNA, because the hydrogeological conditions, allowing periodic groundwater oxygenation, hamper microbial CEs degradation. We estimated that a maximum of 10% combined PCE and TCE mass was dechlorinated, resulting in cDCE accumulation. No NA in terms of total CE mass removal therefore occurred. However, we hypothesized the presence of reducing zones within the aquifer with microbial activity that allow reductive PCE and TCE dechlorination. Therefore, Enhanced or stimulated Natural Attenuation (ENA) might be a promising remediation strategy, as illustrated in a recent study.43 Our study demonstrates the suitability of using δ37Cl for investigation of MNA potential at CEs contaminated field sites. So far, CSIA-based assessment of such sites was limited to δ13C. Only a limited number of laboratories are equipped with the corresponding analytical instruments (GC-C-IRMS). In contrast, the δ37Cl measurements only necessitated a GCqMS system. Note, however, that the method requires isotopic standards molecularly identical to the target compounds, along with an isotopic bracketing of samples and standards of similar concentrations, involving a sequence of at least ten GC runs per δ37Cl measurement.13 The 2D-CSIA approach allowed us to determine isotopic enrichment factors ratios εCl/εC between 0.4 and 1.1 for PCE (Figure 3). This result suggests that εCl and εC lie in the same order of magnitude. Given the similar analytical uncertainty for δ37Cl and δ13C analysis, δ37Cl-CSIA should allow for similar conclusions as the more equipment intensive δ13C−CSIA, making CSIA widely applicable to field site assesment. As a next step, more laboratory degradation studies are needed to better constrain the εCl values for the different CEs dechlorination steps, and facilitate broader field application of this 2D-CSIA approach for chlorinated groundwater contaminants. Furthermore, the presented analytical methods and numerical model also allow including δ13C determination of cDCE, VC, and ethene in order to establish a C isotopic mass

± 0.09 for PCE (with standard error; Figure 3; see Figure S3 for the entire data set). The determined εCl/εC ratios allow the estimation of field εCl values for PCE. Such values are essential to determine degradation degrees from δ37Cl values using eq 1. However, only a few εCl data are so far available in the literature. By applying the εC literature values used for the calculations of the extent of PCE degradation (−2‰ to −7‰, see above), we calculated εCl values ranging from −0.8 to −7.8‰ for PCE hydrogenolysis. Although rather large, this range comprises the literature values for cDCE and VC hydrogenolysis of −1.4 to −1.9‰.20 Our determined εCl values are, however, smaller than values reported for other reaction mechanisms, such as the reductive β-elimination or dehydrochlorination of chloroethanes to CEs with εCl of −10 and −20‰, respectively.37 Regarding the reductive dechlorination pathway, a 2D-CSIA approach has, to date, only been theoretically discussed20 and practically applied21 to cDCE degradation. The latter field studies revealed a calculated slope εCl/εC of 0.48 ± 0.05 for cDCE,21 which is in agreement with our field derived value of 0.42 ± 0.09 for PCE along one selected transect. In contrast, εCl/εC value determined in a laboratory experiment with the commercially available microbial culture KB-1 was 0.081 ± 0.004 for cDCE.20 The difference between laboratory and field studies in the, so far, very limited data set can have many causes. Generally, the 2D-CSIA approach is not sensitive to mass transfer and processes, such as kinetics regulating contaminants’ transport to their degradation site, modifying contaminants’ δ13C and δ37Cl in the same way.38,34 However, physical processes can affect both isotopes in a different way, possibly influencing resulting εCl/εC values. For example, laboratory experiments on chlorinated solvents’ evaporation demonstrated that the remaining substrate becomes enriched in δ37Cl but depleted in δ13C.39 Furthermore, a recent field study on the migration of CEs through an unsaturated zone showed no significant fractionation for either isotopic system.40 Thus, further interpretation of these data awaits more laboratory experiments. Reactive Transport Model and Implication for Natural Attenuation (NA). A generic 1D reactive transport model, based on an approach described in the literature,19 was used to comprehensively evaluate the concentrations and isotope data along the transect described in the previous sections (Figure 4). Despite data scattering due to field heterogeneity, the model reproduced the trends in normalized concentrations (Figure 4B), isotopic ratios (Figure 4C,D), and 2D-CSIA (Figure 4E). The use of two isotopic systems allows inputting more data in the model and therefore helps to choose the best fitting parameters. The result shows that the total mass of PCE and TCE was reduced by only ca. 10% along the selected transect, leading to cDCE accumulation. Based on concentration data, no significant cDCE degradation was observed. Therefore, no NA, in terms of CE mass removal, occurred at this site. Additionally, the model was used to explore the effect of different ratios of PCE and TCE degradation rate constants on the evolution of TCE isotopic signatures (Figure S6). It turned out that this ratio is critical, as discussed below. The model helped to simultaneously interpret multiple isotopic as well as concentration data, and to evaluate different scenarios. CSIA-Based Identification of TCE Origin. We explored the use of CSIA to assess the origin of TCE at the field site. The PCE to TCE reductive dechlorination, which involves the cleavage of a C−Cl bond, results in a δ13C enrichment of the 10923

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balance. Similarly, determining the δ37Cl values of cDCE and VC would add information on their possible reductive or oxidative degradation pathways. Taken together, adding the δ37Cl dimension to δ13C−CSIA investigations of groundwater contamination clearly adds valuable information both with respect to sources and on degradation of compounds such as CEs.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed geological description of the site, the analytical methods used to determine CEs concentrations, tables and figures of the hydrogeochemical and redox parameters, the CEs concentrations, the isotopic signatures, the dual δ13C and δ37Cl plots for the whole data set, and the MATLAB code of the reactive transport model. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +46 (0)8 674 7317; e-mail: [email protected]. Present Address ∥

Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study received funding from the European Community's Seventh Framework Programme (FP7 2009-2012) isoSoil project, under the grant agreement 212781. The work at SU was supported by the Delta Facility of the Faculty of Science. C.A. acknowledges a postdoctoral fellowship from the Swiss National Science Foundation. R.D.P. acknowledges the Royal Society Wolfson Research Merit Award. Ö .G. acknowledges support as an Academy Research Fellow at the Swedish Royal Academy of Sciences through a grant from the Knut and Alice Wallenberg Foundation. Petr Dostal, Monika Kralova, and Monika Stavelova are gratefully acknowledged for groundwater sampling. We thank Yngve Zebühr (SU) for his helpful technical support concerning the GCqMS measurements and Heike Siegmund at SIL (SU) for δ13C determination of the PCE and TCE standards. Prof. Dr.-Ing. Olaf A. Cirpka, at the Centre for Applied Geoscience, University of Tübingen, is acknowledged for support with the numerical model. We thank three anonymous reviewers for constructive comments on a previous version of the manuscript.

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REFERENCES

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