Compound-specific Isotope Analysis Data Research Articles

Hydrogen Isotope Exchange between Trichloroethene and Water under Mild Environmental Conditions─Implications for the Use of Hydrogen CSIA in Contaminated Site Assessment

Trichloroethylene (TCE) is a key legacy environmental contaminant and a major target for contaminant remediation projects. Hydrogen compound-specific isotope analysis (CSIA) of halocarbons is a relatively recent technique, predominantly used as a line of evidence for apportionment of TCE sources. To date, no published data addressed the potential for hydrogen exchange between TCE and hydroxide in a water solution under typical groundwater pH conditions. In this study, we determined the pH- and temperature-dependent rates of exchange in controlled experiments to constrain the scope of applications of hydrogen CSIA at field sites contaminated by TCE. Relatively high rates of the exchange were associated with above-neutral pH typical of carbonate aquifers (half-lives of years to decades) and slower, but not negligible, rates were associated with sub-neutral pH. Hydrogen exchange alters the hydrogen isotope composition of TCE in solution, masking initial source signatures or degradation signatures over environmentally relevant time scales. In effect, the TCE–water exchange restricts the classical applications of hydrogen CSIA data to a subset of distinctly acidic environments. On the other hand, as the rates of the exchange can be predicted for specific combinations of pH and temperature, the process could be potentially utilized in age-dating of TCE contamination.

Sulfidated nano zerovalent iron (S-nZVI) for in situ treatment of chlorinated solvents: A field study

Sulfidated nano zerovalent iron (S-nZVI), stabilized with carboxymethyl cellulose (CMC), was successfully synthesized on site and injected into the subsurface at a site contaminated with a broad range of chlorinated volatile organic compounds (cVOCs). Transport of CMC-S-nZVI to the monitoring wells, both downgradient and upgradient, resulted in a significant decrease in concentrations of aqueous-phase cVOCs. Short-term (0–17 days) total boron and chloride measurements indicated dilution and displacement in these wells. Importantly however, compound specific isotope analysis (CSIA), changes in concentrations of intermediates, and increase in ethene concentrations confirmed dechlorination of cVOCs. Dissolution from the DNAPL pool into the aqueous phase at the deepest levels (4.0–4.5 m bgs) was identifiable from the increased cVOCs concentrations during long-term monitoring. However, at the uppermost levels (∼1.5 m above the source zone) a contrasting trend was observed indicating successful dechlorination. Changes in cVOCs concentrations and CSIA data suggest both sequential hydrogenolysis as well as reductive β-elimination as the possible transformation mechanisms during the short-term abiotic and long-term biotic dechlorination. One of the most positive outcomes of this CMC-S-nZVI field treatment is the non-accumulation of lower chlorinated VOCs, particularly vinyl chloride. Post-treatment soil cores also revealed significant decreases in cVOCs concentrations throughout the targeted treatment zones. Results from this field study show that sulfidation is a suitable amendment for developing more efficient nZVI-based in situ remediation technologies.

Infiltration of Sulfate to Enhance Sulfate‐Reducing Biodegradation of Petroleum Hydrocarbons

