In the framework of the hydrological survey of the Mont Terri anticline (Mont Terri rock Laboratory in the Folded Jura, Switzerland), a 58 m-deep borehole (BHS-1) was drilled through the Jurassic low permeability shale sequence. Dedicated sampling was carried out to characterize gases within a 13 m-thick unit of organic matter-rich Early Jurassic Rietheim black shales and adjacent units, including the underlying Beggingen aquifer. A cone-in-cone calcite, observed at the bottom of the black shale, as identified as an indicator of oil-window conditions and records a maximal burial temperature of similar to 80-90 degrees C. Two fracture zones within the black shales, marked by calcite infillings, provide evidence for at least two episodes of water paleocirculations: (1) an early circulation between fractures at the top of black shales and the Main Fault, and (2) a later circulation between fracture zones within the black shales and the lower Beggingen aquifer. Gas migration was investigated within the complex geological context of regional uplift, Jura folding and thrusting, and associated water flows. Alkane data reveal a partial carbon isotope reversal of thermogenic gases within the black shales between the two fracture zones, strongly suggesting alkane migration linked to the second water paleocirculation episode.

Ground surface analysis of CO2 emissions with 813C determination is experimentally demonstrated to be a po-tential methodology to monitor, on line, the dynamics of petroleum-hydrocarbon biodegradation in soil aquifers, thanks to the improvement of the Isotopic Ratio Infra Red Spectroscopy technique. Biodegradation rate of remaining hydrocarbon substrates in groundwater can be quantified using basic application of the Rayleigh equations, by 813CCO2 analysis released at ground surface above the pollution plume instead of usual approaches based on groundwater hydrocarbons 813C analysis, when physical and chemical properties for the contaminated site meet appropriate conditions. The validation approach for that gasoline contaminated specific site is discussed and verified by comparison of first order attenuation rate constant determined from 813CCO2 analysis emitted at ground surface and from 813CTOLUENE analysis in ground water. A kinetic fractionation factor alpha of 0.9979 (or e value of-2.1 +/- 0.5 parts per thousand) is estimated for the biodegradation of the most reactive hydrocarbon substrates (TEX). The treatment of this Rayleigh equations by linear regression of 813CCO2 values along the predominant direction of groundwater flow leads to the following results and conclusions for that site: (i) first order biodegradation rate constants (and annual variation) are maximum after the activation of a Permeable Reactive Barrier (PRB) in May 2014: 0.92 (+0.29-0.17) year -1, and during July and October: 0.46(+0.14-0.09) year- 1 and minimum in mid-winter in February 2015: 0.17(+0.05-0.03) year -1, given by the estimation range for e. These results are in the lower range with reported in literature for similar contaminated sites (1.6-18 year-1) considering natural attenuation under sulfate reducing conditions and (ii) the seasonal variation of the first order biodegradation rate constant is mainly correlated with the seasonal variation of the CO2 flux, where maximum values are in summers and minimum values in winters. Both seasonal variations are mainly due to the annual cycle of the natural biodegradation activity at the scale of the pollution plume, rather than the activation of the PRB. This work demonstrates that 813CCO2 analysis released at ground surface from biodegradation of groundwater hydrocarbons could provide, under characterized and appropriate conditions, a non-intrusive (without soil samplings), fast, and low-cost online method to monitor and therefore to optimize soil remediation processes in real time. (Monitored Natural Attenuation or Enhanced Bioremediation).

