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Geothermal modeling in complex geological systems with ComPASS
No full textInternational audienceIn deep geothermal reservoirs, faults and fractures play a major role, serving as regulators of fluid flow and heat transfer while also providing feed zones for production wells. To accurately model the operation of geothermal fields, it is necessary to explicitly consider objects of varying spatial scales, from the reservoir scale itself, to that of faults and fractures, down to the scale of the injection and production wells.Our main objective in developing the ComPASS geothermal flow simulator, was to take into account all of these geometric constraints in a flow and heat transfer numerical model using generic unstructured meshes. In its current state, the code provides a parallel implementation of a spatio-temporal discretization of the non-linear equations driving compositional multi-phase thermal flows in porous fractured media on unstructured meshes. It allows an explicit discretization of faults and fractures as 2D hybrid objects, embedded in a 3D matrix. Similarly, wells are modeled as one dimensional graphs discretized by edges of the 3D mesh which allows arbitrary multi-branch wells. The resulting approach is particularly flexible and robust in terms of modeling.Its practical interest is demonstrated by two case studies in high-energy geothermal contexts
Augmented Quantization: Mixture Models for Risk-Oriented Sensitivity Analysis
No full textA central question in risk analysis is to identify the factors that drive the system toward a specific hazardous outcome, such as the exceedance of a given threshold. When relying on numerical simulators, we propose to study the distribution of the inputs, transformed into uniform variables via their cumulative distributions, conditionally on the occurrence of the hazardous event. To represent this multivariate conditional distribution for sensitivity analysis, we introduce an original quantization approach based on estimating a mixture of Dirac and local uniform distributions. For each marginal of this mixture, a Dirac component indicates a strong influence of the corresponding variable, whereas a uniform component with wide support reflects weak influence. A notable advantage of this method is its ability to identify the regions of the input space that most strongly influence the occurrence of the risk event, while also capturing the joint effects of multiple variables. However, learning mixture models typically relies on likelihood-based methods, which are not well suited to mixtures involving singular or Dirac components. To address this, we propose an \emph{Augmented Quantization} method, a reformulation of the classical quantization problem based on the -Wasserstein distance, which can be computed in very general distribution spaces. The performance of Augmented Quantization in estimating such mixture models is first demonstrated on analytical toy problems, and then applied to sensitivity analysis of both an analytical function and a practical flooding case study on a section of the Loire River
Avis du conseil scientifique du comité de bassin Seine-Normandie sur les gestions alternatives des urines et matières fécales humaines.Un potentiel d’amélioration conjointe de la gestion de l’eau et de la souveraineté alimentaire
No full textCet avis vise à remettre en débat le mode de gestion des excrétats humains (urines et matières fécales) vers des alternatives davantage sobres et circulaires, présentant des bénéfices potentiels sur les plans hydriques, environnementaux, sanitaires, économiques et de souveraineté alimentaire. Depuis plusieurs décennies, le tout-à-l’égout est le mode dominant de gestion des excrétats humains dans les villes occidentales, ce qui tend à le faire considérer comme naturel. Pourtant, les limites de ce mode de gestion, majoritairement linéaire, présentent de forts enjeux en termes de soutenabilité et de résilience. Les nombreuses alternatives à ce mode de gestion, communément nommées « séparation à la source des excrétats humains », tendent à se développer depuis dix ans sur le bassin Seine Normandie, au point que ce bassin est désormais devenu une référence mondiale en matière de séparation à la source des excrétats humains (Binz, 2024). Les filières professionnelles de gestion alternative pour une valorisation agricole sont actuellement en émergence. Il y a un intérêt stratégique à favoriser le développement de ces filières, ce qui nécessite le soutien de la puissance publique et la coordination des acteurs. À court terme, de très nombreuses opportunités « sans regret » sont à saisir. À long terme, le déploiement de la séparation à la source doit être pensé en articulation avec la remise en cause plus générale du paradigme actuel de gestion de l’eau et de l’alimentation, incompatible avec le respect des limites planétaires et les adaptations nécessaires aux changements climatiques et géopolitique
Les systemes volcano-sedimentaires ediacariens dans l’Anti-Atlas – Enregistrement d’une transition rift – post-rift
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Review of participatory approaches implemented in different countries to manage groundwater