AbstractThe lack of sufficient electron acceptors, particularly sulfate, can limit the rate of biodegradation of petroleum hydrocarbons (PHCs). Hence, there is a growing interest by remediation practitioners to deliver sulfate to a PHC impacted saturated zone to enhance biodegradation. When shallow contamination is present in a relatively permeable aquifer and site constraints allow, a cost‐effective approach is to apply sulfate on the ground surface. In this investigation a pilot‐scale experiment was conducted to increase our understanding of the delivery of sulfate using a surface‐based method and the resulting impact on a shallow PHC contaminated aquifer. A surficial infiltration pond positioned on the ground surface above a well‐characterized residual PHC source zone was used to control sulfate dosing. A high‐resolution network near the infiltration pond and downgradient of the source zone was employed to monitor relevant geochemical indicators and PHC concentrations. Compound‐specific isotope analysis (CSIA) was used to identify biodegradation patterns and to investigate the occurrence of microbial sulfate reduction. Selected metabolites and reverse‐transcriptase quantitative polymerase chain reaction analyses of expressed biodegradation genes (as mRNA) were also used to characterize the response of indigenous microorganisms (especially sulfate‐reducing bacteria) to the added sulfate. Three sulfate application episodes (5000 L each) at various Na2SO4 concentrations were allowed to infiltrate under a constant hydraulic head. Although the applied sulfate solution was impacted by density‐driven advection, detailed monitoring data indicated that the sulfate‐enriched water mixed with upgradient groundwater as it migrated downward through the residual PHC zone and formed a co‐mingled downgradient plume with the dissolved PHC compounds. The enrichment of δ34S of sulfate in conjunction with a decrease in sulfate concentration showed the occurrence of sulfate reduction due to the applied sulfate. Increased dissolved inorganic carbon (DIC) concentrations associated with a shift toward more depleted values of δ13C of DIC was indicative of an input of isotopically depleted DIC from biodegradation of PHCs. Despite fluctuations in benzene, toluene, and o‐xylene (BTX) concentrations, the CSIA data for BTX showed that these compounds were biodegraded. The biomarker data provided supporting evidence that toluene and o‐xylene were undergoing anaerobic biodegradation due to sulfate reduction. This study provides insight into factors controlling surface‐based delivery of sulfate to shallow PHC impacted groundwater systems, and the value of isotopic and molecular‐biological procedures to augment conventional monitoring tools.

Enrichment with Carbon-13 and Deuterium during Monooxygenase-Mediated Biodegradation of 1,4-Dioxane

Recent technical developments have enabled the application of compound-specific isotope analysis (CSIA) of low (parts per billion) concentrations of 1,4-dioxane that are often found in groundwater at 1,4-dioxane-contaminated sites. However, to quantify 1,4-dioxane biodegradation, isotopic enrichment factors are needed to interpret the CSIA data obtained from field samples. In this study, the carbon and hydrogen isotopic enrichment factors (eC and eH, respectively) for 1,4-dioxane biodegradation have been determined for axenic propane- or isobutane-grown cultures of Rhodococcus rhodochrous ATCC 21198 and for tetrahydrofuran-grown cultures of Pseudonocardia tetrahydrofuranoxidans K1. The enrichment factors for propane-grown (eC = −2.7 ± 0.3‰, and eH = −21 ± 2‰) and isobutane-grown (eC = −2.5 ± 0.3‰, and eH = −28 ± 6‰) cells of strain 21198 were similar and substantially smaller than those determined for tetrahydrofuran-grown cells of strain K1 (eC = −4.7 ± 0.9‰, and eH = −147 ± 22‰). The presence of 1-butyn...

A Nanoscale Study of Carbon and Nitrogen Fluxes in Mats of Purple Sulfur Bacteria: Implications for Carbon Cycling at the Surface of Coastal Sediments.

Mass blooms of purple sulfur bacteria growing seasonally on green stranded macroalgae have a major impact on the microbial composition and functionality of intertidal mats. To explore the active anoxygenic phototrophic community in purple bacterial mats from the Roscoff Aber Bay (Brittany, France), we conducted a combined approach including molecular and high-resolution secondary ion mass spectrometry (NanoSIMS) analyses. To investigate the dynamics of carbon and nitrogen assimilation activities, NanoSIMS was coupled with a stable isotope probing (SIP) experiment and a compound specific isotope analysis (CSIA) of fatty acid methyl ester (FAME). Sediment samples were incubated with 13C- and/or 15N-labeled acetate, pyruvate, bicarbonate and ammonium. NanoSIMS analysis of 13C - and 15N -incubated samples showed elevated incorporations of 13C - and 15N in the light and of 13C -acetate in the dark into dense populations of spherical cells that unambiguously dominated the mats. These results confirmed CSIA data that ranked vaccenic acid, an unambiguous marker of purple sulfur bacteria, as the most strongly enriched in the light after 13C -acetate amendment and indicated that acetate uptake, the most active in the mat, was not light-dependent. Analysis of DNA- and cDNA-derived pufM gene sequences revealed that Thiohalocapsa-related clones dominated both libraries and were the most photosynthetically active members of the mat samples. This study provides novel insights into the contribution of purple sulfur bacteria to the carbon cycle during their seasonal developments at the sediment surface in the intertidal zone.

