Compound-specific Isotope Analysis Research Articles

Decomposition rate and biochemical fate of carbon from natural polymers and microplastics in boreal lakes.

Microbial mineralization of organic compounds is essential for carbon recycling in food webs. Microbes can decompose terrestrial recalcitrant and semi-recalcitrant polymers such as lignin and cellulose, which are precursors for humus formation. In addition to naturally occurring recalcitrant substrates, microplastics have been found in various aquatic environments. However, microbial utilization of lignin, hemicellulose, and microplastics as carbon sources in freshwaters and their biochemical fate and mineralization rate in freshwaters is poorly understood. To fill this knowledge gap, we investigated the biochemical fate and mineralization rates of several natural and synthetic polymer-derived carbon in clear and humic lake waters. We used stable isotope analysis to unravel the decomposition processes of different 13C-labeled substrates [polyethylene, polypropylene, polystyrene, lignin/hemicellulose, and leaves (Fagus sylvatica)]. We also used compound-specific isotope analysis and molecular biology to identify microbes associated with used substrates. Leaves and hemicellulose were rapidly decomposed compared to microplastics which were degraded slowly or below detection level. Furthermore, aromatic polystyrene was decomposed faster than aliphatic polyethylene and polypropylene. The major biochemical fate of decomposed substrate carbon was in microbial biomass. Bacteria were the main decomposers of all studied substrates, whereas fungal contribution was poor. Bacteria from the family Burkholderiaceae were identified as potential leaf and polystyrene decomposers, whereas polypropylene and polyethylene were not decomposed.

Using MixSIAR to quantify mixed contributions of primary producers from amino acid δ15N of marine consumers

Estimations of the trophic position and the food web nitrogen baseline from compound-specific isotope analysis of individual amino acids (CSIA-AA) are challenged when the diet of consumer organisms relies on different proportions of vascular and non-vascular primary producers. Here we propose a method to infer such proportions using mixing models and the δ15N CSIA-AA values from marine herbivores. Combining published and new data, we first characterized CSIA-AA values in phytoplankton, macroalgae and vascular plants, and determined their characteristic β values (i.e. the isotopic difference between trophic and source AA). Then, we applied MixSIAR Bayesian isotope mixing models to investigate the transfer of these isotopic signals to marine herbivores (molluscs, green turtles, zooplankton and fish), and their utility to quantify autotrophic sources. We demonstrated that primary producer groups have distinct δ15NAA fingerprints that can be tracked into their primary consumers, thus offering a rapid solution to quantify resource utilization and estimate βmix values in mixed-sourced environments.

Using Compound Specific Isotope Analysis to decipher the 1,2,3-trichloropropane-to-Allyl chloride transformation by groundwater microbial communities

1,2,3-trichloropropane (TCP), a refractory contaminant, can be reductive dehalogenated to allyl chloride (AC) by microorganisms, which has been shown a potential in situ bioremediation (ISB) strategy for TCP remediation in groundwater. In practice, however, it is hard to monitor the bioreduction extent because the TCP concentrations may also be decreased by non-biodegradation processes. Compound specific isotope analysis (CSIA) can be promising in determining the extent of degradation by quantifying the isotope enrichment factors (ε) of relevant degradation mechanisms. To date, no CSIA study has been reported on TCP degradation. In this study, a novel TCP-to-AC transformation enrichment culture (dominated by Azotobacter, Parabacteroides, Fusibacter, Hydrogenophaga, Trichococcus Desulfovibrio, etc) in the absence of the already identified TCP anaerobic reductive dechlorinating microorganisms (e.g., Dehalogenimonas) was derived from a chlorinated hydrocarbon-contaminated aquifer. A TCP degradation experiment was carried out by adding yeast extract to produce hydrogen as an electron donor. The TCP-to-AC transformation was found to conform to zero-order conversion kinetics with the rate constant 11 ± 0.34 μmol L−1 d−1 during the main biodegradation stage. The bulk carbon isotope enrichment factor (εbulk) of the TCP-to-AC transformation was firstly evaluated as −5.2 ± 0.1‰. This study for the first time characterized the carbon isotope fractionations during TCP biodegradation using a novel enrichment culture, which would provide a promising tool for the incorporation of ISB for TCP removal in the future.