No full textSource Agritrop Cirad (https://agritrop.cirad.fr/615254/)International audienceDeep confined aquifers (CAq) are strategic resources for the supply of drinking water, mainly due to protection against pollution offered by overlying geological layers and their hydrological inertia which reduces their sensitivity to climatic change. This presentation investigates how to develop a shared understanding of those complex aquifers, a pre-requisite to involving stakeholders in the definition of management objectives and rules. Such participation ensures that various knowledge are taken into account and that principles of justice, equity and sustainability are discussed. But actively involving stakeholders, and especially citizens, in CAq management poses specific challenges. Because of its invisible and complex nature, CAq are less known and less appropriated than surface water. This is even truer for CAq which are the focus of the DEESAC project (Sustainability and Exploitability of Groundwater in CAq). Building knowledge about these resources requires technical and scientific mediation and tools. It also requires to mobilize and articulate different types of knowledge and expertise in order to develop common knowledge and build communities of interest around these resources. A second challenge is to support stakeholders in the construction of long-term management strategies by identifying indicators of “good status” which can ensure sustainability of the resources over the long term. Our intervention will present the results of a literature review on participatory approaches implemented in in France and in different countries (i) to define indicators of “good status” for CAq, (ii) to set objective values for these indicators and (iii) to determine associated withdrawal thresholds, with a view to sustainable resource management. The aim is to identify the different ways in which stakeholders were involved in defining these elements: which stakeholders were involved, how and at what stages of the decision-making process? What data and/or tools were used, in what format and according to what principles of use
Petrogenetic link between metasedimentary rocks, migmatites and granitoids in the Variscan basement of the Pontgibaud area, French Massif Central – implications for the crustal structure
No full textInternational audienceThe architecture of the crust in the French Massif Central (FMC) is described as a nappe stack composed, from top to bottom, of the Upper Gneiss Unit (UGU), the Lower Gneiss Unit (LGU), and the Para-Autochthonous Unit (PAU), which are intruded by Carboniferous granitic plutons. In the Pontgibaud region, the Upper Gneiss Unit/Lower Gneiss Unit (UGU/LGU) attribution and the relationship between metamorphic and magmatic rocks are uncertain. This hinders both our understanding of the geodynamic processes that led to the construction of the Variscan orogenic belt, and a precise assessment of the deep geothermal potential of this high-heat flow area. Our field investigations document a metamorphic gradient with, from South to North, micaschists and paragneiss with a gently, dominantly southward–dipping foliation grading into migmatites that display a steeper foliation. A maximum late Ediacaran deposition age of sedimentary protoliths is determined from LA-ICPMS U–Pb dates of the youngest detrital zircon populations that range from 588.9 ± 7.1 to 571.8 ± 8.2 Ma. Equilibrium phase diagrams calculated for selected samples document a metamorphic gradient from 2.3– 5.3 kbar, and 570°–660°C, characterizing a mid- to low pressure-high temperature event dated from 357.6 ± 1.5 to 342.2 ± 4.6 Ma by LA-ICP-MS U–Pb analyses of metamorphic monazite. The contact between the metasedimentary rocks and migmatites is crosscut by peraluminous, cordierite-biotite-bearing Claveix and Gelles granitic plutons dated at 338.0 ± 7.3 Ma and 323.3 ± 9.7 Ma, respectively by U–Pb on zircon. The distribution of U–Pb dates and Lu–Hf isotopic compositions of detrital zircon in metamorphic rocks; the absence of HP mineral relics; the age of HT metamorphism of ca. 357–345 Ma; and the lack of a major mylonitic tectonic contact, which would typify the base of UGU, collectively support that the exposed crust of the Pontgibaud area corresponds to LGU metamorphic rocks. Moreover, the mineralogy, bulk chemistry and zircon Lu–Hf isotopic composition of the granites and migmatites are consistent with an origin by partial melting of LGU metasedimentary rocks. These data show that the Pontgibaud area exposes a section along a Carboniferous MP-LP/HT metamorphic gradient from micaschists to migmatites. Genesis and extraction of a large volume of granitic magma require higher temperatures (>800 °C) that were not reached by the exposed rocks. Accordingly, we propose that the local mid- and lower crustal levels are made of rocks with a similar protolith as those exposed at the surface, but affected by higher metamorphic grade and magma extraction.L’architecture de la croûte dans le Massif central français (FMC) est décrite comme un empilement de nappes composé, de haut en bas, de l’unité supérieure de gneiss (USG), de l’unité inférieure de gneiss (UIG) et de l’unité para-autochtone (UPA), qui sont intrudées par des plutons granitiques du Carbonifère. Dans la région de Pontgibaud, l’attribution USG/UIG et la relation entre les roches métamorphiques et magmatiques sont incertaines. Cela limite à la fois notre compréhension