A Multiple Lines of Evidence Framework to Evaluate Intrinsic Biodegradation of 1,4‐Dioxane

Development of a multiple lines of evidence (MLOE) framework to evaluate the intrinsic biodegradation potential of 1,4‐dioxane is vital to implementing management strategies at groundwater sites impacted by 1,4‐dioxane. A comprehensive MLOE approach was formed to provide significant evidence of natural degradation of 1,4‐dioxane comingled with tetrahydrofuran (THF) within a large, diffuse plume. State‐of‐the art molecular biological analyses and compound‐specific isotope analysis (CSIA) were employed to support more traditional approaches for data analysis (concentration trend analyses, spatial distribution, temporal changes, geochemical biodegradation attenuation indicators, plume mass estimates, and fate and transport modeling). The molecular analyses demonstrated that microorganisms capable of both metabolic and cometabolic degradation of 1,4‐dioxane were present throughout the groundwater plume, whereas the CSIA data provided supporting evidence of biodegradation. 1,4‐Dioxane biomarkers were present and abundant throughout the 1,4‐dioxane plume, and our biomarkers tracked the plume with reasonable accuracy. Evidence also suggests that THF‐driven cometabolic biodegradation as well as catabolic 1,4‐dioxane biodegradation were active at this site. These data supplemented the traditional lines of evidence approaches, which demonstrated that 1,4‐dioxane attenuation was occurring across the groundwater plume and that nondestructive physical processes alone did not account for the observed 1,4‐dioxane attenuation. This MLOE framework combining new and traditional analyses demonstrates that this site has a significant capacity for intrinsic biodegradation of 1,4‐dioxane. ©2016 Wiley Periodicals, Inc.

Evolution of carbon isotope signatures during reactive transport of hydrocarbons in heterogeneous aquifers

Compound-specific isotope analysis (CSIA) of organic pollutants has become a well-established tool for assessing the occurrence and extent of biodegradation processes in contaminated aquifers. However, the precision of CSIA is influenced by the degree to which assumptions underlying CSIA data interpretation hold under realistic field-scale conditions. For the first time this study demonstrates how aquifer analogs combined with reactive transport models offer an underexplored way to develop generic process understanding, evaluate monitoring and quantification strategies in highly heterogeneous subsurface settings. Data from high-resolution aquifer analogs were used in numerical experiments to track the propagation of a representative oxidizable organic compound (toluene) within a variety of realistic heterogeneous aquifers and to investigate its detailed fate. The simulations were used to analyze (1) the effects of physical aquifer heterogeneities on spatiotemporal patterns of contaminant concentrations and isotope signatures, (2) the performance of the commonly applied Rayleigh equation and (3) the applicability of an extension of the Rayleigh equation for complex hydrogeological conditions. The results indicate that if field-derived enrichment factors are applied without corrections for dilution, the conventional Rayleigh equation is inaccurate and estimates for biodegradation are typically overestimated and unreliable in heterogeneous aquifers. Underestimations can occur due to the partial source zone depletion. In contrast, if dilution can be accurately accounted for, field-derived enrichment factors comprise a suitable alternative to laboratory-derived and redox-specific enrichment factors. The study also examines to what extent variations in monitoring/sampling strategies influence the obtained results. Especially measurements from long-screened wells (>1m) reveal to be inappropriate for the application of the Rayleigh equation in the investigated aquifer analogs, as low resolution data sampled from the simulated scenarios only enable a qualitative assessment of biodegradation. Measurements from both long- and short-screened wells employing the Rayleigh equation streamline approach are only partly viable for in situ biodegradation measurements in heterogeneous systems.

CSIA of TCE and Daughter Products With Multiple Sources, Multiple Attenuation Mechanisms, and Low Ethene

One of the most often asked questions from regulators considering monitored natural attenuation (MNA) for a site is if there are mechanisms in situ that can completely detoxify the contaminant of concern. This article describes a method that uses data derived from compound‐specific isotope analysis (CSIA) to investigate if complete in situ degradation of trichloroethene (TCE) has occurred. MNA was the selected remediation strategy at the former England Air Force Base (fEAFB) in Louisiana. Previously declining concentrations of TCE, 1,2‐cis‐dichloroethene, and vinyl chloride had increased in selected wells, and less ethene was observed than expected on the basis of mass balance. Reductive dechlorination, partial or otherwise, could not explain observed trends, so the question at fEAFB had become: Was there an in situ mechanism for complete TCE detoxification (i.e., complete degradation to innocuous end products) at the site? CSIA was used for investigating this question at fEAFB. A previously developed formalism was adapted to interpret the CSIA data to answer the question. Standard interpretation assuming only reductive dechlorination demonstrated detoxification in 9 of the 15 contaminated wells, whereas the interpretation developed here did so in 14. © 2014 Wiley Periodicals, Inc.