Water transfers and processes governing the chemistry of groundwaters and clay pore waters were investigated in the critical zone developed in Te ' gulines Clay in the area of Brienne-Le-chateau, at east of the Aube alluvium plain (France). A pit-digging campaign along two-stepped west-facing hillslopes along the Brevonne valley and the Aube/Brevonne paleo-valley gives evidence of a thin Quaternary clay silty and carbonate-free loams over-lying Te ' gulines Clay, mobile regolith along the slopes and the presence of a carbonate alluvium layer in pits of the Brevonne valley. Groundwaters at the first ridgetop attest of a temporary perched water table, while groundwaters at top of Te ' gulines Clay along the first west-facing steep slope toward the Brevonne River provide evidence of lateral groundwater transfers and runoff of waters toward the valley which is a discharge zone belonging to the present-day Aube alluvium plain. Groundwaters have low concentrations of Total Dissolved Solids (TDS < 600 mg/L) and are calcic-carbonate waters. Their delta 18O and delta D of groundwaters are consistent with local meteoric values. An 18O and D enrichment in groundwaters at the bottom of the second hillslope and in waters from reservoirs indicates evaporation processes. The delta 13CCO2 of dissolved inorganic carbone in ground-waters is essentially due to degradation of organic matter. The composition of the pore waters from Te ' gulines Clay in boreholes are chemically heterogeneous and different from groundwaters. They are Ca-Mg-SO4-rich and show a large range of TDS concentrations (592-6457 mg/L). The highest values are measured in the most intensely weathered clay developed in the first 10-15 m under the first ridge top and the low east-facing slope of the Aube/Brevonne paleo-valley. This latter seems to represent a large intermediate discharge zone between the coarse ridgetops (recharge zones) and the current Aube alluvium plain. The delta 18O and delta D of pore waters are alined along the local meteoric water line, indicating that meteoric waters have diffused through the clay for-mation during post-depositional history and replaced original connate seawater. On contrary the 87Sr/86Sr ratios of pore waters remain almost similar to that of Lower Cretaceous seawater, suggesting a limited diffusion of Sr since the formation deposition. Toward the top of the formation in the Aube/Brevonne paleovalley, major cation and anion concentrations, 18O and D enrichment and a slight increase of the 87Sr/86Sr ratios suggest the diffusion of evaporated waters. The delta 13C, delta 18O, 87Sr/86Sr ratios and 14C activity of concretions at top of Te ' gulines Clay show that they precipitated from evaporated old groundwaters, the oldest one at -34 ky in the TPH2-1 pit at the ridgetop of the second hillslope and a younger age of -8 ky in the TV4-1 pit.

An increasing amount of organic micropollutants (OMPs) are detected in aquatic environments and occur in water supplies worldwide. OMPs pose a severe threat to freshwater resources and present a potential danger to ecosystems and human health, even at trace concentrations. Compound-specific isotope analysis (CSIA) is one of the key techniques for identifying the origin and fate of environmental pol-lutants in aquifers and for characterizing their transformation processes. Considered as relatively mature for environmental monitoring of some well-studied industrial hydrocarbons at contaminated sites, the technique still encounters many analytical challenges in the context of diffuse pollution for other organic compounds. The present review provides a comprehensive overview on the latest technical developments con-cerning isotope ratio analyses of OMPs such as pesticides, various industrial compounds, pharmaceuti-cals and personal care products (PPCPs), and nitroaromatic compounds and organic explosives (NACs/ OEs). Based on a compilation of nearly 200 CSIA-studies of OMPs, we describe the analytical challenges and address common pitfalls that can be encountered in a context of diffuse pollution in wastewater, surface waters or groundwater. Our work gives an update on instrumental developments and advances in analytical approaches for CSIA-techniques for carbon, nitrogen, hydrogen, and chlorine stable isotopes. Furthermore, the review accounts for perspectives provided by associated innovations, such as sample processing for CSIA applications of the different OMPs in the context of diffuse pollution in aquatic environments. (c) 2022 Elsevier B.V. All rights reserved.

Significance Natural gas is a key fossil fuel as the world transitions away from coal toward less polluting energy sources in an attempt to minimize the impact of global climate change. Historically, the origin of natural gas produced from conventional reservoirs has been determined based on gas compositional data and stable isotope fingerprints of methane, ethane, and higher n-alkanes, revealing three dominant sources of natural gas: microbial, thermogenic, and abiotic. In our detailed synthesis of published natural gas data from a variety of unconventional hydrocarbon reservoirs worldwide, we demonstrate that there is a previously overlooked source of natural gas that is generated by radiolysis of organic matter in shales.

