We present an integrated petrological, petrophysical, and hydrogeological study of the critical zone (CZ) developed in the Hercynian granitic basement of the Strengbach watershed ( Vosges Massif, France) to characterize its deep architecture and water circulation levels. For this purpose, six boreholes (50-120mdepth), from which three are cored, and three piezometers (10-15mdepth) were drilled to define the vertical extension and lateral variability of the main CZ horizons. The Strengbach watershed is composed of a topsoil horizon of limited vertical extension (0.81.2 m), a mobile saprolite level, and an in-place fractured bedrock. The latter is subdivided into a few meters thick saprock horizon, defined by open sub-horizontal fractures and a deeper fractured bedrock horizon with steeply dipping fractures (>50 degrees). In the north-facing slope, the vertical extension of the mobile saprolite horizon increases from approximate to 1-2 m at the top of the slope to approximate to 9 m downstream, close to the valley bottom. In contrast, the south-facing and more easterly slope shows a mobile saprolite horizon with limited vertical extension (approximate to 2-3 m thick). Such a difference is associated with the existence of a knickpoint in the river bed, separating a downstream zone marked by currently active erosion from an upstream one, less prone to erosion, with preserved reliefs formed around 20 ka ago. The water circulation scheme within the Strengbach watershed involves two different systems: a subsurface circulation within the shallow aquifer, corresponding to the mobile saprolite horizon and the saprock, and a deeper circulation in the fractured bedrock. The water circulation in the fractured bedrock is controlled by fractures of regional orientations, linked to the Vosges massif and the Rhine Graben Tertiary tectonics, and partly to reactivated Hercynian fracture zones. The unaltered bedrock was not reached by any of the three cores. These results from the Strengbach CZ demonstrate theimportance of integrating geological history of the watershed, either the long-termgeological bedrock evolution or the Quaternary erosion patterns, to better understand and model the CZ hydrological functioning at the watershed scale.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Chapter 3 Developments of auxiliary chemical geothermometers applied to the Los Humeros and Acoculco high-temperature geothermal fields (México) Bernard Sanjuan Sanjuan, Frédérick Gal, Ruth Alfaro

The existence of geological fluids rich in natural hydrogen (H2) raises the question about the energy potential of this carbon-free resource. However, to date there is no exploration strategy based on robust methodologies and pathfinders. Therefore, it is important to develop an exploration guide that is not only focused on surface gas monitoring, but that also considers the local deep geological setting integrating the entire hydrogen system from source to trap or leakage into the atmosphere. The northwestern Pyrenees, and particularly the Maule acute accent on Basin, represent a promising geological environment for natural H2 exploration for at least four reasons. First, an ultramafic mantle body is emplaced at shallow depth below the basin under pressure-temperature conditions favorable to serpentinization. Second, major faults such as the North Pyrenean Frontal Thrust constitute large-scale fluid flow convergence and drainage. Third, hydraulic gradients imposed by sharp reliefs and combined with temperature and pressure gradients trigger fluid migration. Fourth, impermeable sedimentary formations or caprocks such as evaporites or claystones overly porous reservoir rocks that could constitute traps for accumulating H2. To investigate H2 migration at the fault scale, we present new geochemical and geophysical data recorded along the North Pyrenean Frontal Thrust. Based on both soil gas and electromagnetic transects, we reveal the presence of a gas-draining fault. Soil gas concentration (H2, CO2, CH4 and 222Rn) recorded at 1 m depth increases when approaching the North Pyrenean Frontal Thrust. The maximum H2, CO2 and 222Rn concentrations recorded in the fault zone are 822 ppmv, 10.3 vol% and 57 kBq.m? 3, respectively -whereas their local background concentrations are by 1-2 orders of magnitude lower: 10 ppmv, 0.2 vol% and 0.3 kBq.m? 3 respectively. Our geochemical and geophysical data support the concept of a deep-fluid migration along the detected fault plane. In addition, the study of historical well data combined with the most recent geological and geophysical surveys carried out in the region, highlights zones where H2 could accumulate at depth. The Triassic salt formations, located at 2800 to 4000 m deep beneath the Maule acute accent on Basin, represent the most promising trap for H2 in the northwestern Pyrenees.