des processus géodynamiques qui ont conduit à la construction de la ceinture orogénique varisque, et une évaluation précise du potentiel géothermique profond de cette zone à flux de chaleur élevé. Nos résultats de terrain documentent un gradient métamorphique avec, du sud au nord, des micaschistes et des paragneiss avec une foliation légèrement inclinée vers le sud, passant à des metatexites et diatexites avec une foliation plus pentée. L’âge maximum de dépôt des protolithes sédimentaires à la fin est déterminé à partir des âges U–Pb obtenus par LA-ICP-MS avec des populations de zircons détritiques qui vont de 588.9 ± 7.1 à 571.8 ± 8.2 Ma. La modélisation thermodynamique des échantillons sélectionnés donne des conditions de 2.3 à 5.3 kbar et de 570° à 660°C, caractérisant un événement de basse pression-haute température daté par U–Pb (LA-ICP-MS) sur monazite métamorphique entre 357.6 ± 1.5 et 342.2 ± 4.6 Ma. Les roches métamorphiques sont intrudées, par les plutons granitiques peralumineux de Claveix et de Gelles, des granites à cordiérite et biotite, datés respectivement à 338.0 ± 7.3 Ma et à 323.3 ± 9.7 Ma par des analyses U–Pb sur zircon. La distribution des âges U–Pb et des compositions isotopiques Lu–Hf du zircon détritique dans les roches métamorphiques, l’absence de reliques minérales HP, l’âge du métamorphisme HT d’environ 357-345 Ma et l’absence d’une structure majeure reconnue, qui caractériserait la base de l’USG, confirment collectivement que les micaschistes, les paragneiss et les gneiss migmatitiques de la région de Pontgibaud correspondent à l’UIG. De plus, la minéralogie, la chimie en roche totale et la composition isotopique Lu–Hf des zircons des granites sont compatibles avec une origine des roches plutoniques par fusion partielle des roches métasédimentaires de l’UIG. En conséquence, ces données montrent que la région de Pontgibaud expose une section le long d’un gradient métamorphique LP/HT du Carbonifère, allant des micaschistes aux migmatites. La genèse et l’extraction d’un grand volume de magma granitique nécessitent des températures plus élevées (>800°C) qui n’ont pas été atteintes par les roches exposées. Nous proposons donc que les roches métamorphiques locales de la croûte moyenne et inférieure ont un protolithe similaire à celui des roches exposées à la surface, mais de caractère résiduel après extraction de melt, et affecté par un grade métamorphique supérieur
Solar neon dissolution into an ultramafic magma ocean
No full textInternational audienceThe solar-like neon isotopic signature of the mantle suggests the presence of a primitive reservoir. Its origin remains a puzzle, though the literature suggests that a primordial H 2 and He-rich atmosphere captured from the accretion disk was dissolved into a magma ocean. Our study investigates how much neon can be incorporated into the magma ocean based on the basal pressure of such an atmosphere and the neon solubility in mafic to ultramafic melts (49-34 wt. % SiO 2 and 9-21 wt. % MgO). The neon solubility range obtained (3.4 × 10 -4 to 6.5 × 10 -5 cm 3 STP g -1 bar -1 ) cannot match the primitive mantle theoretical neon content in a slow accretion scenario, demanding either fast accretion or alternative models for neon origin on Earth. Considering the planetary embryo mass required to accumulate enough neon (>0.8 times the mass of the Earth), the hypothesis of partial dissolution of the atmosphere into the magma ocean remains difficult to consider.</div
Reconstruction of the Alpine crustal architecture through the Ligurian Flysch nappes: insights from structural, RSCM thermometry and thermokinematic modeling
No full textInternational audienceThe structural and thermal architecture of the Ligurian Flysch Nappes in the southwestern Alps remain poorly constrained, despite their critical role in the late Alpine orogeny. A better understanding of their evolution is essential for reconstructing the kinematic history and pre collisional configuration of the orogenic wedge. This study combines balanced and restored geological cross-sections with Raman Spectroscopy of Carbonaceous Material (RSCM) thermometry to investigate the burial and propagation history of the Ligurian Flysch Nappes. Four NE–SW trending balanced cross-sections, ranging from 13 to 46.5 km in length, were constructed across the para-autochthonous Subalpine foreland, the four main Ligurian Flysch Nappes (Sanremo, Moglio-Testico, Borghetto-Colla Domenica, and Albenga), and the internal Briançonnais units. RSCM thermometry on 67 samples from the Subalpine foreland and internal domains yields peak temperatures (TRSCM) between 140 ± 20 °C and 341 ± 10 °C, increasing systematically with structural depth and toward internal domains. Maximum TRSCM are observed in the inner Subalpine footwall, deeper nappes (e.g., Albenga Nappe), and the Briançonnais domain. A Mesozoic rift-related thermal overprint in the Subalpine Zone is excluded, as most samples were collected from Eocene-Oligocene formations, and no thermal anomalies were observed in the underlying Mesozoic strata. Thermal consistency between nappes and para-autochthonous Subalpine units in the northern sector supports a syn-orogenic burial origin. In contrast, a thermal anomaly in the southwestern frontal part, where a structural window exposes colder para-autochthonous rocks (~180 °C) beneath the