The Chalk aquifer used for drinking-water production in the southwest of the Lille European Metropolis is threatened by the presence of chlorinated aliphatic hydrocarbons (CHCs), their concentrations in groundwater regularly exceeding the regulatory limits for drinking water in France. This hinders its use for drinking-water production. Understanding the dynamics and spatial distribution of CHC in the aquifer is a key factor for resource sustainability. For that purpose, an intensive monitoring was undertaken in several well fields and at different depths over eight years. To assess a possible migration and/or degradation of the compounds, the water column in several wells was sampled at various depths with passive samplers. Furthermore, CHC degradation mechanisms were investigated with compound-specific carbon-isotope analysis. The CHC concentrations and their distributions in the area depend on past and current industrial activity, causing plumes emphasized by pumping in the wells, such plumes being multi-source with no identified origin in most wells. In the south area of Les Ansereuilles, reductive dechlorination of tetrachloroethylene from a former industrial laundry highly impacted the surrounding area with its main degradation product cis-1,2-dichloroethylene. The same area is also affected by tetrachlroroethylene from several industrial laundries, textile factories and dyeing industries with also an anaerobic degradation. In the northern part of Les Ansereuilles, tetrachloroethylene, trichloroethane, trichloroethylene and 1,1-dichloroethylene were found as primary products, whereas cis-1,2-dichloroethylene appears to be an anaerobic degradation product of TCE. The other well fields (Houplin-Ancoisne, Seclin and Emmerin) are less impacted by CHC pollution, and it was shown that no CHC degradation occurred in the wells. However, the stratification of CHCs in the well-water columns, their constant concentration values over time caused by the large amount of available CHCs, and the minor degradation occurring in wells are of concern for water operators in the future.

There is evidence that the emission of 14C –free CO2 during volcanic emissions creates a bias for radiocarbon dating of volcanic events (Holdaway et al., 2018), showing that integration of “dead” carbon by vegetation can serve as indicator of geogenic gas emissions. We tested 14C activities and stable carbon isotope ratios of tree rings and herbal vegetation in the proximity of a natural gas seep in the French Subalpine chains where both methane (<90% in the main vent) and CO2 (<11%) are present (Gal et al., 2018). Wood samples were taken from two alder trees, at different distances and directions from the main gas vent. Grass leaves and roots (Carex sp.) were analysed for two spots with contrasting soil methane concentrations and fluxes within the zone of diffuse gas emanation around the main vent (Gal et al., 2019). Grass and wood samples show contrasting isotope compositions depending on their species, age, and position with respect to the gas seep, some with 14C activities significantly lower than present day values. This offers perspectives of using vegetation carbon isotopes as proxies for present and past gas emanations, including man-induced gas leaks, e.g. from gas storage or natural gas exploitation facilities. This research was co-funded by the EU H2020 Programme (grant 764531 – SECURe “Subsurface Evaluation of Carbon Capture and Storage and Unconventional Risk”) Gal F., Kloppmann W., Proust E., Humez P. (2018) Gas concentration and flow rate measurements as part of methane baseline assessment: Case of the Fontaine Ardente gas seep, Isère, France. Applied Geochemistry, 95, 158-171. Gal F., Proust E., Kloppmann W. (2019) Towards a Better Knowledge of Natural Methane Releases in the French Alps: A Field Approach. Geofluids, 2019, 1-16. Holdaway R. N., Duffy B., Kennedy B. (2018) Evidence for magmatic carbon bias in 14C dating of the Taupo and other major eruptions. Nature Communications, 9, 4110.

Understanding weathering processes in clay formations is an issue of primary importance for the preservation of our natural environment. Reactive-transport modeling used to simulate weathering of clay formations has indicated that reactive gases (CO2 and O-2) are major parameters in controlling weathering processes. The Lower Cretaceous Tegulines marine-clay formation outcropping in the area of Brienne-le-Chateau (northeastern France) has been investigated in the context of a sub-surface waste repository. We developed gas monitoring (CO2, O-2, N-2, alkanes) of core samples from two boreholes that entirely crosscut the Tegulines Clay formation, to define the consequences of weathering and oxidation processes on gases dissolved in pore waters. We discuss amounts of gas and the carbon isotopic composition of CO2 in terms of pore water chemistry including dissolved-inorganic carbon (DIC) and alkalinity, mineral reactivity, organic-matter degradation and oxygen diffusion. Degassing of samples conditioned under He atmosphere provided evidence of very high CO2 production in the soil (0-30 cm), and high CO2 degassing associated with a high oxygen level in the first 2-10 m of the clay. The CO2 degassing increase observed in weathered clay relative to preserved clay resulted from calcite dissolution due to pyrite oxidation and organic matter degradation. The delta C-13 of CO2 indicates that organic matter degradation was a major source of CO2 at shallow depths and down to 10-12 m, which is the maximum depth at which we observed fossil roots. Then the CO2 degassing decreased down to a constant value in preserved clay, where the carbonate system and the mineral assemblage control dissolved carbonates in pore waters. The profile of the delta C-13(CO2) also provides evidence of progressive CO2 diffusion of organic origin from the underlying Greensands aquifer in the lower part of Tegulines Clay up to similar to 40 m in the AUB230 borehole. As a first step toward understanding interactions between Tegulines Clay and near surface waters or water at the Greensands interface, we developed a reactive-transport model to simulate in one dimension weathering processes under ambient temperature, constrained by geochemical reactions in soil (organic matter degradation) and in the clay (pyrite oxidation and calcite dissolution), exchange, DIC and pore water chemistry. The simulation was carried out for 10 kyrs, assuming that weathering and soil formation began after the last glacial maximum. The DIC profile cannot be simulated without considering evaporation processes in agreement with the isotopic data. This type of approach combining a complete field dataset (reactive-gas concentrations, delta C-13 of CO2, major-ion concentrations, delta O-13 and SD of pore waters) and reactive-transport modeling is necessary for better understanding of chemical weathering processes in the critical zone.

