Dual C-Br Isotope Fractionation Indicates Distinct Reductive Dehalogenation Mechanisms of 1,2-Dibromoethane in Dehalococcoides- and Dehalogenimonas-Containing Cultures.

Brominated organic compounds such as 1,2-dibromoethane (1,2-DBA) are highly toxic groundwater contaminants. Multi-element compound-specific isotope analysis bears the potential to elucidate the biodegradation pathways of 1,2-DBA in the environment, which is crucial information to assess its fate in contaminated sites. This study investigates for the first time dual C-Br isotope fractionation during in vivo biodegradation of 1,2-DBA by two anaerobic enrichment cultures containing organohalide-respiring bacteria (i.e., either Dehalococcoides or Dehalogenimonas). Different εbulkC values (-1.8 ± 0.2 and -19.2 ± 3.5‰, respectively) were obtained, whereas their respective εbulkBr values were lower and similar to each other (-1.22 ± 0.08 and -1.2 ± 0.5‰), leading to distinctly different trends (ΛC-Br = Δδ13C/Δδ81Br ≈ εbulkC/εbulkBr) in a dual C-Br isotope plot (1.4 ± 0.2 and 12 ± 4, respectively). These results suggest the occurrence of different underlying reaction mechanisms during enzymatic 1,2-DBA transformation, that is, concerted dihaloelimination and nucleophilic substitution (SN2-reaction). The strongly pathway-dependent ΛC-Br values illustrate the potential of this approach to elucidate the reaction mechanism of 1,2-DBA in the field and to select appropriate εbulkC values for quantification of biodegradation. The results of this study provide valuable information for future biodegradation studies of 1,2-DBA in contaminated sites.

Triple-Element Compound-Specific Stable Isotope Analysis (3D-CSIA): Added Value of Cl Isotope Ratios to Assess Herbicide Degradation.

Multielement isotope fractionation studies to assess pollutant transformation are well-established for point-source pollution but are only emerging for diffuse pollution by micropollutants like pesticides. Specifically, chlorine isotope fractionation is hardly explored but promising, because many pesticides contain only few chlorine atoms so that "undiluted" position-specific Cl isotope effects can be expected in compound-average data. This study explored combined Cl, N, and C isotope fractionation to sensitively detect biotic and abiotic transformation of the widespread herbicides and groundwater contaminants acetochlor, metolachlor, and atrazine. For chloroacetanilides, abiotic hydrolysis pathways studied under acidic, neutral, and alkaline conditions as well as biodegradation in two soils resulted in pronounced Cl isotope fractionation (εCl from -5.0 ± 2.3 to -6.5 ± 0.7‰). The characteristic dual C-Cl isotope fractionation patterns (ΛC-Cl from 0.39 ± 0.15 to 0.67 ± 0.08) reveal that Cl isotope analysis provides a robust indicator of chloroacetanilide degradation. For atrazine, distinct ΛC-Cl values were observed for abiotic hydrolysis (7.4 ± 1.9) compared to previous reports for biotic hydrolysis and oxidative dealkylation (1.7 ± 0.9 and 0.6 ± 0.1, respectively). The 3D isotope approach allowed differentiating transformations that would not be distinguishable based on C and N isotope data alone. This first data set on Cl isotope fractionation in chloroacetanilides, together with new data in atrazine degradation, highlights the potential of using compound-specific chlorine isotope analysis for studying in situ pesticide degradation.

Field-scale monitoring of nitrate leaching in agriculture: assessment of three methods.