Groundwater sampling in boreholes has been carried out for decades according to well-established protocols and regulations. An important requirement in this context is the need to purge the borehole prior to any sampling action, the volume of the purge being adapted to the water flow and the regulatory context. Contaminated site investigations have introduced the concept of water column heterogeneity in the screened section of boreholes, which also occurs in some uncontaminated boreholes or in long-screened boreholes. Specific guidelines and practices were thus introduced, in particular the concept of discrete sampling. This type of sampling can be advantageously used in deep boreholes, provided that there is some renewal of water at the screens or the perforated intervals. The present study aims to take a step forward in the characterization of deep boreholes set up in the Aquitaine basin in France, by defining a protocol for relatively short boreholes (depth < 170 m) and applying it to much deeper boreholes, formerly drilled for oil and gas exploration (sampling up to 1035 m deep). Acquisitions were performed to better characterize water chemistry, including some isotope considerations. They were based on physico-chemical logging and endoscopic inspections where technically possible. After a review of the information provided by the investigations on the characterization of the Paleocene - Eocene aquifer in southwestern France, a methodology is proposed to make sampling in deep boreholes with low-yield more reliable, by combining the abstraction of a reduced water volume and the use of a downhole sampling system.

Monitoring is one of the key activities in a thorough risk management framework, and it is particularly true in the context of underground carbon dioxide storage. The nature of the underground makes the state of the considered system both highly variable and uncertain. One challenge is thus to get a understanding of the system evolution. Another challenge is to take robust decisions in presence of deep uncertainty. In particular, we explore in this work different ways for setting thresholds. Thresholds work as a decision-making aid: while a monitored indicator stays below a threshold, operation can continue as planned. When the indicator is over the threshold, the operator should consider mitigation actions. It is thus fundamental that thresholds are set in a robust manner in order to avoid false alarms or missed alarms. We can distinguish two broad types of monitoring data: the first type is expected to evolve with operations even under normal conditions, for instance the downhole pressure should increase when injecting CO2. On the other hand, the second type of data should not be evolving with operations under normal conditions. This is typically the case for environmental monitoring such as soil gas measurements. In this case, a baseline is needed before the beginning of operations, and the goal of monitoring is then to detect potential deviations from this baseline during operations. In this work, we focus on the second type of monitoring data. Our objective is to compare various methods for detecting deviations from the baseline, and to propose a method for a robust identification of such deviations. In this paper, we apply such methods on soil gas monitoring data, but the findings should be applicable to other types of data as well. The most basic method in this context is a visual comparison between baseline data and operations data. However, this method is subject to numerous biases and can often be impractical due to the quantity and complexity of data to deal with. A slightly method is to set thresholds on a monitored parameter (e.g. CO2 concentration) and to define beforehand the course of action to follow in case the parameter goes outside the safe range. This method is sometimes refer to as a traffic light system when two thresholds are delimiting a safe green parameter range, an amber range and a red range. Even if popular, this method has a number of drawbacks, particularly if the monitored parameter is highly naturally variable, as is the case for soil CO2 concentrations. In several cases, variations of this method has led to false alarms potentially halting injection operations or even false allegations of leakage from external parties. Classical (or frequentists) statistical methods can help in characterizing baselines, and in setting thresholds by computing the probability of false and missed alarms. Recently, some authors have also explored the use of Bayesian methods for characterizing baselines and using the results for setting better thresholds. One of the advantages of Bayesian methods is that the output is usually expressed in probabilities which allows for more informed thresholds and decisions. In addition to these methodological developments, various authors have proposed alternative monitoring strategies in order to get a signal for leakage, such as isotopes monitoring or co-monitoring of O2 and CO2 for a representation of the underlying processes. However, these developments should also be guided by robust decision-making. We test the various methods on synthetic datasets as well as on a real dataset acquired over the course of several years of environmental monitoring in the context of the Rousse storage pilot located in South-West France and operated by Total between 2010 and 2013.