The Alum Shale formation (Mid-Cambrian to Tremadoc, Northern Europe) is a prominent example of uranium-rich black shales, where the contents of U [1] vary with age (highest averages of 100-300 ppm in the U-Cambrian), laterally, and in function of TOC (up to 8000 ppm in discrete TOC-rich nodules or layers). Considering that they were formed (cid:97) 500 Ma ago, the radiation dose obtained by the organic matter is significant, which causes changes in its structure and properties, such as increase of aromaticity, condensation degree, vitrinite reflectance and, to some extent, isotopic signatures. These changes are explained by radiation-initiated cross-linking of organic molecules that results in aromatization and polymerization in the kerogens [2] [3] [4]. The Alum Shale kerogen, of algal or planktonic origin (type I or II), sometimes shows pseudo-type III geochemical signatures [5] [6] in spite of the fact that higher land plants did not exist in the Lower Palaeozoic. This effect could be caused by the radiation-induced aromatization and polymerization of the organic matter leading to the occurrence of more complex organic molecules that are also more prone to gas generation. Indeed, increased U content of the shales leads to increased gas production compared to other marine shales (e.g. [7]). Direct impact of the radiation-induced structural changes on the C-isotopic signatures of organic matter has been shown

Weathering processes in clay environments are of major importance because they participate to regulate elemental cycling and mass transfer in the critical zone with major implications for carbon and nitrogen cycling. In that aim, we measured 1) dissolved CO2, alkanes, O2 and N2 concentrations in clay pore waters by rock degassing, 2) soil gas flux and concentrations, and 3) δ13C of CO2 and alkanes in two contrasted tectonic contexts. The first context is the marine Jurassic black marls in the French Alps, characterized by deep burial, high erosion rates and dominant physical weathering processes. These marls are well-known for their occurrences of natural methane gas seeps. In this area, we carried out rock degassing on outcropping weathered claystone in complement of soil flux measurements to constrain the implication of weathering processes on the natural gas releases. These measurements are also tested as a new component of environmental baseline assessment in the field of unconventional hydrocarbons. The second context is the marine Cretaceous Tégulines Clay of the north-eastern part of the Paris Basin, characterized by low burial, low erosion rates, and dominant chemical weathering processes. In this area, we carried out rock degassing on soil and weathered claystone accessible by deep boreholes, in order to define the depth of the critical zone and major reactions controlling the weathering profile. Oxygen and nitrogen concentrations are the record of the atmospheric diffusion through the formations. Some values are higher than the gas solubility, which could be attributed to rock desaturation and air bubbles, and clay sorption (only for nitrogen). Weathering processes induce a significant CO2 increase and a large range of δ13CCO2, providing evidence of two major CO2 sources: CO2 internally controlled by carbonates and organic-derived CO2 of internal and external origins. In Alpine black marls, field observations suggest a low depth affected by weathering, due to intense erosion. In Tégulines Clay, the CO2 increase provides evidence of a ~ 20 m-thick critical zone. The lowest δ13CCO2 indicates that the highest reactive zone (organic matter degradation, calcite dissolution and pyrite oxidation) is ~10 m deep, in agreement with the depth of the root network. Nature and amounts of alkanes are contrasted in the two contexts. In deep burial environment, alkanes are abundant, in particular, in the “ fontaines ardentes” gas seeps in the French Alps. Composition of hydrocarbon gas and δ13C of methane strongly suggest a thermogenic origin. Outcropping black marls contain methane, suggesting oxidation of higher alkanes. That assumption is supported by δ13CCO2 of soil close to δ13C of alkanes. In low burial environment, small amounts of methane are present that rapidly disappear with weathering. Some methane concentrations could be attributed to diffusion of external methane formed by degradation of organic matter under reducing conditions in soil. Overall those results suggest that dissolved gas and their isotopic signature are good markers of weathering processes in the critical zone. This research was funded by the EU H2020 Programme (grant 764531 - SECURe), ANDRA-BRGM projects.