Deterioration of groundwater quality due to nitrate loss from intensive agricultural systems can only be mitigated if methods for in-situ monitoring of nitrate leaching under active farmers' fields are available. In this study, three methods were used in parallel to evaluate their spatial and temporal differences, namely ion-exchange resin-based Self-Integrating Accumulators (SIA), soil coring for extraction of mineral N (Nmin) from 0 to 90 cm in Mid-October (pre-winter) and Mid-February (post-winter), and Suction Cups (SCs) complemented by a HYDRUS 1D model. The monitoring, conducted from 2017 to 2020 in the Gäu Valley in the Swiss Central Plateau, covered four agricultural fields. The crop rotations included grass-clover leys, canola, silage maize and winter cereals. The monthly resolution of SC samples allowed identifying a seasonal pattern, with a nitrate concentration build-up during autumn and peaks in winter, caused by elevated water percolation to deeper soil layers in this period. Using simulated water percolation values, SC concentrations were converted into fluxes. SCs sampled 30% less N-losses on average compared to SIA, which collect also the wide macropore and preferential flows. The difference between Nmin content in autumn and spring was greater than nitrate leaching measured with either SIA or SCs. This observation indicates that autumn Nmin was depleted not only by leaching but also by plant and microbial N uptake and gaseous losses. The positive correlation between autumn Nmin content and leaching fluxes determined by either SCs or SIA suggests autumn Nmin as a useful relative but not absolute indicator for nitrate leaching. In conclusion, all three monitoring techniques are suited to indicate N leaching but represent different transport and cycling processes and vary in spatio-temporal resolution. The choice of monitoring method mainly depends (1) on the project's goals and financial budget and (2) on the soil conditions. Long-term data, and especially the combination of methods, increase process understanding and generate knowledge beyond a pure methodological comparison.

Dual-Element Isotope Analysis of Desphenylchloridazon to Investigate Its Environmental Fate in a Systematic Field Study: A Long-Term Lysimeter Experiment.

Desphenylchloridazon (DPC), the main metabolite of the herbicide chloridazon (CLZ), is more water soluble and persistent than CLZ and frequently detected in water bodies. When assessing DPC transformation in the environment, results can be nonconclusive if based on concentration analysis alone because estimates may be confounded by simultaneous DPC formation from CLZ. This study investigated the fate of DPC by combining concentration-based methods with compound-specific C and N stable isotope analysis (CSIA). Additionally, DPC formation and transformation processes were experimentally deconvolved in a dedicated lysimeter study considering three scenarios. First, surface application of DPC enabled studying its degradation in the absence of CLZ. Here, CSIA provided evidence of two distinct DPC transformation processes: one shows significant changes only in 13C/12C, whereas the other involves changes in both 13C/12C and 15N/14N isotope ratios. Second, surface application of CLZ mimicked a realistic field scenario, showing that during DPC formation, 13C/12C ratios of DPC were depleted in 13C relative to CLZ, while 15N/14N ratios remained constant. Finally, CLZ depth injection simulated preferential flow and demonstrated the importance of the topsoil for retaining DPC. The combination of the lysimeter study with CSIA enabled insights into DPC transformation in the field that are superior to those of studies of concentration trends.

Compound-Specific Chlorine Isotope Analysis of the Herbicides Atrazine, Acetochlor, and Metolachlor.

A gas chromatography-single quadrupole mass spectrometry method was developed and validated for compound-specific chlorine isotope analysis (Cl-CSIA) of three chlorinated herbicides, atrazine, acetochlor, and metolachlor, which are widespread contaminants in the environment. For each compound, the two most abundant ions containing chlorine (202/200 for atrazine, 225/223 for acetochlor, and 240/238 for metolachlor) and a dwell time of 30 ms were determined as optimized MS parameters. A limit of precise isotope analysis for ethyl acetate solutions of 10 mg/L atrazine, 10 mg/L acetochlor, and 5 mg/L metolachlor could be reached with an associated uncertainty between 0.5 and 1‰. To this end, samples were measured 10-fold and bracketed with two calibration standards that covered a wide range of δ37Cl values and for which amplitudes matched those of the samples within 20% tolerance. The method was applied to investigate chlorine isotope fractionation during alkaline hydrolysis of metolachlor, which showed a shift in δ37Cl of +46‰ after 98% degradation, demonstrating that chlorine isotope fractionation could be a sensitive indicator of transformation processes even when limited degradation occurs. This method, combined with large-volume solid-phase extraction (SPE), allowed application of Cl-CSIA to environmentally relevant concentrations of widespread herbicides (i.e., 0.5-5 µg/L in water before extraction). Therefore, the combination of large-volume SPE and Cl-CSIA is a promising tool for assessing the transformation processes of these pollutants in the environment.