The objective of this paper is to test different methods for detecting anomalies in environmental monitoring of CO2 geological storage sites. This type of monitoring relies on a baseline that is performed before injection. Data collected during injection are then compared to this baseline in order to detect deviations from a normal behavior. A robust method for detecting anomalous measurements would have few false positives (i.e. when an anomaly is wrongly detected) and false negatives (when a real anomaly is not detected). We use a dataset of soil gas measurements that were collected in 36 different locations in the surrounding of a CO2 storage pilot in the South-West of France. We consider CO2 and O2 concentrations in soil. The dataset is modified in order to simulate a leak. We compare the performance of four different methods. The first method uses a threshold in CO2 concentrations. The second method compares measurements with a threshold in the CO2 and O2 relationship (respiration line). The third method fits a Bayesian hierarchical model to the data, accounting for variability between measurements locations. The fourth method fits a Bayesian model to the CO2 / O2 relationship. The Bayesian models both use the same method: a “normal” model is fitted to the baseline data. An additional “anomalous” model (same model but with higher deviation) is tested with the measurements collected during injection. Bayesian inference is used in order to check which of these two models fits best to each measurement. With the data used in this paper, the Bayesian version of the CO2 / O2 relationship has the best results in terms of false positives and false negatives. One advantage of using a Bayesian approach is that the result is a probability that a data point is an anomaly, allowing for a natural interpretation. However, Bayesian approaches may be harder to communicate to lay people.

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.

The first analytical results relative to the native geothermal brine discharged from the two deep wells drilled at Vendenheim, in the Rhine Graben, in Alsace (France), obtained within the framework of the Vendenheim FONROCHE geothermal project and the H2020 European DEEP-EGS project, suggest the geochemical composition of this brine is very similar to that of the fluid which was discharged from the neighboring Cronenbourg deep well, in the past. It is also close to that of the brines discharged from the other deep wells located in more northern areas of the Rhine Graben, such as Soultz-sous-Forets, Rittershoffen, in France, and Landau, Insheim, in Germany. This Na-Cl brine has a TDS value around 100 g/l and a pH value close to 5, before cooling and degassing. Except for the calcium and strontium concentration values, which are much lower than those in the Soultz-sous-Forets, Rittershoffen, Landau and Insheim brines, the other concentration values of major and trace species are comparable. Given the similarity of the geochemical composition of all these waters, their origin is probably similar but also multiple, because it results from processes of mixing between primary brines formed by advanced evaporation of seawater (probably until the stage of halite precipitation) and meteoric freshwaters, plus contributions from halite dissolution following successive marine transgression-regression cycles from the Triassic to Oligocene. As for the other deep brines, the main solute cation geothermometers give estimations of reservoir temperature close to 225 ± 25°C for the Vendenheim native brine. This estimation probably corresponds to the temperature of equilibrium at which this brine acquires its chemical composition by interaction with the reservoir rocks. The mineralogical assemblage in equilibrium with the brine at this temperature was described in previous studies. The concordant estimations of reservoir temperature, using thermometric relationships such Na-Li and Mg/Li, especially developed for oil-field and sedimentary basin brines, and existing thermal gradients from 40 to 60°C/km, suggest that the deep brines discharged from the granite basement could probably originate from Triassic sedimentary formations (Buntsandstein, for example) located at great depth ( ≥ 4 km), in the centre of the Rhine Graben, in which they acquire their high salinity and chemical composition at temperatures close to 225 ± 25°C. This assumption seems to be also supported by their Li, B and Sr isotopic signatures. These hot brines would then migrate through a complex, but still poorly defined system of deep faults (probably NE-SW but also NW-SE faults), from the sedimentary centre of the Rhine Graben to the granite-fractured basement and the Graben’s NW borders.

