Use of dual carbon-chlorine isotope analysis to assess the degradation pathways of 1,1,1-trichloroethane in groundwater

Variable dual C-Cl isotope slopes of trichloromethane transformation by alkaline-activated persulfate under different simulated field conditions

Laboratory experiments were conducted to evaluate the potential of δ13C and δ37Cl isotopic values of trichloromethane (TCM) to monitor and quantify its transformation during alkaline persulfate (PS) activation. Batch experiments were designed to replicate different TCM:PS molar ratios, pH values, the presence of CO32- ion and the simulation of an alkaline interception trench. Results revealed three distinct C-Cl isotopic trends; First, despite differences in degradation kinetics, isotopic trends were consistent across TCM:PS molar ratios (ΛC-Cl between 23 ± 10 and 33 ± 6), suggesting that radical activation remained unaffected. Conversely, at pH 12.8, alkaline hydrolysis (AH) became the predominant degradation process (ΛC-Cl of 9 ± 1 and 11 ± 1) over reaction with PS derived radical species. Finally, in the presence of excess CO32- ion, which acts as radical scavenger probably affecting the radical species involved in TCM degradation, a ΛC-Cl value of 5.5 ± 0.6 was observed, suggesting a reductive degradation reaction. Therefore, our results reveal, for the first time, that the dual C-Cl isotope slope during TCM degradation by PS varies significantly depending on field conditions. The unexpected accumulation of higher chlorinated byproducts, such as hexachloroethane, during TCM degradation by alkaline-activated PS was observed for the first time and further research is needed in real open-systems to assess its potential environmental implications.

Tracking abiotic transformation of 1,1,1-trichloroethane to 1,1-dichloroethylene in contaminated groundwater on a national scale

1,1,1-trichloroethane (TCA) is a chlorinated aliphatic compound that was increasingly applied as a replacement for trichloroethylene. TCA can simultaneously follow two abiotic degradation processes: hydrolysis and dehydrochlorination, where the latter lead to the formation of a relatively recalcitrant product 1,1-dichloroethylene (DCE). The abiotic processes are relatively rapid and can be assessed based on the ratio between TCA and DCE. Using national data collected in Israel since the year 2000, this work aimed to examine the abiotic degradation extent of TCA nationally and whether it is affected by the aquifer type and unsaturated zone thickness. We have also examined temporal shifts in TCA and DCE concentrations and tested whether they follow expected trends. Our results show that the abiotic degradation of TCA is significant on a national scale, with higher DCE over TCA concentrations in ≈ 89 % of the wells. Furthermore, in ≈ 85 % of the data points, TCA over DCE concentrations indicate that TCA underwent three or more half-lives. Comparing the different lithologies of contaminated groundwater, higher concentrations are observable in karst relative to unconsolidated aquifers. Nevertheless, the ratio between the two and correspondingly the degradation rates are not affected by lithology. Finally, temporal shifts in TCA and DCE concentrations, as well as the ratio between the two, are different than expected in an idealized closed system. Complexities such as lithological heterogeneity, multiple sources, transport parameters, and more must be considered in interpreting these trends when quantification of the degradation rate is attempted.

Carbon and Nitrogen Isotope Fractionations During Biotic and Abiotic Transformations of 2,4-Dinitroanisole (DNAN)

2,4-Dinitroanisole (DNAN) is a main constituent in various new insensitive munition formulations. Although DNAN is susceptible to biotic and abiotic transformations, in many environmental instances, transformation mechanisms are difficult to resolve, distinguish, or apportion on the basis solely of analysis of concentrations. We used compound-specific isotope analysis (CSIA) to investigate the characteristic isotope fractionations of the biotic (by three microbial consortia and three pure cultures) and abiotic (by 9,10-anthrahydroquinone-2-sulfonic acid [AHQS]) transformations of DNAN. The correlations of isotope enrichment factors (ΛN/C) for biotic transformations had a range of values from 4.93 ± 0.53 to 12.19 ± 1.23, which is entirely distinct from ΛN/C values reported previously for alkaline hydrolysis, enzymatic hydrolysis, reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, and UV-driven phototransformations. The ΛN/C value associated with the abiotic reduction by AHQS was 38.76 ± 2.23, within the range of previously reported values for DNAN reduction by Fe2+-bearing minerals and iron-oxide-bound Fe2+, albeit the mean ΛN/C was lower. These results enhance the database of isotope effects accompanying DNAN transformations under environmentally relevant conditions, allowing better evaluation of the extents of biotic and abiotic transformations of DNAN that occur in soils, groundwaters, surface waters, and the marine environment.