13C- and 15N-Isotope Analysis of Desphenylchloridazon by Liquid Chromatography-Isotope-Ratio Mass Spectrometry and Derivatization Gas Chromatography-Isotope-Ratio Mass Spectrometry.

The widespread application of herbicides impacts surface water and groundwater. Metabolites (e.g., desphenylchloridazon from chloridazon) may be persistent and even more polar than the parent herbicide, which increases the risk of groundwater contamination. When parent herbicides are still applied, metabolites are constantly formed and may also be degraded. Evaluating their degradation on the basis of concentration measurements is, therefore, difficult. This study presents compound-specific stable-isotope analysis (CSIA) of nitrogen- and carbon-isotope ratios at natural abundances as an alternative analytical approach to track the origin, formation, and degradation of desphenylchloridazon (DPC), the major degradation product of the herbicide chloridazon. Methods were developed and validated for carbon- and nitrogen-isotope analysis (δ13C and δ15N) of DPC by liquid chromatography-isotope-ratio mass spectrometry (LC-IRMS) and derivatization gas chromatography-IRMS (GC-IRMS), respectively. Injecting standards directly onto an Atlantis LC-column resulted in reproducible δ13C-isotope analysis (standard deviation <0.5‰) by LC-IRMS with a limit of precise analysis of 996 ng of DPC on-column. Accurate and reproducible δ15N analysis with a standard deviation of <0.4‰ was achieved by GC-IRMS after derivatization of >100 ng of DPC with 160-fold excess of (trimethylsilyl)diazomethane. Application of the method to environmental-seepage water indicated that newly formed DPC could be distinguished from "old" DPC by the different isotopic signatures of the two DPC sources.

Solid-phase extraction method for stable isotope analysis of pesticides from large volume environmental water samples.

Compound-specific isotope analysis (CSIA) is a valuable tool for assessing the fate of organic pollutants in the environment. However, the requirement of sufficient analyte mass for precise isotope ratio mass spectrometry combined with prevailing low environmental concentrations currently limits comprehensive applications to many micropollutants. Here, we evaluate the upscaling of solid-phase extraction (SPE) approaches for routine CSIA of herbicides. To cover a wide range of polarity, a SPE method with two sorbents (a hydrophobic hypercrosslinked sorbent and a hydrophilic sorbent) was developed. Extraction conditions, including the nature and volume of the elution solvent, the amount of sorbent and the solution pH, were optimized. Extractions of up to 10 L of agricultural drainage water (corresponding to up to 200 000-fold pre-concentration) were successfully performed for precise and sensitive carbon and nitrogen CSIA of the target herbicides atrazine, acetochlor, metolachlor and chloridazon, and metabolites desethylatrazine, desphenylchloridazon and 2,6-dichlorobenzamide in the sub-µg L-1-range. 13C/12C and 15N/14N ratios were measured by gas chromatography-isotope ratio mass spectrometry (GC/IRMS), except for desphenylchloridazon, for which liquid chromatography (LC/IRMS) and derivatization-GC/IRMS were used, respectively. The method validated in this study is an important step towards analyzing isotope ratios of pesticide mixtures in aquatic systems and holds great potential for multi-element CSIA applications to trace pesticide degradation in complex environments.

Compound-Specific Chlorine Isotope Analysis of Tetrachloromethane and Trichloromethane by Gas Chromatography-Isotope Ratio Mass Spectrometry vs Gas Chromatography-Quadrupole Mass Spectrometry: Method Development and Evaluation of Precision and Trueness.

