Towards a Better Knowledge of Natural Methane Releases in the French Alps: A Field Approach

International audienceWe 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 60x20cm, characterized by a strong hydrocarbon flux (release of about 100kg of CH4 per year). At a second site, hydrocarbon emissions are measured over a surface of 12m(2). For this site, methane emission is evaluated at 235kg per year and CO2 emission is 600kg per year, 210kg being related to gas seepage. At the third site, hydrocarbons are released over a 60m(2) 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

Identifying the origin of geomaterials of original and restored parts of a 14th century alabaster annunciation group through stable isotopes

International audienceThe origin of raw materials for sculpture is often obscure before the 17th century due to the scarcity of written sources. Identifying this origin provides hints to economic exchanges but also, potentially, allows for attributing sculptures to a specific context of creation (regional workshops, artists). Another challenge for art historians is the identification of restorations and their potential chronology. We present an example of a 14th century group of two statues, made of gypsum alabaster, representing an annunciation group, with the Virgin Mary and the angel Gabriel. Their original position was a near Troyes in the eastern Paris Basin, they are now separated being conserved at the Louvre Museum (Virgin Mary) and the Cleveland Museum of Art (Gabriel). Our multi-isotope study revealed the common origin of the material used for both sculptures, their isotope fingerprints being identical within the analytical error. These fingerprints are highly specific and point to an origin in a historical gypsum and alabaster quarry in the northern part of Provence, France, first mentioned at the end of the 13th century. We were also able to identify an unknown restoration of lower part of the Virgin Mary statue with an optically undistinguishable material, using Tuscan alabaster, most likely in the 19th century. Two other 14th century statues of the Virgin Mary originally situated in eastern and southeastern France can also be linked to this quarry. This underlines the potential and usefulness of independent geochemical evidence to underpin stylistic hypotheses on grouping of individual artworks, historical economic relationships between regions and on past restoration activities

Natural gas of radiolytic origin: An overlooked component of shale gas.

SignificanceNatural 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

Faire face à la salinisation et à la contamination des aquifères côtiers dans le contexte des changements environnementaux planétaires et sociétaux : Approches géochimique et isotopique appliquées aux ressources en eau de Recife (Brésil)

National audienceEn raison d'une pression démographique croissante, la région métropolitaine de Recife a subi d'importants changements d'utilisation des terres et de l'eau au cours des dernières décennies. Ces évolutions ont notamment généré une baisse spectaculaire des niveaux piézométriques, la salinisation et la contamination des eaux souterraines. Cette dégradation des ressources naturelles est liée à l'augmentation de la demande en eau, ponctuellement amplifiée par des périodes de sécheresse qui ont conduit à la construction de milliers de puits privés. La région de Recife apparaît ainsi comme un "point chaud" typique illustrant les problèmes des pays émergents tels que l'urbanisation, la répartition inégale de la richesse, la faiblesse des structures décisionnelles, les rapides développements industriel et touristique, induisant des pressions fortes sur les ressources en eau (quantité et qualité) dans le contexte des changements globaux sociétaux et environnementaux. Le projet COQUEIRAL a pour objectif d'étudier l'impact des activités humaines sur les aquifères côtiers surexploités en (1) analysant les pressions sur les ressources en eau souterraine et leurs raisons sociales et structurelles,(2) identifiant les sources et les mécanismes de la dégradation des ressources en eau souterraine en termes de qualité et de quantité, en se concentrant sur les processus physiques et chimiques en tant que vecteurs de la réaction du système aux pressions extérieures et (3) évaluant les impacts des changements globaux sur les ressources en eau à l'échelle régionale

Managed aquifer recharge in weathered crystalline basement aquifers in India: Monitoring of the effect of tank infiltration on water quality over several monsoon events

International audienceManaged aquifer recharge (MAR) structures like percolation tanks are considered by the Indian national and regional governments as major option for tackling declining groundwater levels due to overexploitation for irrigation purposes (Boisson et al., 2014). Their main purpose is to restore groundwater availability under strong climatic and anthropogenic pressure. Furthermore, MAR-induced dilution with fresh surface water is generally expected to improve groundwater quality with respect to both anthropogenic and geogenic contaminants (total mineralisation, nitrates, chlorides, sulphates and fluoride contents). The impact of a percolation tank on groundwater quality was investigated in a context that is typical for hydro-climatic and geological settings in southern and eastern India: fractured crystalline basement aquifers overlain by a weathering zone under semi-arid climate. Water level data and geochemical indicators (stable isotopes and major ions) were monitored for both groundwater and surface water, over several successive monsoon events. In case of high to very high water levels, the groundwater quality is globally improved. However, in a few cases, the quality of the groundwater can be negatively impacted due to leaching of salts under the tank, particularly during the first rain events of the monsoon. Geogenic fluoride contents in groundwater, induced by water-rock interaction and enhanced by recycling of agricultural return flow under paddy fields, is found to be relatively stable over the year. This finding points out that the underlying processes, mainly dissolution of F-bearing phases like fluorapatites combined with Ca/Na cation exchange and calcite precipitation , both limiting the possibility of F-removal via fluorite precipitation (Pettenati et al., 2013, 2014), are not impacted by the hydrological conditions. This work highlights the complexity of the recharge processes in crystalline aquifers, enhanced by the variability of hydrological conditions. It also provides insights into the possible risk for groundwater quality deterioration in cases of light and short monsoons periods