A Novel Multi-ion Evaluation Scheme to Determine Stable Chlorine Isotope Ratios (37Cl/35Cl) of Chlordecone by LC-QTOF

Organochlorinated pesticides are highly persistent organic pollutants having important adverse effects in the environment. To study their fate, compound-specific isotope analysis (CSIA) may be used to investigate their degradation pathways and mechanisms but is currently limited to 13C isotope ratios. The assessment of 37Cl isotope ratios from mass spectra is complicated by the large number of isotopologues of polychlorinated compounds. For method development, chlordecone (C10Cl10O2H2; hydrate form), an organochlorine insecticide that led to severe contamination of soils and aquatic ecosystems of the French West Indies, was taken as a model analyte. Chlorine isotope analysis of chlordecone hydrate was evaluated using high-resolution liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS), enabling smooth ionization to detect the molecular ion. First, a new evaluation scheme is presented to correct for multiple isotope presence in polychlorinated compounds. The scheme is based on probability calculations of the most frequent isotopologues, distributions by binomial probability functions, and corrections for the presence of nonchlorine heavy isotopes. Second, mobile-phase modifiers, ionization energy (sampling cone tension) and scan time were optimized for accurate chlorine isotope ratios. Chlordecone standard samples were measured up to 10-fold and bracketed with a second chlordecone external standard. δ37Cl values were obtained after conversion to the SMOC scale by a two-point calibration. The robustness of the analysis method and evaluation scheme were tested and gave satisfactory results with standard errors (σm) of ±0.34‰ for precision and ±0.89‰ for long-term accuracy of chlorine isotope ratios of chlordecone hydrate. This work opens perspectives for applications of the C-Cl CSIA approach to investigate the fate of highly toxic and low reactive polychlorinated compounds in the environment.

Chlorinated solvents source identification by nonlinear optimization method

In this work, chloride ions were used as conservative tracers and supplemented with conservative amounts of chloroethenes (PCE, TCE, Cis-DCE, 1,1-DCE), chloroethanes (1,1,1-TCA, 1,1-DCA), and the carbon isotope ratios of certain compounds, the most representative on the sites studied, which is a novelty compared to the optimization methods developed in the scientific literature so far. A location of the potential missing sources is then proposed in view of the balances of the calculated mixing fractions. A test of the influence of measurement errors on the results shows that the uncertainties in the calculation of the mixture fractions are less than 11%, indicating that the source identification method developed is a robust tool for identifying sources of chlorinated solvents in groundwater.

Using 1,1,1-Trichloroethane degradation data to understand NAPL dissolution and solute transport at real sites

Analytical and numerical models describing the evolution of contaminant concentrations in the plume associated with the dissolution of NAPL source and degradation processes were presented in the literature. At real sites and particularly in complex aquifers like chalk, it is difficult to understand how the sources of contaminants evolve with time. 1,1,1-Trichloroethane (1,1,1-TCA) is one of the few compounds with a well-known hydrolysis constant, that can help to improve knowledge of the contaminant sources and transport rates of dissolved contaminants in groundwater by dating the spill. In this work, different scenarios that could explain the evolution of the concentrations of 1,1,1-TCA and its degradation product 1,1-Dichloroethene (1,1-DCE) at a real contaminated site were investigated by analytical and numerical modelling. The results show that (1) the peaks of concentration time series do not correspond to a single contamination event even in the case of a complex medium, (2) the multiphasic behavior of the concentration time series is dictated by the dissolution in a heterogeneous medium, and (3) the persistence of the concentrations can arise from a small residual organic phase or transport in dual domain medium.