Multiple trophic pathways support fish on floodplains of California's Central Valley.

We used compound-specific isotope analysis of carbon isotopes in amino acids (AAs) to determine the biosynthetic source of AAs in fish from major tributaries to California's Sacramento-San Joaquin river delta (i.e., the Sacramento, Cosumnes and Mokelumne rivers). Using samples collected in winter and spring between 2016 and 2019, we confirmed that algae are a critical component of floodplain food webs in California's Central Valley. Results from bulk stable isotope analysis of carbon and nitrogen in producers and consumers were adequate to characterize a general trophic structure and identify potential upstream and downstream migration into our study site by American shad Alosa sapidissima and rainbow trout Oncorhynchus mykiss, respectively. However, owing to overlap and variability in source isotope compositions, our bulk data were unsuitable for conventional bulk isotope mixing models. Our results from compound-specific carbon isotope analysis of AAs clearly indicate that algae are important sources of organic matter to fish of conservation concern, such as Chinook salmon Oncorhynchus tshawytscha in California's Central Valley. However, algae were not the exclusive source of energy to metazoan food webs. We also revealed that other sources of AAs, such as bacteria, fungi and higher plants, contributed to fish as well. While consistent with the well-supported notion that algae are critical to aquatic food webs, our results highlight the possibility that detrital subsidies might intermittently support metazoan food webs.

Carbon and hydrogen stable isotope fractionation of sulfamethoxazole during anaerobic transformation catalyzed by Desulfovibrio vulgaris Hildenborough

The fate of antibiotics in aquatic environments is of high concern and approaches are needed to assess the transformation of antibiotics in wastewater treatment plants. Here we used the model organism Desulfovibrio vulgaris Hildenborough to analyze compound specific isotope fractionation associated with anaerobic transformation of the antibiotic sulfamethoxazole (SMX). The results show that the rearrangement of the isoxazole ring in SMX is leading to significant carbon and hydrogen isotopic fractionation (εC = −5.8 ± 0.7‰, εH = −34 ± 9‰) during anaerobic transformation. The observed carbon isotopic fractionation is significantly higher than the values reported for aerobic degradation (εC = −0.6 ± 0.1‰) or abiotic reactions (εC = −0.8 to −4.8‰ for photolysis, εC = −0.8 to −2.2‰ for advanced oxidation). This indicates that carbon isotope fractionation can be used as a parameter to differentiate reaction mechanisms of SMX transformation. The corresponding apparent kinetic isotope effect (AKIEC) for anaerobic transformation of SMX was 1.029 ± 0.003, suggesting that the mechanism for anaerobic transformation is distinct from the mechanism reported for microbial aerobic degradation (AKIEC = 1.006 ± 0.001). In addition, dual-element (C–H) isotope analysis of SMX was performed in the present study, which was achieved by utilizing gas chromatography (GC) as the separation method instead of routine liquid chromatography. This dual-element isotope analysis resulted in a Λ value of 4.5 ± 2.2. Overall, compound specific isotope analysis can be a feasible tool to monitor the mitigation of SMX in wastewater treatment plants.

Mercury isotopic evidence for the importance of particles as a source of mercury to marine organisms.