Compound-specific chlorine isotope analysis of tetrachloromethane (CCl4) and trichloromethane (CHCl3) was explored by both, gas chromatography-isotope ratio mass spectrometry (GC-IRMS) and GC-quadrupole MS (GC-qMS), where GC-qMS was validated in an interlaboratory comparison between Munich and Neuchâtel with the same type of commercial GC-qMS instrument. GC-IRMS measurements analyzed CCl isotopologue ions, whereas GC-qMS analyzed the isotopologue ions CCl3, CCl2, CCl (of CCl4) and CHCl3, CHCl2, CHCl (of CHCl3), respectively. Lowest amount dependence (good linearity) was obtained (i) in H-containing fragment ions where interference of 35Cl- to 37Cl-containing ions was avoided; (ii) with tuning parameters favoring one predominant rather than multiple fragment ions in the mass spectra. Optimized GC-qMS parameters (dwell time 70 ms, 2 most abundant ions) resulted in standard deviations of 0.2‰ (CHCl3) and 0.4‰ (CCl4), which are only about twice as large as 0.1‰ and 0.2‰ for GC-IRMS. To compare also the trueness of both methods and laboratories, samples from CCl4 and CHCl3 degradation experiments were analyzed and calibrated against isotopically different reference standards for both CCl4 and CHCl3 (two of each). Excellent agreement confirms that true results can be obtained by both methods provided that a consistent set of isotopically characterized reference materials is used.

Hydrogen Isotope Fractionation during the Biodegradation of 1,2-Dichloroethane: Potential for Pathway Identification Using a Multi-element (C, Cl, and H) Isotope Approach.

Even though multi-element isotope fractionation patterns provide crucial information with which to identify contaminant degradation pathways in the field, those involving hydrogen are still lacking for many halogenated groundwater contaminants and degradation pathways. This study investigates for the first time hydrogen isotope fractionation during both aerobic and anaerobic biodegradation of 1,2-dichloroethane (1,2-DCA) using five microbial cultures. Transformation-associated isotope fractionation values (εbulkH) were -115 ± 18‰ (aerobic C-H bond oxidation), -34 ± 4‰ and -38 ± 4‰ (aerobic C-Cl bond cleavage via hydrolytic dehalogenation), and -57 ± 3‰ and -77 ± 9‰ (anaerobic C-Cl bond cleavage via reductive dihaloelimination). The dual-element C-H isotope approach (ΛC-H = Δδ2H/Δδ13C ≈ εbulkH/εbulkC, where Δδ2H and Δδ13C are changes in isotope ratios during degradation) resulted in clearly different ΛC-H values: 28 ± 4 (oxidation), 0.7 ± 0.1 and 0.9 ± 0.1 (hydrolytic dehalogenation), and 1.76 ± 0.05 and 3.5 ± 0.1 (dihaloelimination). This result highlights the potential of this approach to identify 1,2-DCA degradation pathways in the field. In addition, distinct trends were also observed in a multi- (i.e., Δδ2H versus Δδ37Cl versus Δδ13C) isotope plot, which opens further possibilities for pathway identification in future field studies. This is crucial information to understand the mechanisms controlling natural attenuation of 1,2-DCA and to design appropriate strategies to enhance biodegradation.

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 E1CB elimination mechanism during alkaline hydrolysis) where a secondary AKIECl (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.

Quantification of Degradation of Chlorinated Hydrocarbons in Saturated Low Permeability Sediments Using Compound-Specific Isotope Analysis.

This field and modeling study aims to reveal if degradation of chlorinated hydrocarbons in low permeability sediments can be quantified using compound-specific isotope analysis (CSIA). For that purpose, the well-characterized Borden research site was selected, where an aquifer-aquitard system was artificially contaminated by a three component chlorinated solvent mixture (tetrachloroethene (PCE) 45 vol %, trichloroethene (TCE) 45 vol %, and chloroform (TCM) 10 vol %). Nearly 15 years after the contaminant release, several high-resolution concentration and CSIA profiles were determined for the chlorinated hydrocarbons that had diffused into the clayey aquitard. The CSIA profiles showed large shifts of carbon isotope ratios with depth (up to 24‰) suggesting that degradation occurs in the aquitard despite the small pore sizes. Simulated scenarios without or with uniform degradation failed to reproduce the isotope data, while a scenario with decreasing degradation with depth fit the data well. This suggests that nutrients had diffused into the aquitard favoring stronger degradation close to the aquifer-aquitard interface than with increasing depth. Moreover, the different simulation scenarios showed that CSIA profiles are more sensitive to different degradation conditions compared to concentration profiles highlighting the power of CSIA to constrain degradation activities in aquitards.

Investigating chloroperoxidase-catalyzed formation of chloroform from humic substances using stable chlorine isotope analysis.