Quantification of Lambda (Λ) in multi-elemental compound-specific isotope analysis

In multi-elemental compound-specific isotope analysis the lambda (Λ) value expresses the isotope shift of one element versus the isotope shift of a second element. In dual-isotope plots, the slope of the regression lines typical reveals the footprint of the underlying isotope effects allowing to distinguish degradation pathways of an organic contaminant molecule in the environment. While different conventions and fitting procedures are used in the literature to determine Λ, it remains unclear how they affect the magnitude of Λ. Here we generate synthetic data for benzene δ²H and δ¹³C with two enrichment factors ε_H and ε_C using the Rayleigh equation to examine how different conventions and linear fitting procedures yield distinct Λ. Fitting an error-free data set in a graph plotting the δ²H versus δ¹³C overestimates Λ by 0.225%⋅ε_H/ε_C, meaning that if ε_H/ε_Cis larger than 22, Λ is overestimated by more than 5%. The correct fitting of Λ requires a natural logarithmic transformation of δ²H versus δ¹³C data. Using this transformation, the ordinary linear regression (OLR), the reduced major-axis (RMA) and the York methods find the correct Λ, even for large ε_H/ε_C. Fitting a dataset with synthetic data with typical random errors let to the same conclusion and positioned the suitability of each regression method. We conclude that fitting of non-transformed δ values should be discontinued. The validity of most previous Λ values is not compromised, although previously obtained Λ values for large ε_H/ε_C could be corrected using our error estimation to improve comparison.

Simultaneous determination of stable chlorine and bromine isotopic ratios for bromochlorinated trihalomethanes using GC-qMS

Bromochlorinated compounds are organic contaminants originating from different natural and anthropic sources and increasingly found in different environmental compartments. This work presents an online approach for compound specific stable isotope analysis of chlorine and bromine isotope ratios for bromochlorinated trihalomethanes using gas chromatography coupled to quadrupole mass spectrometry (GC-qMS). An evaluation scheme was developed to simultaneously determine stable chlorine and bromine isotope ratios based on the mass spectral data of two target compounds: dibromochloromethane and dichlorobromomethane. The analytical technique was optimized by assessing the impact of different instrumental parameters, including dwell time, split ratios, and ionization energy. Successively, static headspace samples containing the two target compounds at aqueous concentrations ranging from 0.1 mg/L to 5 mg/L were analyzed in order to test the precision and reproducibility of the proposed approach. The results showed a good precision under the optimized instrumental conditions, with relative standard deviations ranging between 0.05% and 0.5% for chlorine and bromine isotope analysis. Finally, the method was tested in a source identification problem in which the simultaneous determination of chlorine and bromine stable isotope ratios allowed the clear distinction of dibromochloromethane from three different manufacturers.

Two distinct Dehalobacter strains sequentially dechlorinate 1,1,1-trichloroethane and 1,1-dichloroethane at a field site treated with granular zero valent iron and guar gum

Chlorinated ethanes are environmental pollutants found frequently at many contaminated industrial sites. 1,1,1-Trichloroethane (1,1,1-TCA) can be dechlorinated and detoxified via abiotic transformation or biologically by the action of dechlorinating microorganisms such as Dehalobacter (Dhb). At a field site, it is challenging to distinguish abiotic vs. biotic mechanisms as both processes share common transformation products. In this study, we evaluated using the Dhb 16S rRNA gene and specific reductive dehalogenase genes as biomarkers for 1,1,1-TCA and 1,1-dichloroethane (1,1-DCA) dechlorination. We analyzed samples from laboratory groundwater microcosms and from an industrial site where a mixture of granular zero valent iron (ZVI) and guar gum was injected for 1,1,1-TCA remediation. Abiotic and biotic transformation products were monitored and the changes in dechlorinating organisms were tracked using quantitative PCR (qPCR) with primers targeting the Dhb 16S rRNA gene and two functional genes cfrA and dcrA encoding enzymes that dechlorinate 1,1,1-TCA to 1,1-DCA and 1,1-DCA to chloroethane (CA), respectively. The abundance of the cfrA- and dcrA-like genes confirmed that the two dechlorination steps were carried out by two distinct Dhb populations at the site. The biomarkers used in this study proved useful for monitoring different Dhb populations responsible for step-wise dechlorination and tracking biodegradation of 1,1,1-TCA and 1,1-DCA where both abiotic (e.g., with ZVI) and biotic processes co-occur.