The origin of methylmercury in pelagic fish remains unclear, with many unanswered questions regarding the production and degradation of this neurotoxin in the water column. We used mercury (Hg) stable isotope ratios of marine particles and biota to elucidate the cycling of methylmercury prior to incorporation into the marine food web. The Hg isotopic composition of particles, zooplankton, and fish reveals preferential methylation of Hg within small (< 53 µm) marine particles in the upper 400 m of the North Pacific Ocean. Mass-dependent Hg isotope ratios (δ202Hg) recorded in small particles overlap with previously estimated δ202Hg values for methylmercury sources to Pacific and Atlantic Ocean food webs. Particulate compound specific isotope analysis of amino acids (CSIA-AA) yield δ15N values that indicate more-significant microbial decomposition in small particles compared to larger particles. CSIA-AA and Hg isotope data also suggest that large particles (> 53 µm) collected in the equatorial ocean are distinct from small particles and resemble fecal pellets. Additional evidence for Hg methylation within small particles is provided by a statistical mixing model of even mass-independent (Δ200Hg and Δ204Hg) isotope values, which demonstrates that Hg within near-surface marine organisms (0-150 m) originates from a combination of rainfall and marine particles. In contrast, in meso- and upper bathypelagic organisms (200-1,400 m), the majority of Hg originates from marine particles with little input from wet deposition. The occurrence of methylation within marine particles is supported further by a correlation between Δ200Hg and Δ199Hg values, demonstrating greater overlap in the Hg isotopic composition of marine organisms with marine particles than with total gaseous Hg or wet deposition.

Compound-specific stable isotope analysis of amino acid nitrogen reveals detrital support of microphytobenthos in the Dutch Wadden Sea benthic food web

The Wadden Sea is the world’s largest intertidal ecosystem and provides vital food resources for a large number of migratory bird and fish species during seasonal stopovers. Previous work using bulk stable isotope analysis of carbon found that microphytobenthos (MPB) was the dominant resource fueling the food web with particulate organic matter making up the remainder. However, this work was unable to account for the trophic structure of the food web or the considerable increase in δ15N values of bulk tissue throughout the benthic food web occurring in the Eastern regions of the Dutch Wadden Sea. Here, we combine compound-specific and bulk analytical stable isotope techniques to further resolve the trophic structure and resource use throughout the benthic food web in the Wadden Sea. Analysis of δ15N for trophic and source amino acids allowed for better identification of trophic relationships due to the integration of underlying variation in the nitrogen resources supporting the food web. Baseline-integrated trophic position estimates using glutamic acid (Glu) and phenylalanine (Phe) allow for disentanglement of baseline variations in underlying δ15N sources supporting the ecosystem and trophic shifts resulting from changes in ecological relationships. Through this application, we further confirmed the dominant ecosystem support by MPB-derived resources, although to a lesser extent than previously estimated. In addition to phytoplankton-derived particulate, organic matter and MPB supported from nutrients from the overlying water column there appears to be an additional resource supporting the benthic community. From the stable isotope mixing models, a subset of species appears to focus on MPB supported off recycled (porewater) N and/or detrital organic matter mainly driven by increased phenylalanine δ15N values. This additional resource within MPB may play a role in subsidizing the exceptional benthic productivity observed within the Wadden Sea ecosystem and reflect division in MPB support along green (herbivory) and brown (recycled/detrital) food web pathways.

Amino acid δ13C and δ15N fingerprinting of sea ice and pelagic algae in Canadian Arctic and Subarctic Seas

The on-going decline in Arctic sea ice represents a significant loss of habitat for sea ice algae, which are a major contributor to primary production in the Arctic. Data on sea ice algal production is limited due to difficulties in both accessing sea-ice and sampling under-ice algae. Compound-specific isotope analysis (CSIA) of amino acids (AAs) is emerging as a powerful tool to trace element origins and biogeochemical processes in marine food webs and may address the knowledge gaps in sea ice algal productivity dynamics. Here we measured δ13C-AA and δ15N-AA in natural communities of sea ice and pelagic algae collected from regions offshore Labrador and Nunavut, Canada. Significant difference in δ13C-AA patterns between sea ice and pelagic algae was observed in different size classes. This difference was further supported by multivariate analyses based on normalized δ13C of essential amino acids (EAAs), which demonstrated a clear separation between sea ice and pelagic algae. Beta (β) values and trophic position (TP) calculated from δ15N of Glutamic Acid and Phenylalanine (Phe) and ∑V parameter for microbial resynthesis indicated a slightly higher heterotrophic biomass in pelagic and sea ice samples as compared to cultured samples. This finding is consistent with the Phe-normalized δ15N of Alanine and Threonine, which provided better separations between sea ice/pelagic algae and other end-member groups. Overall, our study provides first insights into the potential differences in δ13C-AA and δ15N-AA patterns between sea ice and pelagic algae and suggests carbon of sea ice origins may be distinguished from pelagic sources using CSIA-AA approach. These observations highlight the potential of CSIA-AA to estimate proportional contributions of sea ice and pelagic algae to export production and efficiency of benthic-pelagic coupling in polar marine environments.