Chloroperoxidase (CPO) is suspected to play an important role in the biosynthesis of natural chloroform. The aims of the present study are to evaluate the variability of the δ(37)Cl value of naturally produced chloroform and to better understand the reaction steps that control the chlorine isotope signature of chloroform. The isotope analyses have shown that the chlorination of the humic substances (HS) in the presence of high H3O(+) and Cl(-) concentrations induces a large apparent kinetic isotope effect (AKIE = 1.010-1.018) likely associated with the transfer of chlorine between two heavy atoms, whereas in the presence of low H3O(+) and Cl(-) concentrations, the formation of chloroform induces a smaller AKIE (1.005-1.006) likely associated with the formation of an HOCl-ferriprotoporphyrin IX intermediate. As the concentration of H3O(+) and Cl(-) in soils are generally at submillimolar levels, the formation of the HOCl-ferriprotoporphyrin IX intermediate is likely rate-limiting in a terrestrial environment. Given that the δ(37)Cl values of naturally occurring chloride tend to range between -1 and +1‰, the δ(37)Cl value of natural chloroform should vary between -5‰ and -8‰. As the median δ(37)Cl value of industrial chloroform is -3.0‰, the present study suggests that chlorine isotopic composition of chloroform might be used to discriminate industrial and natural sources in the environment.

Carbon and chlorine isotope analysis to identify abiotic degradation pathways of 1,1,1-trichloroethane.

This study investigates dual C-Cl isotope fractionation during 1,1,1-TCA transformation by heat-activated persulfate (PS), hydrolysis/dehydrohalogenation (HY/DH) and Fe(0). Compound-specific chlorine isotope analysis of 1,1,1-TCA was performed for the first time, and transformation-associated isotope fractionation ε bulk C and ε bulk Cl values were -4.0 ± 0.2‰ and no chlorine isotope fractionation with PS, -1.6 ± 0.2‰ and -4.7 ± 0.1‰ for HY/DH, -7.8 ± 0.4‰ and -5.2 ± 0.2‰ with Fe(0). Distinctly different dual isotope slopes (Δδ13C/Δδ37Cl): ∞ with PS, 0.33 ± 0.04 for HY/DH and 1.5 ± 0.1 with Fe(0) highlight the potential of this approach to identify abiotic degradation pathways of 1,1,1-TCA in the field. The trend observed with PS agreed with a C-H bond oxidation mechanism in the first reaction step. For HY/DH and Fe(0) pathways, different slopes were obtained although both pathways involve cleavage of a C-Cl bond in their initial reaction step. In contrast to the expected larger primary carbon isotope effects relative to chlorine for C-Cl bond cleavage, ε bulk C < ε bulk Cl was observed for HY/DH and in a similar range for reduction by Fe(0), suggesting the contribution of secondary chlorine isotope effects. Therefore, different magnitude of secondary chlorine isotope effects could at least be partly responsible for the distinct slopes between HY/DH and Fe(0) pathways. Following this dual isotope approach, abiotic transformation processes can unambiguously be identified and quantified.

Multiple dual C-Cl isotope patterns associated with reductive dechlorination of tetrachloroethene.

Dual isotope slopes are increasingly used to identify transformation pathways of contaminants. We investigated if reductive dechlorination of tetrachloroethene (PCE) by consortia containing bacteria with different reductive dehalogenases (rdhA) genes can lead to variable dual C-Cl isotope slopes and if different slopes also occur in the field. Two bacterial enrichments harboring Sulfurospirillum spp. but different rdhA genes yielded two distinct δ(13)C to δ(37)Cl slopes of 2.7 ± 0.3 and 0.7 ± 0.2 despite a high similarity in gene sequences. This suggests that PCE reductive dechlorination could be catalyzed according to at least two distinct reaction mechanisms or that rate-limiting steps might vary. At two field sites, two distinct dual isotope slopes of 0.7 ± 0.3 and 3.5 ± 1.6 were obtained, each of which fits one of the laboratory slopes within the range of uncertainty. This study hence provides additional insight into multiple reaction mechanisms underlying PCE reductive dechlorination. It also demonstrates that caution is necessary if a dual isotope approach is used to differentiate between transformation pathways of chlorinated ethenes.