The metropolis of Lille (more than 1 million inhabitants) produces 40% of its drinking water through well fields tapping the chalk aquifer to the agricultural, urban, and industrial region southwest of Lille. The groundwater quality is threatened by the presence of chlorinated solvents amongst others pollutants. In fact, many industries using chlorinated solvents are or were established on the well field’s territory as paint factories, industrial laundries and metallurgical plants. The chlorinated solvent concentrations in groundwater often exceed the regulation limit for drinking water usage in France ([perchloroethene] + [trichloroethene] < 10 µg.L-1) and then limit its use for drinking water production. The understanding of the chlorinated solvents dynamics and space distribution in the aquifer is a major issue for the metropolis of Lille. In addition, the quantities of available water with good quality is currently decreasing due to repeated annual droughts in the recent years. Thus, the metropolis of Lille, the French Geological Survey and the University of Lille explored the transfer and degradation mechanisms of the chlorinated solvents in the well fields in the two research projects RESEAU (2016-2019) and COHMET (2017-2020).18 wells and 9 piezometers were sampled during 3 years in order to evaluate the chlorinated solvents concentrations. In order to assess a possible migration of the compounds, three piezometers were sampled along the water column using passive samplers. In addition, a more detailed hydrochemical characterisation of groundwaters (chemical elements markers of the reducing conditions, physico-chemical parameters) was performed in the same three piezometers. Furthermore, the possible chlorinated solvent sources were identified with the databases BASIAS and BASOL, which list the past and current industrial plants, polluted soils and sites on the French territory. Finally, the chlorinated solvent degradation mechanisms were investigated with a compound-specific carbon isotope analysis.The three-year concentration monitoring highlights complex dynamics of the chlorinated solvents in the aquifer. A wide variety of compounds is detected in the well fields (perchloroethene, trichloroethene, cis and trans-1,2-dichloroethene, 1,1-dichloroethene, 1,1,1-trichloroethane, 1,1-dichloroethane, 1,2-dichloroethane and vinyl chloride) with maximal concentrations ranging from 1.2 (vinyl chloride) to 155 µg.L-1 (cis-1,2-dichloroethene). The highest concentrations are measured downstream three former industrial laundries in the south of the territory. The chlorinated solvent concentrations are stratified along the wells water columns and increased with depth. These concentration increases are consistent with water inlets along the wells originated from the fractured chalk. Despite the measure of favourable physico-chemical conditions, the δ13C ratios comparison do not indicate biodegradation of the chlorinated solvents, except in two wells. The concentrations changes are essentially due to the migration of compounds in depth. Then, the δ13C ratios indicate the presence of several major sources of chlorinated solvents.

Natural gas emanations are part of the geochemical baseline to take into account when assessing global greenhouse gas emissions and potential impacts of conventional and unconventional gas exploration and exploitation on groundwater. Examples of such natural gas macro-seeps are known in several parts of the world (Etiope et al., 2009). Only a limited number of them have been characterized for their gas and isotopic compositions. Such analyses can provide essential information for baseline studies, providing insight in the sources (biogenic vs. ther-mogenic or modified thermogenic) and pathways of such seeps and may allow for distinction of natural seeps from stray gas leakage associated with human activities. Here, we report gas concentrations and multi-isotope data (δ 13 C and δ 2 H of methane and ethane, δ 13 C and δ 18 O of CO 2 , 3 He/ 4 He ratio) of two gas seeps in the French subalpine chains, both in a similar geological and structural position within Middle Jurassic claystones along the eastern border of the large synclinal structures of the Vercors and the Chartreuse massifs (Moss, 1992). The ardent fountain (fontaine ardente) of Le Gua, 30 km south of Grenoble has most likely the longest continuous written record of existence of any individual natural gas seep, mentioned explicitly as early as the first quarter of the 5 th century (Augustin of Hippo (St. Augustin), approx. 426) This natural seep was described in the past as a wet seep associated with a spring, whereas the second investigated seep, Rochasson near Meylan north of Grenoble, is a dry seep.