Reactive transport of micropollutants in laboratory aquifers undergoing transient exposure periods

Groundwater quality is of increasing concern due to the ubiquitous occurrence of micropollutant mixtures. Stream-groundwater interactions near agricultural and urban areas represent an important entry pathway of micropollutants into shallow aquifers. Here, we evaluated the biotransformation of a micropollutant mixture (i.e., caffeine, metformin, atrazine, terbutryn, S-metolachlor and metalaxyl) during lateral stream water flow to adjacent groundwater. We used an integrative approach combining concentrations and transformation products (TPs) of the micropollutants, compound-specific isotope analysis (δ13C and δ15N), sequencing of 16S rRNA gene amplicons and reactive transport modeling. Duplicate laboratory aquifers (160 cm × 80 cm × 7 cm) were fed with stream water and subjected over 140 d to three successive periods of micropollutant exposures as pulse-like (6000 μg L−1) and constant (600 μg L−1) injections under steady-state conditions. Atrazine, terbutryn, S-metolachlor and metalaxyl persisted in both aquifers during all periods (<10 % attenuation). Metformin attenuation (up to 14 %) was only observed from 90 d onwards, suggesting enhanced degradation over time. In contrast, caffeine dissipated during all injection periods (>90 %), agreeing with fast degradation rates (t1/2 < 3 d) in parallel microcosm experiments and detection of TPs (theobromine and xanthine). Significant stable carbon isotope fractionation (Δδ13C ≥ 6.6 ‰) was observed for caffeine in both aquifers, whereas no enrichment in 15N occurred. A concentration dependence of caffeine biotransformation in the aquifers was further suggested by model simulations following Michaelis-Menten kinetics. Changes in bacterial community composition reflected long-term bacterial adaptation to micropollutant exposures. Altogether, the use of an integrative approach can help to understand the interplay of subsurface hydrochemistry, bacterial adaptations and micropollutants biotransformation during stream-groundwater interactions.

Investigating preservation of stable isotope ratios in subfossil deep-sea proteinaceous coral skeletons as paleo-recorders of biogeochemical information over multimillennial timescales

Paleoproxy records in deep-sea proteinaceous coral skeletons can reconstruct past ocean conditions on centennial to millennial time scales. Commonly recovered subfossil specimens could potentially extend these archives through the Holocene. However, protein matrix stability and integrity of stable isotope proxies over multi-millennial timescales in such specimens have never been examined. Here we compare amino acid (AA) composition together with bulk and AA compound-specific carbon (δ13C) and nitrogen (δ15N) isotopes in live-collected and subfossil (∼9.6–11.6 kyrs BP) Kulamanamana haumeaae deep-sea coral specimens from the central Pacific to understand the effects of long-duration benthic oxic exposure on primary coral chemistry. We find large coupled shifts in bulk δ15N (∼7‰) and δ13C (∼2‰) in the outermost portion (0–10 mm) of the subfossil coral, coincident with extensive alteration of the protein matrix. Microstructural changes in skeletal texture coincide with higher C/N ratios (+0.8) and isotope-based amino acid degradation parameters (e.g. ΣV ≥ 3), indicating extensive degradation of seawater-exposed gorgonin. However, interior gorgonin (>10 mm) retained amino acid molecular compositions (with exception of major Glycine loss) and bulk and amino acid-specific isotopic values that were similar to live-collected specimens. These results indicate that compound-specific isotope analysis of amino acids can reconstruct paleo-oceanographic biogeochemical and ecosystem information in subfossil corals beyond a clear diagenetic horizon, which is easily identifiable from an evaluation of C/N ratios together with the ΣV degradation proxy.