C and Cl isotope fractionation of 1,2-dichloroethane displays unique δ¹³C/δ³7Cl patterns for pathway identification and reveals surprising C-Cl bond involvement in microbial oxidation.

This study investigates dual element isotope fractionation during aerobic biodegradation of 1,2-dichloroethane (1,2-DCA) via oxidative cleavage of a C-H bond (Pseudomonas sp. strain DCA1) versus C-Cl bond cleavage by S(N)2 reaction (Xanthobacter autotrophicus GJ10 and Ancylobacter aquaticus AD20). Compound-specific chlorine isotope analysis of 1,2-DCA was performed for the first time, and isotope fractionation (ε(bulk)(Cl)) was determined by measurements of the same samples in three different laboratories using two gas chromatography-isotope ratio mass spectrometry systems and one gas chromatography-quadrupole mass spectrometry system. Strongly pathway-dependent slopes (Δδ13C/Δδ37Cl), 0.78 ± 0.03 (oxidation) and 7.7 ± 0.2 (S(N)2), delineate the potential of the dual isotope approach to identify 1,2-DCA degradation pathways in the field. In contrast to different ε(bulk)(C) values [-3.5 ± 0.1‰ (oxidation) and -31.9 ± 0.7 and -32.0 ± 0.9‰ (S(N)2)], the obtained ε(bulk)(Cl) values were surprisingly similar for the two pathways: -3.8 ± 0.2‰ (oxidation) and -4.2 ± 0.1 and -4.4 ± 0.2‰ (S(N)2). Apparent kinetic isotope effects (AKIEs) of 1.0070 ± 0.0002 (13C-AKIE, oxidation), 1.068 ± 0.001 (13C-AKIE, S(N)2), and 1.0087 ± 0.0002 (37Cl-AKIE, S(N)2) fell within expected ranges. In contrast, an unexpectedly large secondary 37Cl-AKIE of 1.0038 ± 0.0002 reveals a hitherto unrecognized involvement of C-Cl bonds in microbial C-H bond oxidation. Our two-dimensional isotope fractionation patterns allow for the first time reliable 1,2-DCA degradation pathway identification in the field, which unlocks the full potential of isotope applications for this important groundwater contaminant.

Soil and unsaturated zone as a long-term source for pesticide metabolites in groundwater.

Pesticide metabolites are frequently detected in groundwater, often exceeding the concentrations of their parent pesticides. Ceasing the application of certain pesticides has often not led to the expected decrease in metabolite concentrations in groundwater, which is potentially caused by residues in soil. Whereas pesticide residues in soils are well-documented, there are only few studies about metabolite residues. We investigated if the soil/unsaturated zone can act as a long-term source for metabolites in groundwater by combining soil analysis, groundwater analysis and numerical modelling. The field study focused on the herbicide chloridazon (CLZ) and its frequently detected metabolites desphenyl-chloridazon (DPC) and methyl-desphenyl-chloridazon (MDPC) while in the model additional pesticides and metabolites were considered. In soil samples from an agricultural area, where the last CLZ application was 5 to 10 years ago, we observed 10 times (DPC: 0.22 - 7.4 µg kg-1) and 6 times (MDPC: 0.12 - 3.1 µg kg-1) higher metabolite concentrations compared to CLZ (< 0.050 - 1.0 µg kg-1). Calculations suggested that the majority of the metabolites (DPC: 63 - 96%, MDPC: 74 - 97%) were sorbed despite their lower sorption tendency. The metabolite retention was in particular related to the organic carbon content. The calculated pore water concentrations were highest in the deepest part of the soil profile (75 - 100 cm) with median concentrations of 3.6 and 1.7 µg L-1 for DPC and MDPC, respectively. The groundwater concentrations of DPC and MDPC were 3 to 3.5 times higher in monitoring wells downgradient from the agricultural zone than upgradient of it. This increase highlights the potential of soil and unsaturated zone as a long-term metabolite source after the application stop of pesticides, consistent with the calculated elevated pore water concentrations. Numerical flow and transport model simulations suggested that this input from soil and unsaturated zone can cause elevated metabolite concentrations (> 0.1 µg L-1) in groundwater over more than one decade. The study highlights that soil and unsaturated zone can act as a long-term source of pesticide metabolites even if they have much higher mobility than the parent compound.