warmer allochthonous Sanremo Nappe (~250 °C), suggests an inherited thermal history during nappe emplacement. Assuming a ~30 °C/km geothermal gradient, we estimate up to ~10 km of eroded overburden in the hinterland and ~3 km in the foreland. To better constrain the thermal effect of nappe emplacement on peak temperatures, we use a 1D thermo-kinematic model simulating its emplacement with variable thickness and velocity above the autochthonous. This model takes into account crustal heat generation, basal shear heating, and heat dissipation by conduction, and allows reconstruction of maximum temperatures at different depths and distances beneath the nappe. These results provide new constraints on nappe emplacement, paleo-thicknesses, and the kinematic architecture of the southwestern Alpine orogenic wedg
Investigation of PFAS/nPd/μZVI interactions: mechanism, performance, and nPd/μZVI reuse
No full textInternational audienceDue to their persistence, toxicity, and environmental spread, PFAS have been extensively studied for removal since the early 2000s. This study investigates the interactions between nano-sized palladium-coated zero-valent iron microparticles (nPd/μZVI) and perfluorooctanoic acid (PFOA) or perfluorooctanesulfonic acid (PFOS) to examine the mechanism and evaluate the performance of these two PFAS abatement.Batch experiments were performed under anoxic conditions (1–72 h, 90 °C) using 1 g of ultrasonicated (45 Hz) slurries of nPd/μZVI and 9.29 to 485 mg L−1 of PFAS across a pH range of 1–14. After the reaction and subsequent settling, samples of the aqueous phase were collected for pH measurement, fluoride analysis using a fluoride ion-selective electrode (F-ISE), and quantification of residual PFOA or PFOS and potential byproducts using ultra-high-performance liquid chromatography coupled with mass spectrometry (UHPLC-MS). Methanol used to wash nPd/μZVI under oxic conditions was also analyzed to differentiate between PFAS adsorption and their defluorination. The washing step enabled reuse of the nPd/μZVI, which were characterized by XRD and SEM-EDS before interaction, after interaction, and following methanol washing.PFAS sorption, governed by surface complexation and hydrophobic interactions, is the predominant PFAS abatement mechanism. It is driven by the anoxic galvanic corrosion of nPd/μZVI (AGC-nPd/μZVI), which modifies the physical and chemical conditions of the liquid phase and the surface chemistry of nPd/μZVI as a function of pH. Under the investigated conditions, a linear sorption model accurately described PFAS abatement at pH 4, where nPd/μZVI were shown to be reusable for at least 4–5 cycles without significant loss of sorption efficienc
Hydrogen, single-carbon compounds, and thermal regime in the oceanic ultramafic-dominated lithosphere: insights from a deep borehole on the Atlantis Massif
No full textInternational audienceWater that flows through permeable ultramafic rocks produces high abundances of molecular hydrogen (H 2 ), methane (CH 4 ), and other small organic molecules. Such products can fuel life in the rocky subseafloor, be extracted for energy, and may have played a role in pre-biological chemical synthesis on early Earth or other planetary bodies. The International Ocean Discovery Program drilled a new 1268-m-deep borehole (U1601C) into serpentinized mantle with minor gabbroic rocks on the Atlantis Massif, ~800-m north of the Lost City hydrothermal field (30°N, Mid-Atlantic Ridge). Measured temperatures of the disturbed borehole reached 91.3°C, and equilibrated temperatures of the deepest section are estimated to be between 110 -140°C. Water collected every ~5-m during drilling operations had H 2 concentrations that were regularly > 200 nM and spiked to > 10 µM at multiple depths. In these waters, carbon monoxide was only present in deeper, hotter sections, and potentially associated with gabbroic intrusions into the peridotite host. Open borehole fluids were sampled after drilling and samples recovered from the deepest portion contained elevated short-lived 222-Radon and strontium isotope ratios similar to Lost City fluids, pointing to the presence of in situ subseafloor formation waters that have equilibrated with the host rock. The deepest samples were actively degassing upon recovery and contained 740 ± 360 µM H 2 , 340 ± 36 µM CH 4 , and 25.5 µM ∑formate (= formate and formic acid). The shallowest fluids from the open borehole also contain micromolar H 2 and ∑formate concentrations, the presence of which cannot be attributed to the upward migration of the deeper, higher concentration fluids. We interpret these data as reflecting two distinct and interconnected regimes of fluid flow and composition. Deep waters that are channelized along faults, lithologic contacts, and other high permeability pathways host high H 2 and CH 4 concentrations plus micromolar ∑formate that closely mirror the chemistry and isotopic signatures of LCHF vent fluids. Pervasive fluid flow permeates the mesh texture and microfracture network of the serpentinized peridotite and sustains H 2 and ∑formate even in the shallowest subseafloor intervals at mild temperatures. These findings demonstrate that both focused and pervasive fluid flow contribute to the transport, and potentially the generation, of reduced volatiles and C1 compounds within the Atlantis Massif.</div