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 first analytical results relative to the native geothermal brine discharged from the two deep wells drilled at Vendenheim, in the Rhine Graben, in Alsace (France), obtained within the framework of the Vendenheim FONROCHE geothermal project and the H2020 European DEEP-EGS project, suggest the geochemical composition of this brine is very similar to that of the fluid which was discharged from the neighboring Cronenbourg deep well, in the past. It is also close to that of the brines discharged from the other deep wells located in more northern areas of the Rhine Graben, such as Soultz-sous-Forets, Rittershoffen, in France, and Landau, Insheim, in Germany. This Na-Cl brine has a TDS value around 100 g/l and a pH value close to 5, before cooling and degassing. Except for the calcium and strontium concentration values, which are much lower than those in the Soultz-sous-Forets, Rittershoffen, Landau and Insheim brines, the other concentration values of major and trace species are comparable. Given the similarity of the geochemical composition of all these waters, their origin is probably similar but also multiple, because it results from processes of mixing between primary brines formed by advanced evaporation of seawater (probably until the stage of halite precipitation) and meteoric freshwaters, plus contributions from halite dissolution following successive marine transgression-regression cycles from the Triassic to Oligocene. As for the other deep brines, the main solute cation geothermometers give estimations of reservoir temperature close to 225 ± 25°C for the Vendenheim native brine. This estimation probably corresponds to the temperature of equilibrium at which this brine acquires its chemical composition by interaction with the reservoir rocks. The mineralogical assemblage in equilibrium with the brine at this temperature was described in previous studies. The concordant estimations of reservoir temperature, using thermometric relationships such Na-Li and Mg/Li, especially developed for oilfield and sedimentary basin brines, and existing thermal gradients from 40 to 60°C/km, suggest that the deep brines discharged from the granite basement could probably originate from Triassic sedimentary formations (Buntsandstein, for example) located at great depth (≥ 4 km), in the centre of the Rhine Graben, in which they acquire their high salinity and chemical composition at temperatures close to 225 ± 25°C. This assumption seems to be also supported by their Li, B and Sr isotopic signatures. These hot brines would then migrate through a complex, but still poorly defined system of deep faults (probably NE-SW but also NW-SE faults), from the sedimentary centre of the Rhine Graben to the granite-fractured basement and the Graben's NW borders.

Soil-gas concentrations and flux were measured during 20 separate measurement campaigns at the TOTAL Lacq-Rousse carbon capture and storage (CCS) pilot site, southern France, where 51,000 tons of CO2 were injected in a depleted natural gas field. Baseline data (September 2008 to December 2009) are compared to monitoring data from the injection (March 2010 to March 2013) and post-injection (February 2014 to December 2015) periods. CO2 soil-gas concentrations varied from atmospheric concentrations to more than 16% vol. with 1.4% as median value. Summer data showed high CO2 concentrations in the soil that remained quite high during winter. Median CO2 flux at the soil/atmosphere interface was close to 4.4 cm3·min−1·m−2. Carbon-isotope ratios measured on CO2 in soil gas had a mean value of −23.5 ± 3.1‰, some deviation being due to atmospheric CO2. Comparison between different gas species and the influence of temperature, pressure and soil-water content suggest that gases in near-surface environments are produced locally and naturally, and are unrelated to CO2 ascending from the storage reservoir. Monitoring of CO2 injection and the use of threshold levels is discussed as part of a practical approach considering specific regulations for the Lacq-Rousse CCS pilot experiment and constraints for the site operator.

We report investigations performed at some hydrocarbon gas seeps located in the French Subalpine Chains in zones of outcropping Jurassic black shales, increasing the reported number of such occurrences in this part of the Alps. We present the characteristics of each of the seeps, based on soil flux measurements and soil gas measurements. Gases emitted are CH4-rich (87–94%) with the exception of one site (78.5% CH4 + 8.2% CO2) where an active landslide may induce dilution by atmospheric air. CO2 is generally measured at low levels (<1.6%). Concentrations in C2H6 are more variable, from less than 1% to more than 2.3%. Gas is emitted over areas of various sizes. The smallest gas emission area measures only 60×20 cm, characterized by a strong hydrocarbon flux (release of about 100 kg of CH4 per year). At a second site, hydrocarbon emissions are measured over a surface of 12 m2. For this site, methane emission is evaluated at 235 kg per year and CO2 emission is 600 kg per year, 210 kg being related to gas seepage. At the third site, hydrocarbons are released over a 60 m2 area but strong gas venting is restricted to localized seeps. Methane emission is evaluated at 5.1 tons per year and CO2 emission at 1.58 tons per year, out of which 0.53 tons are attributed to gas seepage. Several historical locations remain uninvestigated at present, and numerous others may still be unknown. We outline strategies to search for such unrecorded sites. Considering the topography of the potential alpine and perialpine emission areas, the possibilities to detect gas emissions appear of the size recorded so far seem to be restricted to ground-based methods or to methods offering the possibility to point orthogonally to the soil towards the seep maximum. If such sites are to be investigated in the future in the frame of Environmental Baseline Assessment (EBA), even establishing appropriate monitoring protocols will be challenging.