Geochemistry of organic matter by multi-proxy analyses and temperature sensitivity of methanogenesis in clearwater Amazonian lake sediments

Despite the relevance of bottom sediments from clearwater floodplain lakes in organic matter preservation and degradation, there is a scarcity of data regarding the composition of molecular biomarkers and responses of benthic methanogenesis in Amazonian lakes in global warming scenarios. Here, surface sediments from a small clearwater floodplain lake (Jacundá Lake) in the Brazilian Amazon (Tapajós River) were assessed in a combined approach between molecular biomarker composition and the sensitivity of methanogenesis under two experimental temperatures: 25 °C as control vs. + 4 °C as an Intergovernmental Panel on Climate Change (IPCC) global warming scenario for 2081–2100. Lipid biomarker composition (n-alkanes, sterols) in a 12-cm sediment core revealed a strong predominance of terrestrial-derived organic matter over aquatic contributions, indicating more refractory preserved organic matter, while aquatic organic matter represents substrates to biological degradation. Moreover, alkanoic acids and microbial triterpenoids indicated an intense microbial contribution and elevated organic matter processing. Compound specific isotopic analysis confirmed the presence of methanotrophic microorganisms (diploptene δ13C = −53.5‰) and an uptake of depleted 13C in aquatic organic matter. Diagenetic transformation of biomarkers also indicated a highly anoxic depositional environment throughout the core. Methanogenesis in the upper 15-cm sediment layer under 25 °C (39.3 mg C–CH4 m−2 d−1) was similar to higher rates reported in Amazonian lakes and increased about 60% in the treatment when the temperature increased by 4 °C. Despite the predominant terrestrial sources and subsequent refractory composition of organic matter, our findings integrating biomarkers and metabolic processes support that biomass stocks in clearwater lake sediments of the Amazon basin may be sensitive to global warming.

The world's largest omnivore is a fish.

The evolution of very large body size requires a ubiquitous and abundant source of food. In marine environments, the largest animals such as whale sharks are secondary consumers that filter feed on nekton, which is plentiful, although patchy. Consequently, feeding in coastal environments requires cost-efficient foraging that focuses on oceanographic features that aggregate both nektonic prey and marine debris such as floating macroalgae. Consumption of this algae could present an energetic challenge for these animals, unless some component can be digested. Here, we use a multi-technique approach involving amino acid compound-specific stable isotope analysis (CSIA) and fatty acid analysis to determine the trophic level of whale sharks and to identify likely items in the diet. CSIA analyses showed that the species has a trophic level consistent with omnivory. Fatty acid profiles of whale shark tissues, feces and potential prey items suggest that the floating macroalgae, Sargassum, and its associated epibionts is a significant source of food. Although this overcomes the energetic challenge of consumption of floating algae, this mode of feeding and the need to focus on oceanographic features that aggregate prey also increases the threat to the species posed by pollutants such as plastic.

