Field demonstration for the solvent-based sampling method to perform compound-specific isotope analysis on gas-phase VOC.

The solvent-based sampling method for collecting gas-phase volatile organic compounds (VOCs) and conducting compound-specific isotope analysis (CSIA) was deployed during a controlled field study. The solvent-based method used methanol as a sink to accumulate petroleum hydrocarbons during the sampling of soil air and effluent gas. For each gaseous sample collected, carbon isotope analysis (δ13C) was conducted for a selection of five VOCs (benzene, toluene, o-xylene, cyclopentane and octane) emitted by a synthetic hydrocarbon source emplaced in the subsurface. The δ13C values obtained for gaseous VOCs (collected from soil gas and effluent gas) were compared to measurements obtained for the same VOCs present in the source material (none aqueous phase liquid - NAPL) and dissolved in groundwater to evaluate the reliability of the solvent-based sampling method in providing accurate isotope measurements. Since the NAPL source was composed of only 12 VOCs, potential bias related to the analytical procedure (such as co-elution) were avoided, hence emphasizing on field-related bias. This field evaluation demonstrated the capacity of the solvent-based method to produce precise and accurate δ13C measurements. The isotopic discrepancies between the gaseous and the NAPL values were < 1 ‰ for 39 out of the 41 comparison points, thus deemed not statistically different based on a common isotopic uncertainty error of ±0.5 ‰. Moreover, the current field study is the first field study to report δ13C measurements for up to five gas-phase VOCs obtained from the same sample, which appears to be of interest for VOC fate or forensic studies. The possibility to use several VOC isotopic measurements enabled by the sampling method would contribute to strengthen the connection assessment between gaseous VOCs and the suspected emitting source. Accordingly, the field results presented herein support the application of this sampling methodology to conduct CSIA assessment in the frame of VOC vapor studies.

Unravelling long-term source removal effects and chlorinated methanes natural attenuation processes by C and Cl stable isotopic patterns at a complex field site.

The effects of contaminant sources removal in 2005 (i.e. barrels, tank, pit and wastewater pipe sources) on carbon tetrachloride (CT) and chloroform (CF) concentration in groundwater were assessed at several areas of a fractured multi-contaminant aquifer (Òdena, Spain) over a long-term period (2010-2014). Changes in redox conditions, in these chlorinated methanes (CMs) concentration and in their carbon isotopic compositions (δ13C) were monitored in multilevel wells. δ13C values from these wells were compared to those obtained from sources (barrels, tank and pit before their removal, 2002-2005) and to commercial solvents values in literature. Additionally, CMs natural attenuation processes were identified by C-Cl isotope slopes (Λ). Analyses revealed the downstream migration of the pollutant focus and an efficient removal of DNAPLs in the pit source's influence area. However, the removal of the contaminated soil from former tank and wastewater pipe was incomplete as leaching from unsaturated zone was proved, evidencing these areas are still active sources. Nevertheless, significant CMs degradation was detected close to all sources and Λ values pointed to different reactions. For CT in the tank area, Λ value fitted with hydrogenolysis pathway although other possible reduction processes were also uncovered. Near the wastewater pipe area, CT thiolytic reduction combined with hydrogenolysis was derived. The highest CT degradation extent accounted for these areas was 72 ±â€¯11% and 84 ±â€¯6%, respectively. For CF, the Λ value in the pit source's area was consistent with oxidation and/or with transport of CF affected by alkaline hydrolysis from upstream interception trenches. In contrast, isotope data evidenced CF reduction in the tank and wastewater pipe influence areas, although the observed Λ slightly deviates from the reference values, likely due to the continuous leaching of CF degraded in the non-saturated zone by a mechanism different from reduction.

37Cl-compound specific isotope analysis and assessment of functional genes for monitoring monochlorobenzene (MCB) biodegradation under aerobic conditions.

A laboratory approach was adopted in this study to explore the potential of 37Cl-CSIA in combination with 13C-CSIA and Biological Molecular Tools (BMTs) to estimate the occurrence of monochloroenzene (MCB) aerobic biodegradation. A new analytical method for 37Cl-CSIA of MCB was developed in this study. This methodology using a GC-IRMS allowed to determine δ37Cl values within an internal error of ±0.3‰. Samples from a heavily MCB contaminated site were collected and MCB aerobic biodegradation microcosms with indigenous cultures in natural and enhanced conditions were set up. The microcosms data show a negligible fractionation for 13C associated to MCB mass decrease of >95% over the incubation time. Conversely, an enrichment factor of -0.6±0.1‰ was estimated for 37Cl, which is a reflection of a secondary isotope effect. Moreover, the dual isotope approach showed a pattern for aerobic degradation which differ from the theoretical trend for reductive dehalogenation. Quantitative Polymerase Chain Reaction (qPCR) results showed a significant increase in todC gene copy number with respect to its initial levels for both natural attenuation and biostimulated microcosms, suggesting its involvement in the MCB aerobic degradation, whereas phe gene copy number increased only in the biostimulated ones. Indeed, 37Cl fractionation in combination with the dual carbon­chlorine isotope approach and the todC gene copy number represent valuable indicators for a qualitative assessment of MCB aerobic biodegradation in the field.

Multi-isotope (carbon and chlorine) analysis for fingerprinting and site characterization at a fractured bedrock aquifer contaminated by chlorinated ethenes.