Good practices of wellbore purging advice to draw three to five times the volume of the water column prior to sample. An extensive literature in the past 20 years has established the biases that may be linked to this procedure with emphasis on contaminant sampling and the benefit low-flow sampling may have to avoid redistribution of contaminants in boreholes because of the flow-weighted average character of pumping (Einarson, 2006, Handbook Environmental Site Characterization and GroundWater Monitoring). Low-flow sampling may produce flow-biased samples if operated in long-screened boreholes where vertical gradients exist (McMillan et al., 2014, J. Contam. Hydrol. 1699, 50-61) and is better suited for high permeability boreholes (ISO Standards). Water sampling in low permeability aquifers remains challenging especially for boreholes drilled for water table level monitoring (long-screened boreholes; pumps may not be lowered down in the screened interval). Fluid Electrical Conductivity logging prior to pumping and sampling may help in locating the productive levels albeit the information obtained under ambient flow conditions may be of less relevance than the data collected using salt injection (e.g. Lasher and Nel, 2013, Groundwater Division Conference, Durban). Fiber optic Distributed Temperature Sensing may also resolve hydraulic (McMillan, 2015, PhD dissertation) but is not of common use. We refer to investigations performed in two boreholes (Labruguiere and Valdurenque) of low permeabilities (10 −6 to 10 −7 m.s −1), located in detritic formations in SW France. These two boreholes, of 170 m and 123 m depth respectively, have long screened sections (128-170 m and 75-123 m respectively). With such geometries, the sub-mersible pump cannot be placed in the screened interval to perform volume purge. This raises the question of how long the pumping has to be done to get water representative of the downhole chemistry. The purge of three times the volume of the water column is unrealistic. For Labruguiere borehole, it would take 12 hours cumulated at ≤1 m 3 .h −1 pumping rate, each session cannot last more than 2 hours (dewatering) and 12 hours are needed to recover the water table level. We thus refer to deep sampling to assess the usefulness of such a method. Several levels were determined on the basis of ambient logging (temperature, conductivity, pH, dissolved oxygen, redox potential). In parallel we use pumping to assess the purge process inside the borehole and determine the minimum amount of drawn water needed to get water from the screens. This also highlights that alternative method to judge of representativeness, such as stabilization of physico-chemical parameters, may lead to false positives, i.e. the parameters were stabilized but the water chemistry was not that of the screened section. Three cycles of logging – deep sampling – pumping were done in each borehole. Based on field data and laboratory analyses, it appears that a protocol for deep boreholes characterization may refer to 1) borehole logging (information on ambient structure of the water column), 2) slight solicitation of the borehole by pumping (renewal of water at productive levels), and 3) deep sampling at the depth(s) suggested by borehole logs.

Methane and CO2 gas emissions from a gas seep located in the French Alps, documented over two millenaries, have been quantified along with gas emissions from the miniseepage area contouring the main vent. Several tons per year of both gas phases are released in the atmosphere from the main seep (18 tons of CH4 and 5.5 tons of CO2) which has been modified by an old borehole. Diffuse seepage brings additional CH4 (1 ton per year) and CO2 (2.3 tons per year) which are emitted from a small area of 240 m(2). Secondary CH4 oxidation processes are likely to occur near the surface. Biologically produced CO2 is also found in the soil in the seepage area and the biological component becomes predominant as CO2 concentrations drop lower than 2% vol. Soil gas CH4 + CO2 enrichments are oriented along N45 and N170 pathways that represent the main structural directions of the area. Soil flux anomalies are more scattered because surface clay formations have been strongly reworked at surface. Variety of methods (IRGA, micro-GC, accumulation chamber flux measurements, IR-camera) were tested and are discussed in connection with environmental impact monitoring and the role of sampling conditions, with emphasis on shale gas and carbon storage baseline assessments The role of potential leakage pathways represented by the old borehole structure is also discussed as an analogue with leaky shale gas wells.