Symbiont starvation affects the stability of the coral–Symbiodiniaceae symbiosis

Coral bleaching, the breakdown of the coral-Symbiodiniaceae association has been identified as a major cause of coral reef decline worldwide. When symbiont functions are compromised, corals receive fewer photosynthetic products from their symbionts and suffer significant starvation along with changes in nutrient cycling. Not all coral species are equally susceptible to bleaching, but despite intensive research, our understanding of the causes for coral bleaching remains incomplete. Here, we investigated nutrient exchange between host and symbionts of two coral- Symbiodiniaceae associations that are differentially susceptible to bleaching when maintained under heterotrophy in the dark. We followed the fate of heterotrophic nutrients using bulk isotope and compound-specific (amino acid) isotope analyses. We showed that symbiont starvation is a major cause of symbiotic breakdown in the dark. WhileOculina patagonicatransferred almost all heterotrophically-acquired amino acids within two weeks in the dark to its symbionts and did not bleach,Turbinaria reniformis, transferred only 2 amino acids to its symbionts after 4 weeks in the dark, and experienced significant bleaching. These results pave the way for future studies on the role of nutrition in coral stress response and the importance of maintaining a healthy symbiont population to avoid coral bleaching.

Stable isotopes in monitoring terrestrial arthropods

Monitoring of arthropods focuses typically on changes in population and range size over time. Yet, there are a myriad of other aspects that could and should be monitored under the ongoing global and local environmental change. Stable isotope analysis, widely employed in short-term ecological studies, has potential in long-term monitoring of arthropods. Here we discuss the use of stable isotopes in monitoring terrestrial arthropods, provide some empirical examples of the use of bulk tissue samples in stable isotope analysis, and outline future directions in using compound-specific stable isotope analysis in monitoring. We performed a literature search for 2012–2021 to see if stable isotopes have been specifically used in monitoring of terrestrial arthropods. The literature shows that stable isotopes have been successfully used to reveal ecological phenomena (dispersal, trophic interactions, resource use) that would have been difficult or impossible to detect by other means. Yet, stable isotopes have been underused in arthropod monitoring programs, but the growing number of basic studies on stable isotope ecology and methodology provides crucial basis needed for developing monitoring programs. Stable isotopes provide technically, economically and ecologically feasible addition to the traditional monitoring methods of terrestrial arthropods.

Amino acid isotopes in functional assemblages of Collembola reveal the influence of vertical resource heterogeneity and root energy supply on trophic interactions in soil food webs

The vertical heterogeneity of trophic interactions in soil food webs in temperate forest systems and its response to deprivation of root energy supply are poorly understood. In the present study, community-level trophic niches of Collembola functional groups (epedaphic, hemiedaphic and euedaphic) were analyzed using compound specific isotope analysis (CSIA) of carbon (C) and nitrogen (N) in amino acids (AAs) in a root-trenching experiment. The δ15N values of phenylalanine, serving as proxy for N resources, were higher in euedaphic Collembola than in ep-/hemiedaphic Collembola, suggesting that euedaphic Collembola mainly utilized N from processed organic substrate in deeper soil, while leaf litter was the dominant N source for ep-/hemiedaphic Collembola. The trophic position calculated from CSIA (TPCSIA) of euedaphic Collembola was higher than that of epedaphic Collembola in most of the studied forest sites, reflecting a higher number of trophic transfers in the soil than in the litter layer. For the first time, we found evidence that the deprivation of root energy supply can decrease the TPCSIA of soil omnivores, suggesting that root-derived C may increase the number of trophic transfers and food chain length in soil food webs. However, effects varied among functional groups of Collembola in different forest stands, indicating that the association between root energy supply and trophic interactions is only partially in line with the ‘productivity hypothesis’ and strongly mediated by vertical resource heterogeneity, regional conditions and feeding strategies of consumers. Finally, our results are in line with recent studies, suggesting that saprotrophic microorganisms, especially saprotrophic fungi, are the predominant resources for Collembola, whereas the contribution of mycorrhizal fungi and plants is subordinate. Overall, by using CSIA of AAs our study provides novel information on trophic niches of microarthropods in forest soils, extending our understanding of the influences of vertical resource heterogeneity and root energy supply on trophic interactions in soil food webs.

Expanding the calibration range of compound-specific chlorine isotope analysis by the preparation of a 37 Cl-enriched tetrachloroethylene.