Electron competition and electron selectivity in abiotic, biotic, and coupled systems for dechlorinating chlorinated aliphatic hydrocarbons in groundwater: A review

Chlorinated aliphatic hydrocarbons (CAHs) have been frequently detected in aquifers in recent years. Owing to the bioaccumulation and toxicity of CAHs, it is essential to explore high-efficiency technologies for their complete dechlorination in groundwater. At present, the most widely used abiotic and biotic remediation technologies are based on zero-valent iron (ZVI) and functional anaerobic bacteria (FAB), respectively. However, the main obstacles to the full potential of both technologies in the field include their lowered efficiencies and increased economic costs due to the co-existence of a variety of natural electron acceptors in the environment, such as dissolved oxygen (DO), nitrate (NO₃^-), sulfate (SO₄^2-), ferric iron (Fe (III)), bicarbonate (HCO₃^-), and even water, which compete for electrons with the target contaminants. Therefore, a clear understanding of the mechanisms governing electron competition and electron selectivity is significant for the accurate evaluation of the effectiveness of both technologies under natural hydrochemical conditions. We collected data from both abiotic and biotic CAH-remediation systems, summarized the dechlorination and undesired reactions in groundwater, discussed the characterization methods and general principles of electron competition, and described strategies to improve electron selectivity in both systems. Furthermore, we reviewed the emerging ZVI-FAB coupled system, which integrates abiotic and biotic processes to enhance dechlorination performance and electron utilization efficiency. Lastly, we propose future research needs to quantitatively understand the electron competition in abiotic, biotic, and coupled systems in more detail and to promote improved electron selectivity in groundwater remediation.

Tracking chlorinated contaminants in the subsurface using compound-specific chlorine isotope analysis: A review of principles, current challenges and applications

Many chlorinated hydrocarbons have gained notoriety as persistent organic pollutants in the environment. Engineered and natural remediation efforts require a monitoring tool to track the progress of degradation processes. Compound-specific isotope analysis (CSIA) is a robust method to evaluate the origin and fate of contaminants in the environment and does not rely on concentration measurements. While carbon CSIA has established itself in the routine assessment of contaminated sites, studies incorporating chlorine isotopes have only recently become more common. Although some aspects of chlorine isotope analysis are more challenging than carbon isotope analysis, having additional isotopic data yields valuable information for contaminated site management. This review provides an overview of chlorine isotope fractionation of chlorinated contaminants in the subsurface by different processes and presents analytical techniques and unresolved challenges in chlorine isotope analysis. A summary of successful field applications illustrates the potential of using chlorine isotope data. Finally, approaches in modelling chlorine isotope fractionation due to degradation, diffusion, and sorption processes are discussed.

Understanding Groundwater Mineralization Changes of a Belgian Chalky Aquifer in the Presence of 1,1,1-Trichloroethane Degradation Reactions

An abandoned industrial site in Belgium, located in the catchment of a chalk aquifer mainly used for drinking water, has been investigated for groundwater pollution due to a mixture of chlorinated solvents with mainly 1,1,1-trichloroethane (1,1,1-TCA) at high concentrations. The observed elevated groundwater mineralization was partly explained by chemical reactions associated with hydrolysis and dehydrohalogenation (HY/DH) of 1,1,1-TCA in the chalky aquifer. Leaching of soluble compounds from a backfilled layer located in the site could also have influenced the groundwater composition. In this context, the objective of this study was to investigate the hydrochemical processes controlling groundwater mineralization through a characterization of the backfill and groundwater chemical composition. This is essential in the context of required site remediation to define appropriate remediation measures to soil and groundwater. Groundwater samples were collected for chemical analyses of chlorinated aliphatic hydrocarbons, major ions, and several minor ones. X-Ray Diffraction Analysis (XRD), Scanning Electron Microscopy (SEM) and a leaching test according to CEN/TS 14405 norm were carried out on the backfill soil. δ34S and δ18O of sulphate in groundwater and in the backfill eluates were also compared. Both effects influencing the groundwater hydrochemistry around the site were clarified. First, calcite dissolution under the 1,1,1-TCA degradation reactions results in a water mineralization increase. It was assessed by geochemical batch simulations based on observed data. Second, sulphate and calcium released from the backfill have reached the groundwater. The leaching test provided an estimation of the minimal released quantities.