Reconstructing bulk isotope ratios from compound‐specific isotope ratios

Carbon isotope analysis by bulk elemental analysis coupled with isotope ratio mass spectrometry has been the mainstay of delta(13)C analyses both at natural abundance and in tracer studies. More recently, compound-specific isotope analysis (CSIA) has become established, whereby organic constituents are separated online by gas or liquid chromatography before oxidation and analysis of CO(2) for constituent delta(13)C. Theoretically, there should be concordance between bulk delta(13)C measurements and carbon-weighted delta(13)C measurements of carbon-containing constituents. To test the concordance between the bulk and CSIA, fish oil was chosen because the majority of carbon in fish oil is in the triacylglycerol form and approximately 95% of this carbon is amenable to CSIA in the form of fatty acids. Bulk isotope analysis was carried out on aliquots of oil extracted from 55 fish samples and delta(13)C values were obtained. Free fatty acids (FFAs) were produced from the oil samples by saponification and derivatised to fatty acid methyl esters (FAMEs) for CSIA by gas chromatography/combustion/isotope ratio mass spectrometry. A known amount of an internal standard (C15:0 FAME) was added to allow analyte quantitation. This internal standard was also isotopically calibrated in both its FFA (delta(13)C = -34.30 per thousand) and FAME (delta(13)C = -34.94 per thousand) form. This allowed reporting of FFA delta(13)C from measured FAME delta(13)C values. The bulk delta(13)C was reconstructed from CSIA data based on each FFA delta(13)C and the relative amount of CO(2) produced by each analyte. The measured bulk mean delta(13)C (SD) was -23.75 per thousand (1.57 per thousand) compared with the reconstructed bulk mean delta(13)C of -23.76 (1.44 per thousand) from CSIA and was not significantly different. Further analysis of the data by the Bland-Altman method did not show particular bias in the data relative to the magnitude of the measurement. Good agreement between the methods was observed with the mean difference between methods (range) of 0.01 per thousand (-1.50 to 1.30).

Effects of Volatilization on Carbon and Hydrogen Isotope Ratios of MTBE

Contaminant attenuation studies utilizing CSIA (compound-specific 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. A conceptual model for interpretation of those IEs is proposed, based on the physics of liquid-air partitioning. Normal or inverse IEs were observed for different volatilization scenarios. The range of carbon enrichment factors (epsilon) was from +0.7 per thousand (gasoline vapor extraction) to -1 per thousand (diffusive volatilization of MTBE from gasoline), the range of hydrogen epsilon was from +7 per thousand (gasoline vapor extraction) to -12 per thousand (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.

Beyond the Rayleigh Equation: Reactive Transport Modeling of Isotope Fractionation Effects to Improve Quantification of Biodegradation

The Rayleigh equation is commonly applied to evaluate the extent of degradation at contaminated sites for which compound-specific isotope analysis (CSIA) data are available. However, it was shown recently that (i) the Rayleigh equation systematically underestimates the extent of biodegradation in physically heterogeneous systems, while (ii) it overestimates biodegradation if sorption-based carbon isotope fractionation is relevant. This paper further explores these two isotope effects not captured by the Rayleigh equation by means of a numerical modeling approach. The reactive multicomponent transport simulations show that the systematic underestimation is considerably larger for fringe-controlled and Monod-type degradation reactions than for previously assumed redox-insensitive first-order degradation kinetics, while forthe nonsteady state front portion of plumes, the Rayleigh equation may falsely indicate the occurrence of and/or overestimate biodegradation. The latter anomaly results from carbon isotope fractionation during sorption. It occurs for both supply-controlled degradation at the plume fringe and slow, reaction-controlled degradation inside the plume core. The numerical model approach enables a more accurate interpretation of CSIA data and thereby improves the quantification of biodegradation processes.