One of the current scientific and societal challenges concerning the characterization of the spatial and temporal evolution of the Earth continental surfaces is to define all the active water circulations and water-rock interactions within the critical zone. The vertical extension of the critical zone, beyond the few meters classically studied through the analysis of weathering profiles, is still a very open question. The understanding of the deep critical zone functioning will certainly be improved by the information obtained from the semi-deep boreholes (10m to 100m deep) drilled in the substratum of several critical zone observatories. This is the case for the Strengbach watershed, one of the French environment observatory labeled “Service National d’Observation” by the INSU-CNRS, France, and integrated to the French Research Infrastructure OZCAR. The Strengbach catchment has been equipped with 6 boreholes from 50 to 100m deep, drilled along the two slopes of the catchment, in its upper part. Some of them have been cored. Several piezometers (≈15m deep) have been also drilled. The petro-geochemical analysis of the drill cores, the loggings of the wells, the monitoring of water levels, along with the geochemical characteristics of the borehole waters allow us to give a conceptual diagram for the localization and the nature of the water circulations and the related weathering processes in the Strengbach basin. Comparing these data with the hydrogeochemical data of the springs emerging on the Strengbach watershed makes it possible to quantify the part of the deep hydrogeochemical fluxes relative to the sub-surface ones. The results are also important to better understand the role of the deep critical zones in the constitution of the water resources in such granitic catchments.

Natural CO2 emissions from the volcanic rocks of the French Massif Central are poorly constrained. It is of interest better to assess the emission of such non-anthropogenic gases that may significantly contribute to the global carbon budget. We quantified the CO2 emissions to the atmosphere in a small area (0.052km2) located in the Massif Central close to Lake Pavin, the most recent volcanic edifice in metropolitan France. The specific character of this area, known as the Escarot mofette, was earlier studied for soil-gas concentrations only. In June 2017, we used the accumulation chamber method for measuring CO2 flux and related O2 depletion in the gases emitted at the soil/atmosphere interface, resulting in 176 data acquisitions over four days. In addition, 44 soil-gas concentration measurements were made at selected locations. CO2 emission rates are estimated at 8100±1800 tons/year of deep-seated CO2 and at 660±440 tons/year of biologically produced CO2. The uncertainty on these evaluations comes from the high-frequency variability of CO2 efflux in the more emissive areas and from the occurrence of heavy precipitation events. Though unexpected, these events were used for quantifying the decreases in CO2 efflux, which were as high as 500% over a few hours or even days in some locations. However, repeat acquisitions performed under more favourable weather conditions showed errors of commonly accepted amplitude (±15%). The area showed several degassing centres aligned along a NNW-SSE direction that correlates well with known geological structures, proving the ability of soil-gas methods to map hidden faults. The whole area is characterized by strong CO2 enrichment and related O2 depletion, but it is nonetheless possible to detect areas influenced by the rise of deep-seated gases and a few peripheral areas where biological processes dominate (CO2 up to 10% vol.). This study of gas emissions in a non-urban area also provides complementary information that is of use when extrapolated to similar structures in urban areas, where the occurrence of such gas releases, and its potential hazard may be more difficult to assess.

Two CH4-rich (> 80% vol.) gas seeps situated in the French Alps have been characterized in order to get a detailed overview of the gas phases and their spatial variability. Albeit set in similar geological settings, the Le Gua and Rochasson gas seeps present some differences in C2H6 or CO2 contents. Gas fluxes emitted into the atmosphere suggest that few hundreds of kg of CH4 are emitted each day from the seep area of Le Gua together with tens of kg of CO2. Emissions at Rochasson are one order of magnitude lower and only consist in CH4 venting. Site monitoring techniques and strategies are discussed.