A review of threats to groundwater quality in the anthropocene

Awareness concerning sustainable groundwater consumption under the context of land use and climate change is gaining traction, raising the bar for adequate understanding of the complexities of natural and anthropogenic processes and how they affect groundwater quality. The heterogeneous characteristics of aquifers have hampered comprehensive source, transport and contaminant identification. As questions remain about the behavior and prediction of well-known groundwater contaminants, new concerns around emerging contaminants are on the increase. This review highlights some of the key contaminants that originate from anthropogenic activities, organized based on land use categories namely agricultural, urban and industrial. It further highlights the extensive overlap, in terms of both provenance as well as contaminant type, between the different land use sectors. A selection of case studies from literature that describe the continued concern of established contaminants, as well as new and emerging compounds, are presented to illustrate the many qualitative threats to global groundwater resources. In some cases, the risk of groundwater contamination lacks adequate gravity, while in others the underlying physical and societal processes are not fully understood and activities may commence without adequately considering potential impacts. In the agricultural context, the historic and current application of fertilizers and plant protectants, use of veterinary pharmaceuticals and hormones, strives to safeguard the growing food demands. In the context of a sprawling urban environment, waste, human pharmaceuticals, and urban pesticide outputs are increasing, with adequate runoff and sanitation infrastructure often lagging. Finally, industrial activities are associated with accidental leaks and spills, while the large-scale storage of industrial byproducts has led to legacy contaminants such as those stemming from raw mineral extraction. With this review paper, we aim to underscore the need for transdisciplinary research, along with transboundary communication, using sound science and adaptive policy and management practice in order to procure sustainable groundwater quality.

Sequential coupling of bio-augmented permeable reactive barriers for remediation of 1,1,1-trichloroethane contaminated groundwater

Sequential coupling of high-density luffa sponge (HDLS) immobilized microorganism and permeable reactive barriers (IM Bio-PRBs) was superior to intimate coupling of free microorganism and permeable reactive barriers (FM Bio-PRBs) for remediation of 1,1,1-trichloroethane contaminated groundwater. IM Bio-PRBs had much better performance to removal 1,1,1-trichloroethane (1,1,1-TCA) and prevent the transport of 1,1,1-TCA and inorganic ions (NO₃^-, PO₄^3-, and SO₄^2-). The majority of them were prevented and accumulated in upgradient of IM Bio-PRBs. 1,1,1-TCA and inorganic ions in there contributed to the much faster growth of microorganism in upgradient aquifer. Therefore, the removal of 1,1,1-TCA and consumption of inorganic ions in upgradient of Bio-PRBs played a constructive role in reducing the processing load of following zero-valent iron (ZVI) PRBs and the negative effect of free microorganism cells (biological clogging) and inorganic ions (chemical clogging) on Bio-PRB permeability. In addition, IM Bio-PRBs were more conducive to accelerate the removal of 1,1,1-TCA in long-term remediation and 1,1,1-TCA residual concentration significantly lower than the safety standard of 0.2 mg L^-1. The change of terminal by-products of 1,1,1-TCA contaminated groundwater in Bio-PRBs showed that 1,1,1-TCA could be effectively de-chlorinated and mineralized in Bio-PRBs. The reductant H₂S (prolong the service life of ZVI-PRBs) was much more produced and utilized in IM Bio-PRBs. Taken together, sequentially coupled IM Bio-PRBs had a better overall performance, and its service life could be prolonged. It was a different design and idea to update conventional PRB remediation technology and theory.

Reductive Dehalogenation of Trichloromethane by Two Different Dehalobacter restrictus Strains Reveal Opposing Dual Element Isotope Effects

Trichloromethane (TCM) is a frequently detected and persistent groundwater contaminant. Recent studies have reported that two closely related Dehalobacter strains (UNSWDHB and CF) transform TCM to dichloromethane, with inconsistent carbon isotope effects (ε¹³C_UNSWDHB = -4.3 ± 0.45‰; ε¹³C_CF = -27.5 ± 0.9‰). This study uses dual element compound specific isotope analysis (C; Cl) to explore the underlying differences. TCM transformation experiments using strain CF revealed pronounced normal carbon and chlorine isotope effects (ε¹³C_CF = -27.9 ± 1.7‰; ε³⁷Cl_CF = -4.2 ± 0.2‰). In contrast, small carbon and unprecedented inverse chlorine isotope effects were observed for strain UNSWDHB (ε¹³C_UNSWDHB = -3.1 ± 0.5‰; ε³⁷Cl_UNSWDHB = 2.5 ± 0.3‰) leading to opposing dual element isotope slopes (λ_CF = 6.64 ± 0.14 vs λ_UNSWDHB = -1.20 ± 0.18). Isotope effects of strain CF were identical to experiments with TCM and Vitamin B₁₂ (ε¹³C_Vitamin B12 = -26.0 ± 0.9‰, ε³⁷Cl_Vitamin B12 = -4.0 ± 0.2‰, λ_Vitamin B12 = 6.46 ± 0.20). Comparison to previously reported isotope effects suggests outer-sphere-single-electron transfer or S_N2 as possible underlying mechanisms. Cell suspension and cell free extract experiments with strain UNSWDHB were both unable to unmask the intrinsic KIE of the reductive dehalogenase (TmrA) suggesting that enzyme binding and/or mass-transfer into the periplasm were rate-limiting. Nondirected intermolecular interactions of TCM with cellular material were ruled out as reason for the inverse isotope effect by gas/water and gas/hexadecane partitioning experiments indicating specific, yet uncharacterized interactions must be operating prior to catalysis.