Groundwater salinization in a coastal multilayer aquifer: Preliminary results on origins and mechanisms. Example of Recife (Brazil)

International audienceThe Recife Metropolitan Region (PE, Brazil) appears as a typical "hot spot" illustrating the problems of southern countries such as urbanization or rapid industrial and touristic development. All these factors induce high pressures on water resources both on quantity and quality in the context of global social and environmental changes. This paper focuses on the groundwater geochemistry in a costal multilayer aquifer aiming at investigating the sources and processes of salinization. The preliminary results of 62 groundwater samples from the five main aquifers are reported

Tracing the origins and processes of groundwater salinization in coastal aquifers with a multi-isotopes approach. Example of Recife, Northeast of Brazil

International audienceThe Recife Metropolitan Region (PE, Brazil) is a typical "hot spot" illustrating the problems of southern countries on water issues inducing high pressures on water resources both on quantity and quality in the context of global social and environmental changes. By focusing on the groundwater geochemistry in a costal multilayer aquifer, this work aims at investigating the sources and processes of salinization. Two different Precambrian blocks separated by a large lineament area constitute the site basement. The sedimentary fillings of the two basins present different origins that were distinguished by the Sr isotope composition. The northern deep Beberibe aquifer displays very high 87Sr/86Sr with a large range of values (0.7102-0.7233) illustrating the main continental origin of sediments whereas the southern deep Cabo aquifer showed lower values (0.7097-0.7141) indicating the contribution of the marine sedimentation. Although sulfate isotopes, Electrical Conductivity and Cl contents indicate a mixing with seawater for some samples of the deep Cabo and Beberibe aquifers, all 87Sr/86Sr values are above the present-day seawater composition. This can be related to the complex local history of transgression/regression phases that induced alternatively salinisation and freshening with gains and losses of cations and Sr, together with water-rock interactions. δ18O-δ2H clearly evidence the local present day recharge in the surficial aquifer, some samples being affected by in situ evaporation processes and/or recharge with evaporated water from dams used for water supply. The deep aquifers display a high range of B (20-600µg/L) and δ11B (6.7-68.5‰) with some of the highest values known to date. Multiple sources and processes affect the B behavior, among which mixing with saline water, B sorption on clays/organic matter and mixing with wastewater. The surficial aquifers are locally salinized possibly due to present seawater intrusion, and highly contaminated with wastewater

Potential leakage between aquifers in a deeply anthropized coastal sedimentary basin (Recife, Brazil): Strontium isotope constraints

Code ADS 2013EGUGA..15.7691PInternational audienceDue to an increasing demographic pressure, the Metropolitan Region of Recife (RMR) went through remarkable changes of water and land uses over the last decades. These evolutions gave rise to numerous environmental consequences, such as a dramatic decline of the piezometric levels, groundwater salinization and contamination. This degradation of natural resources is linked to the increase of water demand, punctually amplified by drought periods which induced the construction of thousands of private wells. The preliminary results of 62 groundwater samples from the five main aquifers are reported. This part of the study focuses on the major elements together with Sr isotopes to (1) characterize the signature of the different aquifer compartments of the 5 main aquifers, and (2)assess the potential connections between aquifers, e.g. vertical exchanges between the aquifers that can be modified or amplified by overexploitation or triggered through failed or improperly constructed wells

Past Hydrological Conditions in a Fluvial Valley: Records from C-O Isotope Signatures of Holocene Sediments in the Loire River (France)

Multi-proxy indices (grainsize distribution, mineralogy, &delta;18O, &delta;13C) in sediments from a meander infill in the Middle Loire alluvial plain of central France are used to highlight some aspects of the basin evolution over the period from 0 to 10,000 years BP. During the Late-Glacial and Holocene period, the lacustrine carbonate substratum of the alluvial plain was incised by the Loire River, creating numerous oxbows and channels related to meander migration. The channel fills consist mainly of fine clayey sediments deposited during flooding of the river, with an almost total absence of coarse-clastic and sandy material, except in the basal part. The record of isotope ratio variations together with the distribution of particle sizes allows the evolution of the river dynamics to be constrained. The strong decrease of carbonate &delta;13C in the upper part of the record is ascribed to a progressive closure of the meander and, thus, an increasing control of the C-isotope signature by biological activity in a local environment. Variations in carbonate &delta;18O, rather, reflect paleohydrological/paleoclimatic changes at the basin scale. The isotope record of the river dynamics also agrees with the variations in clay mineralogy

Lead isotopes tracing the origin of lead in an irrigated agricultural soil in Crete

International audienceMeasurements of the concentrations and isotopic composition of Pb in soil fractions in an irrigated cultivated soil profile developed on Miocene sediments in Crete are used to estimate the significance of soil-derived versus man-induced sources. In soil samples, lead concentrations are 7.1 ± 1 μg/g at shallow depth (40 cm), and reach 9.1 μg/g near the surface (10 cm). 206 Pb/ 207 Pb values range from 1.20 in shallow samples to 1.19 in the uppermost samples. In all the samples, the isotopic composition of Pb in the residual soil fraction, i.e. essentially silicate Pb, is slightly more radiogenic than in the soil mobile fraction. Pb in the EDTA-extracted fraction is anthropogenic Pb issuing most probably from Pb emitted in Greece