The use of compound specific multi-isotope approach (C and Cl) in the characterization of a chlorinated ethenes contaminated fractured aquifer allows the identification of several sources and contaminant plumes, as well as the occurrence of biodegradation and mixing processes. The study site is located in Spain with contamination resulting in groundwater concentrations of up to 50mg/L of trichloroethene (TCE), the most abundant chlorinated ethene, and 7 mg/L of tetrachloroethene (PCE). The potential sources of contamination including abandoned barrels, an underground tank, and a disposal lagoon, showed a wide range in δ(13)C values from -15.6 to -40.5‰ for TCE and from -18.5 to -32.4‰ for PCE, allowing the use of isotope fingerprinting for tracing of the origin and migration of these contaminants in the aquifer. In contrast, there is no difference between the δ(37)Cl values for TCE in the contaminant sources, ranging from +0.53 to +0.66‰. Variations of δ(37)Cl and δ(13)C in the different contaminant plumes were used to investigate the role of biodegradation in groundwater. Moreover, the isotopic data were incorporated into a reactive transport model for determination of whether the isotope pattern observed downstream from the tank's source could be explained by the simultaneous effect of mixing and biodegradation. The results demonstrate that a multi-isotope approach is a valuable tool for characterization of complex sites such as fractured bedrock aquifer contaminated by multiple sources, providing important information which can be used by consultants and site managers to prioritize and design more successful remediation strategies.

The use of alkaline hydrolysis as a novel strategy for chloroform remediation: the feasibility of using construction wastes and evaluation of carbon isotopic fractionation.

Laboratory and field-scale pilot experiments were performed to evaluate the feasibility of chloroform degradation by alkaline hydrolysis and the potential of δ(13)C values to assess this induced reaction process at contaminated sites. In batch experiments, alkaline conditions were induced by adding crushed concrete (pH 12.33 ± 0.07), a filtered concrete solution (pH 12.27 ± 0.04), a filtered cement solution (pH 12.66 ± 0.02) and a pH 12 buffer solution (pH 11.92 ± 0.11). The resulting chloroform degradation after 28 days was 94, 96, 99, and 72%, respectively. The experimental data were described using a pseudo-first-order kinetic model, resulting in pseudo-first-order rate constant values of 0.10, 0.12, 0.20, and 0.05 d(-1), respectively. Furthermore, the significant chloroform carbon isotopic fractionation associated with alkaline hydrolysis of chloroform (-53 ± 3‰) and its independence from pH in the admittedly limited tested pH range imply a great potential for the use of δ(13)C values for in situ monitoring of the efficacy of remediation approaches based on alkaline hydrolysis. The carbon isotopic fractionation obtained at the lab scale allowed the calculation of the percentage of chloroform degradation in field-scale pilot experiments where alkaline conditions were induced in two recharge water interception trenches filled with concrete-based construction wastes. A maximum of approximately 30-40% of chloroform degradation was achieved during the two studied recharge periods. Although further research is required, the treatment of chloroform in groundwater through the use of concrete-based construction wastes is proposed. This strategy would also imply the recycling of construction and demolition wastes for use in value-added applications to increase economic and environmental benefits.

Carbon isotope fractionation of 1,1,1-trichloroethane during base-catalyzed persulfate treatment.

The extent of carbon isotope fractionation during degradation of 1,1,1-trichloroethane (1,1,1-TCA) by a base-catalyzed persulfate (S2O8(2-)) treatment system was investigated. Significant destruction of 1,1,1-TCA was observed at a pH of ∼12. An increase in the NaOH:S2O8(2-) molar ratio from 0.2:1 to 8:1 enhanced the reaction rate of 1,1,1-TCA by a factor of ∼5 to yield complete (>99.9%) destruction. An average carbon isotope enrichment fractionation factor which was independent of the NaOH:S2O8(2-) molar ratio of -7.0 ± 0.2‰ was obtained. This significant carbon isotope fractionation and the lack of dependence on changes in the NaOH:S2O8(2-) molar ratio demonstrates that carbon isotope analysis can potentially be used in situ as a performance assessment tool to estimate the degradation effectiveness of 1,1,1-TCA by a base-catalyzed persulfate system.

Carbon isotope fractionation of chlorinated ethenes during oxidation by Fe2+ activated persulfate.

The increased use of persulfate (S(2)O(8)(2-)) for in situ chemical oxidation to treat groundwater and soils contaminated by chlorinated hydrocarbon compounds (CHCs) requires unbiased methods to assess treatment performance. Stable carbon isotope analysis offers a potential tool for assessing the in situ treatment performance of persulfate at sites contaminated with CHCs. This study investigated the extent of C isotope fractionation during oxidation of tetrachloroethene (PCE), trichloroethene (TCE) and cis-dichloroethene (cis-DCE) by persulfate activated by ferrous ion (Fe(2+)). An average carbon isotope enrichment factor ε(bulk) of -4.9‰ for PCE, -3.6‰ for TCE and -7.6‰ for cis-DCE were obtained in batch experiments. Variations in the initial S(2)O(8)(2-)/Fe(2+)/CHC molar ratios did not result in any significant differences in carbon isotope fractionation. The occurrence of carbon isotope fractionation during oxidation and the lack of dependence of enrichment factors upon the S(2)O(8)(2-)/Fe(2+)/CHC molar ratio demonstrate that carbon isotope analysis can potentially be used at contaminated sites as an additional technique to estimate treatment efficacy during oxidation of CHCs by Fe(2+) activated persulfate.