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 ± 11% and 84 ± 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.

Stable carbon isotope fractionation of chlorinated ethenes by a microbial consortium containing multiple dechlorinating genes

The study aimed to determine the possible contribution of specific growth conditions and community structures to variable carbon enrichment factors (Ɛ-_carbon) values for the degradation of chlorinated ethenes (CEs) by a bacterial consortium with multiple dechlorinating genes. Ɛ-_carbon values for trichloroethylene, cis-1,2-dichloroethylene, and vinyl chloride were -7.24% ± 0.59%, -14.6% ± 1.71%, and -21.1% ± 1.14%, respectively, during their degradation by a microbial consortium containing multiple dechlorinating genes including tceA and vcrA. The Ɛ-_carbon values of all CEs were not greatly affected by changes in growth conditions and community structures, which directly or indirectly affected reductive dechlorination of CEs by this consortium. Stability analysis provided evidence that the presence of multiple dechlorinating genes within a microbial consortium had little effect on carbon isotope fractionation, as long as the genes have definite, non-overlapping functions.

Carbon and Chlorine Isotope Fractionation Patterns Associated with Different Engineered Chloroform Transformation Reactions

To use compound-specific isotope analysis for confidently assessing organic contaminant attenuation in the environment, isotope fractionation patterns associated with different transformation mechanisms must first be explored in laboratory experiments. To deliver this information for the common groundwater contaminant chloroform (CF), this study investigated for the first time both carbon and chlorine isotope fractionation for three different engineered reactions: oxidative C-H bond cleavage using heat-activated persulfate, transformation under alkaline conditions (pH ∼ 12) and reductive C-Cl bond cleavage by cast zerovalent iron, Fe(0). Carbon and chlorine isotope fractionation values were -8 ± 1‰ and -0.44 ± 0.06‰ for oxidation, -57 ± 5‰ and -4.4 ± 0.4‰ for alkaline hydrolysis (pH 11.84 ± 0.03), and -33 ± 11‰ and -3 ± 1‰ for dechlorination, respectively. Carbon and chlorine apparent kinetic isotope effects (AKIEs) were in general agreement with expected mechanisms (C-H bond cleavage in oxidation by persulfate, C-Cl bond cleavage in Fe(0)-mediated reductive dechlorination and E1_CB elimination mechanism during alkaline hydrolysis) where a secondary AKIE_Cl (1.00045 ± 0.00004) was observed for oxidation. The different dual carbon-chlorine (Δδ¹³C vs Δδ³⁷Cl) isotope patterns for oxidation by thermally activated persulfate and alkaline hydrolysis (17 ± 2 and 13.0 ± 0.8, respectively) vs reductive dechlorination by Fe(0) (8 ± 2) establish a base to identify and quantify these CF degradation mechanisms in the field.

Simulating stable carbon and chlorine isotope ratios in dissolved chlorinated groundwater pollutants with BIOCHLOR-ISO

BIOCHLOR is a well-known simple tool for evaluating the transport of dissolved chlorinated solvents in groundwater, ideal for rapid screening and teaching. This work extends the BIOCHLOR model for the calculation of stable isotope ratios of carbon and chlorine isotopes in chloroethenes. An exact solution for the three-dimensional reactive transport of a chain of degrading compounds including sorption is provided in a spreadsheet and applied for modeling the transport of individual isotopes ¹²C, ¹³C, ³⁵Cl, ³⁷Cl from a constant source. The model can consider secondary isotope effects that can occur in the breaking of CCl bonds. The model is correctly reproducing results for δ¹³C and δ³⁷Cl modeled by a previously published 1-D numerical model without secondary isotope effects, and is also reproducing results from a microcosm experiment with secondary chlorine isotope effects. Two applications of the model using field data from literature are further given and discussed. The new BIOCHLOR-ISO model is distributed as a spreadsheet (MS EXCEL) along with this publication.