The recent development of reliable GC/qMS methods for δ37 Cl compound-specific stable isotope analysis (CSIA) paves the way for dual carbon-chlorine isotope analysis of chlorinated ethenes and thus allows deeper insights into underlying transformation processes/mechanisms. A two-point calibration is indispensable for the precise and correct conversion of raw data to the international δ37 ClSMOC scale. The currently available calibration standards for tetrachloroethylene (PCE) span only a very narrow range from -2.52‰ (EIL2) to +0.29‰ (EIL1), which is considerably smaller than observed δ37 Cl isotope enrichment in (bio-)transformation studies (up to 12‰). We describe the preparation and evaluation of a new 37 Cl-enriched PCE standard to avoid bias in δ37 Cl CSIA arising from extrapolation beyond the calibration range. The preparation comprised: (i) partial PCE reduction by zero-valent zinc in a system of PCE, ethanol (initial volume ratio 3/5) and trace amounts of water followed by (ii) liquid-liquid extraction and (iii) a subsequent fractional distillation to purify the 37 Cl-enriched PCE. The obtained PCE (PCEenriched ) showed a purity of 98.8% (mole fraction) and a δ37 ClSMOC value of +10.8±0.5‰. The evaluation of an experimental dataset with and without extrapolation showed no significant variation. The new PCE standard (PCEenriched ) expands the calibration range to 13.3‰ (previously 2.8‰) and thus prevents potential bias introduced by extrapolation beyond the calibration range.

Identification of chlorohydrocarbon degradation pathways in aquitards using dual element compound-specific isotope measurements in aquifers

Compound-specific isotope analysis (CSIA) has been increasingly used to understand and quantify the (bio)degradation processes affecting chlorohydrocarbons in aquifer-aquitard systems. In this study, we aimed at investigating through reactive transport simulations if dual element (C, Cl) CSIA in aquifer samples can provide information about the occurring (bio)degradation pathways in the underlying aquitard. To that end, we modeled the continous dissolution of a 1,1,2,2-tetrachloroethane (TeCA) dense nonaqueous phase liquid (DNAPL) source in an aquifer as well as the resulting TeCA groundwater plume formation and diffusion into the underlying aquitard. The (bio)degradation of TeCA in the aquifer-aquitard system was simulated in four scenarios: TeCA biodegradation via dehydrohalogenation to trichloroethene (TCE) and TeCA dichloroelimination to dichloroethene (DCE) in the aquifer as well as in the aquitard. The simulations revealed that dual element (C, Cl) CSIA in the aquifer allows the disentanglement of whether TeCA degradation occurs in the aquifer or the aquitard and which (bio)degradation pathways occur in the aquitard. This demonstrates that chlorohydrocarbon (bio)degradation pathways in aquitards can be identified based on CSIA aquifer measurements only, which is an advantage as aquifers are easier to monitor than aquitards.

Stable carbon and hydrogen isotope fractionation of volatile organic compounds caused by vapor-liquid equilibrium

Several types of laboratory experiments were conducted to evaluate isotope fractionation caused by phase transfer process for a selection of common environmental contaminants. Carbon and hydrogen isotope fractionation caused by vaporization of non-aqueous phase liquid (NAPL), by volatilization from water and by dissolution into an organic solvent (tetraethylene glycol dimethylether or TGDE) under equilibrium conditions was investigated with closed system experimental setups to isolate the air-liquid partitioning process. A selection of aromatic, aliphatic and chlorinated compounds along with one fuel oxygenate (methyl tert-butyl ether or MTBE) were evaluated to determine isotope enrichment factor related to respective phase transfer process. During NAPL vaporization, the residual mass of aromatic compounds, aliphatic compounds and MTBE became progressively depleted in heavy carbon and hydrogen isotopes. In contrast, during volatilization from water, the residual mass of aromatic compounds and MTBE dissolved in the water became progressively enriched in heavy hydrogen isotopes, whereas no significant change in carbon isotope was observed, except for MTBE showing a significant depletion. For the air-TGDE partitioning process, most of the aromatic compounds tested led to no significant carbon (except ethylbenzene) or hydrogen (except toluene and o-xylene) isotope fractionation. In contrast, significant carbon isotope fractionation was observed for aliphatic and chlorinated compounds and hydrogen isotope fractionation for aliphatic compounds, and are comparable to progressive NAPL vaporization in direction and magnitude. The isotope fractionation factors determined in this study are key for interpreting the change in isotope ratios when assessing the fate of gas-phase VOCs present in the soil air or when gas-phase VOCs are sampled using TGDE as the sink matrix. The results of this study contribute to expand the list of common environmental contaminants that can be assessed by the compound-specific isotope analysis (CSIA) method deployed in the frame of gas-phase studies.