Compound-Specific Isotope Analysis Coupled with Multivariate Statistics to Source-Apportion Hydrocarbon Mixtures

Compound Specific Isotope Analysis (CSIA) has been shown to be a useful tool for assessing biodegradation, volatilization, and hydrocarbon degradation. One major advantage of this technique is that it does not rely on determining absolute or relative abundances of individual components of a hydrocarbon mixture which may change considerably during weathering processes. However, attempts to use isotopic values for linking sources to spilled or otherwise unknown hydrocarbons have been hampered by the lack of a robust and rigorous statistical method for testing the hypothesis that two samples are or are not the same. Univariate tests are prone to Type I and Type II error, and current means of correcting error make hypothesis testing of CSIA source-apportionment data problematic. Multivariate statistical tests are more appropriate for use in CSIA data. However, many multivariate statistical tests require high numbers of replicate measurements. Due to the high precision of IRMS instruments and the high cost of CSIA analysis, it is impractical, and often unnecessary, to perform many replicate analyses. In this paper, a method is presented whereby triplicate CSIA information can be projected in a simplified data-space, enabling multivariate analysis of variance (MANOVA) and highly precise testing of hypotheses between unknowns and putative sources. The method relies on performing pairwise principal components analysis (PCA),then performing a MANOVA upon the principal component variables (for instance, three, using triplicate analyses) which capture most of the variability in the original data set. A probability value is obtained allowing the investigator to state whether there is a statistical difference between two individual samples. A protocol is also presented whereby results of the coupled pairwise PCA-MANOVA analysis are used to down-select putative sources for other analysis of variance methods (i.e., PCA on a subset of the original data) and hierarchical clustering to look for relationships among samples which are not significantly different. A Monte Carlo simulation of a 10 variable data set; tanks used to store, distribute, and offload fuels from Navy vessels; and a series of spilled oil samples and local tug boats from Norfolk, VA (U.S.A.) were subjected to CSIA and the statistical analyses described in this manuscript, and the results are presented. The analysis techniques described herein combined with traditional forensic analyses provide a collection of tools suitable for source-apportionment of hydrocarbons and any organic compound amenable to GC-combustion-IRMS.

Analysis of anaerobic BTX biodegradation in a subarctic aquifer using isotopes and benzylsuccinates

In situ biodegradation of benzene, toluene, and xylenes in a petroleum hydrocarbon contaminated aquifer near Fairbanks, Alaska was assessed using carbon and hydrogen compound specific isotope analysis (CSIA) of benzene and toluene and analysis of signature metabolites for toluene (benzylsuccinate) and xylenes (methylbenzylsuccinates). Carbon and hydrogen isotope ratios of benzene were between − 25.9‰ and − 26.8‰ for δ 13C and − 119‰ and − 136‰ for δ 2H, suggesting that biodegradation of benzene is unlikely at this site. However, biodegradation of both xylenes and toluene were documented in this subarctic aquifer. Biodegradation of xylenes was indicated by the presence of methylbenzylsuccinates with concentrations of 17–50 μg/L in three wells. Anaerobic toluene biodegradation was also indicated by benzylsuccinate concentrations of 10–49 μg/L in the three wells with the highest toluene concentrations (1500–5000 μg/L toluene). Since benzylsuccinate typically accounts for a very small fraction of the toluene present in groundwater (generally < 1 mol%), the signature metabolite approach works best at higher toluene concentrations when it is not constrained by detection limits. In wells with lower toluene concentrations (410–640 μg/L), carbon and hydrogen isotopic values were enriched by up to ∼2‰ for δ 13C and ∼70‰ for δ 2H. This evidence of isotopic fractionation verifies the effects of biodegradation in these low concentration wells where metabolites may already be below detection limits. The combined use of signature metabolite and CSIA data is particularly valuable given the challenge of verifying biodegradation in subarctic environments where degradation rates are typically much slower than in temperate environments.

Compound-specific isotope analysis of the organic constituents in carbonaceous chondrites.

The carbonaceous chondrite meteorites contain evidence of the formation of the solar system, part of which is present within organic matter. In recent years, compound-specific isotope analysis (CSIA) has been useful in deciphering this complex record. The current published data set provides an insight into the source environments and reaction mechanisms that have contributed to the final state of the organic constituents in carbonaceous chondrites. This review summarises the CSIA data and related key interpretations.