Real-time methods to monitor stable isotope ratios of CO2 are needed to identify biogeochemical origins of CO2 emissions from the soil–air interface. An isotope ratio infra-red spectrometer (IRIS) has been developed to measure CO2 mixing ratio with δ13C isotopic signature, in addition to mixing ratios of other greenhouse gases (CH4, N2O). The original aspects of the instrument as well as its precision and accuracy for the determination of the isotopic signature δ13C of CO2 are discussed. A first application to biodegradation of hydrocarbons is presented, tested on a hydrocarbon contaminated site under aerobic bio-treatment. CO2 flux measurements using closed chamber method is combined with the determination of the isotopic signature δ13C of the CO2 emission to propose a non-intrusive method to monitor in situ biodegradation of hydrocarbons. In the contaminated area, high CO2 emissions have been measured with an isotopic signature δ13C suggesting that CO2 comes from petroleum hydrocarbon biodegradation. This first field implementation shows that rapid and accurate measurement of isotopic signature of CO2 emissions is particularly useful in assessing the contribution of contaminant degradation to the measured CO2 efflux and is promising as a monitoring tool for aerobic bio-treatment.

Hydrocarbon-contaminated aquifers can be successfully remediated through enhanced biodegradation. However, in situ monitoring of the treatment by piezometers is expensive and invasive and might be insufficient as the information provided is restricted to vertical profiles at discrete locations. An alternative method was tested in order to improve the robustness of the monitoring. Geophysical methods, electrical resistivity (ER) and induced polarization (IP), were combined with gas analyses, CO2 concentration, and its carbon isotopic ratio, to develop a less invasive methodology for monitoring enhanced biodegradation of hydrocarbons. The field implementation of this monitoring methodology, which lasted from February 2014 until June 2015, was carried out at a BTEX-polluted site under aerobic biotreatment. Geophysical monitoring shows a more conductive and chargeable area which corresponds to the contaminated zone. In this area, high CO2 emissions have been measured with an isotopic signature demonstrating that the main source of CO2 on this site is the biodegradation of hydrocarbon fuels. Besides, the evolution of geochemical and geophysical data over a year seems to show the seasonal variation of bacterial activity. Combining geophysics with gas analyses is thus promising to provide a new methodology for in situ monitoring.

RATIONALE:Compound-specific isotope analysis (CSIA) of persistent organic contaminants can be used for source apportioning in the environment if appropriate sensitivity can be achieved. This paper describes the optimization and validation of a sensitive analytical approach for the determination of the carbon isotope composition of semi-volatile organic compounds, such as polycyclic aromatic hydrocarbons (PAHs).METHODS:Analyses are based on the introduction of up to 150 μL of organic extracts by means of programmed temperature vaporization-large-volume injection combined with gas chromatography coupled to isotope ratio mass spectrometry (PTV-LVI-GC/IRMS). To allow for the analysis of more volatile, low-molecular-weight PAHs, the PTV injector was equipped with a sub-ambient/cryogenic cooling. Accuracy, precision, linearity and determination limits for application in isotope analysis were evaluated for a set of individual PAHs ranging from two- to five-ring molecular structures. The method was exemplified by determining the δ(13) C values of individual PAHs in soil samples in a source apportionment study at a contaminated site.RESULTS:The choice of PTV injection parameters is crucial to prevent isotope fractionation during injection and largely depends on the analytes to be determined. The observed isotope fractionation effect on semi-volatiles depends on the applied solvent and injection temperature and demonstrates the importance of performing appropriate tests with given PTV parameters for each of the compounds of interest. The proposed PTV-LVI-GC/IRMS method allows the carbon isotope ratio (δ(13) C value) of individual PAHs to be determined accurately and precisely at concentrations of 0.04-0.1 ng μL(-1) even for volatile PAHs such as naphthalene or acenaphthene.CONCLUSIONS:LVI with PTV injector cooling allows for the isotopic analysis of volatile and semi-volatile PAHs at trace concentrations, thus considerably expanding the applicability of CSIA in environmental studies.