Sulfate Land Application Enhances Biodegradation in a Petroleum Hydrocarbon Smear Zone

AbstractDelivery of sulfate to petroleum hydrocarbons (PHCs) source zones and groundwater plumes is desirable to enhance biodegradation rates when treatment has become limited due to depletion of sulfate. Sulfate land application involves spreading sulfate salts on ground surface and allowing their dissolution and infiltration of sulfate into subsurface. The objectives of this pilot‐scale investigation were to capture the vertical transport of sulfate beneath an application area, confirm that sulfate reduction was occurring, and explore how the added sulfate affected biodegradation of benzene and toluene. Approximately 4000 kg of gypsum was spread over a 30 m × 30 m study area above a smear zone located approximately 2 m below‐ground surface. Precipitation was augmented by two irrigation events. Groundwater samples, collected over 1058 days from multilevel wells and a conventional long‐screened monitoring well, were analyzed for benzene, toluene, ethylbenzene, and xylenes (BTEX), sulfate, bromide, dissolved inorganic carbon (DIC) and methane. Compound‐specific isotope analyses (CSIA) for benzene and toluene, and isotope analyses of13C‐DIC and34S‐SO42−were performed. Following application, an increase in sulfate concentration was noted in the smear zone.34S‐SO42−enrichment and13C‐DIC depletion indicated that sulfate reduction and mineralization of PHCs were enhanced. CSIA results provided unequivocal evidence of anaerobic biodegradation of benzene and toluene. After 1058 days when sulfate was depleted, methane concentrations were about three times greater than baseline conditions suggesting syntrophic benefit of the delivered sulfate. Observations from this investigation support the viability of sulfate land application to enhance biodegradation rates in shallow PHC smear zones.

Compound-specific isotopic analysis to characterize the photocatalytic reaction of TiO2 nanoparticles with diethyl phthalate

In this study compound-specific isotope analysis (CSIA) has been used to explore the degradation mechanism of nano titanium dioxide (TiO2) catalyzes photodegradation of diethyl phthalate (DEP). TiO2 is a popular photosensitizer with potential in waste water treatment and application in advanced oxidation processes. The degradation process of DEP can be described with a first-order kinetics in the applied concentration ranges. The larger degradation rate constant has been found at neutral conditions. The 13C and 2H isotope fractionation associated with the nano TiO2 catalyzes photodegradation of DEP at pH 3, 7 and 11 yield normal isotope effects. In the TiO2/UV/DEP and TiO2/H2O2/UV/DEP systems, the correlation of 13C and 2H fractionation (Λ) were calculated to be 2.7 ± 0.2, 2.8 ± 0.2 at pH 3, 2.2 ± 0.4, 2.5 ± 0.2, 2.3 ± 0.6 at pH 7 and 2.6 ± 0.3, 2.2 ± 0.3, 2.7 ± 0.2 and 2.3 ± 0.3 at pH11, respectively. The dominant free radical species in studied systems were explored by combining free radical quenching method and electron paramagnetic resonance analysis. The hydroxyl radicals have been found as the main radical species at all pH conditions studied. Furthermore, the 13C and 2H fractionation suggested that the addition of •OH on the benzene ring of DEP is the main conversion pathway. Therefore, CSIA is a promising technology for the identification of reaction pathways of DEP for example in water treatment systems.