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.

Climate change impacts on groundwater discharge-dependent streamflow in an alpine headwater catchment.

Climate change will have-and, in much of the world, is already having-a pronounced impact on alpine water resources. A deeper understanding of the future role of groundwater in alpine catchments, including quantification of climate change impacts on groundwater discharge, is vital for understanding the future of alpine water resources as a whole. Here, we develop and couple a geophysics-informed groundwater model with a net recharge model to investigate the impacts of climate change on a nival-regime alpine headwater catchment with significant unconfined Quaternary aquifer coverage. Flow in the groundwater-fed stream at the catchment outlet is analysed to determine changes in its annual dynamics. Comparing the periods 2020-2040 and 2080-2100 under ten RCP-8.5 climate models, we find a 35 % decrease in mean groundwater discharge and an increase in no-flow periods from ~0 % to 4.3 %. We also observe significant changes to the timing of monthly mean discharge maxima and minima, which shift ~1 month and ~5 months earlier, respectively. While groundwater has the potential to dampen the impacts of snow cover loss, currently perennial nival-regime alpine streams could be at risk of becoming intermittent by the end of the century. Our study underscores the increasingly critical role that groundwater will play in alpine catchments and emphasizes the need for quantitative understanding of the limits to its buffering capacity.

Exploring the reliability of 222Rn as a tracer of groundwater age in alluvial aquifers: Insights from the explicit simulation of variable 222Rn production.

Knowledge of groundwater residence times (GRT; the time elapsed since surface water infiltration) between losing rivers and pumping wells is crucial for management of water resources in alluvial aquifers. The radioactive noble gas radon-222 (222Rn) has been used for decades as a natural indicator of surface water infiltration, as it can provide quantitative information on GRT. However, models using 222Rn as a tracer of GRT are often based on a set of highly simplifying assumptions, including spatially homogenous 222Rn production and exclusively advective mass transport within the aquifer. In this paper, we use the integrated surface-subsurface hydrological model HydroGeoSphere (HGS) to simulate 222Rn transport, production, and decay in a bank filtration context. Spatially variable 222Rn production, based on experimental data, is explicitly considered. We show that variable 222Rn production rates, coupled with hydrodispersive mixing of groundwater, may lead to large biases in GRT estimates. Under certain transient conditions however, changes in tracer-derived GRTs correlate well with changes in mean groundwater age. Whereas 222Rn-derived GRTs may only be reliable under a narrow range of field conditions, 222Rn may serve as a powerful tracer of changes in mean GRT even in complex and heterogenous environments.

Chlorine and carbon isotopes fractionation during volatilization and diffusive transport of trichloroethene in the unsaturated zone.

To apply compound-specific isotope methods to the evaluation of the origin and fate of organic contaminants in the unsaturated subsurface, the effect of physicochemical processes on isotope ratios needs to be known. The main objective of this study is to quantify chlorine and carbon isotope fractionation during NAPL-vapor equilibration, air-water partitioning, and diffusion of trichloroethene (TCE) and combinations of these effects during vaporization in porous media. Isotope fractionation is larger during NAPL-vapor equilibration than air-water partitioning. During NAPL-vapor equilibration, carbon, and chlorine isotope ratios evolve in opposite directions although both elements are present in the same bond, with a normal isotope effect for chlorine (ε(Cl) = -0.39 ± 0.03‰) and an inverse effect for carbon (ε(C) = +0.75 ± 0.04‰). During diffusion-controlled vaporization in a sand column, no significant carbon isotope fractionation is observed (ε(C) = +0.10 ± 0.05‰), whereas fairly strong chlorine isotope fractionation occurs (ε(Cl) = -1.39 ± 0.06‰) considering the molecular weight of TCE. In case of carbon, the inverse isotope fractionation associated with NAPL-vapor equilibration and normal diffusion isotope fractionation cancel, whereas for chlorine both processes are accompanied by normal isotope fractionation and hence they cumulate. A source of contamination that aged might thus show a shift toward heavier chlorine isotope ratios.