The aim of this study was to develop an easy-to-use, reliable method for measuring the isotopic composition of individual alkanes present in natural gases at concentrations below 1000 ppmv without the need of expensive or complex pre-concentration devices. The proposed method combines pre-concentration of the gaseous compounds (C1 to C5) in a cooled, programmable temperature injector equipped with a Carbosieve-packed liner followed by compound-specific isotope analysis (CSIA) by gas chromatography – isotope ratio mass spectrometry (GC-IRMS). This method requires only small sample volumes of up to 10mL and allows reliable isotopic measurements at low concentrations (< 100 ppmv) within the typical precision of GC-IRMS: ≤ 0.5‰ for carbon and ≤ 5‰ and hydrogen isotope analysis, respectively. The proposed injection techniques have been developed and validated using gaseous hydrocarbon samples of different origins.

Characterization of dissolved CO2 and alkane gas in clayrocks may help assessing the confinement properties of geological barriers considered as potential host rocks for a deep geological disposal as well as for caprocks of gas storages.A monitoring of alkanes with CO2, combined with carbon isotopes was performed on core samples coming from Underground ResearchLaboratories (Bure, Mont Terri, Tournemire) and the Schlattingen borehole in France and Switzerland. Compositionof hydrocarbon gas and δ13Cof methane strongly suggest a dominant thermogenic origin of methane which ismixed with a bacterial origin for the Toarcian shales, Pliensbachien and Callovian-Oxfordian clayrocks. Results also evidence the contrasted behavior of CO2, which is controlled by chemical equilibrium between pore water and carbonate mineralogy, compared to the alkanes which are present in the porosity as a stock of dissolved gases which can be depleted during degassing experiments.

Since the 1970ies, isotope fingerprinting on CO2 and gaseous alkanes proved to be a powerful tool in hydrocarbon exploitation [1]. In particular, isotope fingerprinting allows distinguishing between methane of biogenic or thermogenic origin [2]. Recently, alkanes confined in low permeability rocks (shales) have drawn economical attention as non-conventional energy resource. The same rocks are equally of interest as targets for CO2 sequestration (as caprocks), and for solid waste disposal (as host rocks), since they form natural barriers for fluid and gas migration . Characterisation of carbon, oxygen and hydrogen isotopes on CO2 and alkanes can reveal information on the origin and mobility of gases in the pore space of argillaceous sedimentary rock series thus providing additional constraints on their aptitude to act as barriers for the confinement of fluids and gases. Earlier work in the frame of research on radioactive waste disposal had allowed the prototyping of degassing cells for gas characterisation of shale rock drilling cores [3, 4, 5].Here we present a method making use of steel degassing cells specifically designed for large core samples and of CF-IRMS to determine C, H and O isotope ratios on the extracted gases [7]. After placing the core sample in the cell, atmospheric contamination is eliminated by several flushing cycles with an inert gas. The cell is then sealed to allow degassing over several months until the gas composition is stabilised. This process is monitored for gas pressure, temperature and relative humidity; regular measurements of bulk chemical gas composition are performed using gas chromatography. Once stabilised, the gas is recovered from the cell and transferred into He-conditioned vials. Carbon and oxygen of CO2 are measured by Gasbench-IRMS on a Thermo Finnigan Delta plus XP continuous flow mass spectrometer. delta13C and delta2H of alkanes are determined after manual injection by GC-IRMS combining a Thermo Electron Trace GC Ultra and a Delta plus XP GC-IRMS. Standard deviations are less than 0.5 ‰ for delta13C, and 5 ‰ for delta2H, respectively. Required concentrations for determinations of the different alkanes vary between 300 and 900 ppmv. The influence of sample volume and time lapse before analysis was investigated. For concentrations < 500 ppmv (1 to10 ml injection volume) a N2-cooled PTV system including a Carbosieve liner was tested for different trapping-and desorption temperatures. The method was tested on a synthetic gas mixture and on natural samples obtained from cores from different depths (780 to 974 m) in the Opalinus Clay (Middle Jurassic) of the Mont Terri underground rock laboratory (Jura, Switzerland).