HAL-BRGM, les publications scientifiques en libre accès du BRGM
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Isotopic fractionation of neon during magma degassing
No full textInternational audienceDetermining the neon isotope composition of the Earth's mantle is key to unravelling how light noble gases became part of the primordial Earth. However, accurately measuring neon abundance and isotopic composition in primary mantle melts is challenging due to the low concentration of neon, coupled with potential isotopic fractionation during transport and degassing. Interestingly, natural samples that exhibit solar-like neon isotopic compositions, ranging between the values calculated for the Sun ( 20 Ne/ 22 Ne = 13.36 ± 0.09; Heber et al., 2012) and those resulting from solar wind implantation and sputtering ( 20 Ne/ 22 Ne = 12.73; Moreira and Charnoz, 2016), support the idea that the Earth's mantle might have trapped a primordial nebula in its early formation stages. Analyses of three synthetic vesiculated glasses, produced at ∼1.7 kbar and 1200 °C using a starting material with an air-like isotopic composition ( 20 Ne/ 22 Ne = 9.81 and 21 Ne/ 22 Ne = 0.0287) fluxed with CO 2 , reveal significant isotopic fractionation of Ne within trapped vesicles. Measured values reach 20 Ne/ 22 Ne = 10.50 ± 0.14. The isotopic variations among individual vesicles align with expectations for kinetic fractionation, suggesting that degassing processes affect Ne isotope composition of basaltic melts.</div
Near real time groundwater forecast for the MAR projects of Cape Town, South-Africa
No full textInternational audienceThe water supply of the City of Cape Town is highly dependent on surface water and highly sensitive to drought. Since the 2015-2018 drought period, there has been a significant drive to identify and operationalize alternative water sources for the City of Cape Town such as the development of managed aquifer recharge (MAR) projects of Cape Flats and Atlantis. This has necessitated the equipping of existing monitoring points in order to monitor the behaviour of groundwater before and after the implementation of the MAR schemes. A new tool developed by the French Geological survey was implemented to enhance the information acquired by the monitoring points in a short term predictive assessment of groundwater situation. The Cape Flats Aquifer Management Scheme will rely on MAR for emergency supply and long-term resilience. Five MAR wellfield will inject advanced treated effluent thereby improving the groundwater yield and quality for abstraction from seven wellfields. The Atlantis MAR scheme, operational for 40 years, is a water recycling scheme in the sandy aquifer. Treated domestic effluent (from maturation ponds) is blended with urban storm-water runoff and discharged into recharge basins in the coastal dune area.In the frame of a project funded by the French Agency for Development, the MeteEau Nappes tool was set up for the City of Cape Town for two monitoring stations, one in each scheme. The tool reads in near real-time data of all the parameters needed to run a lump model calibrated on historical data. The model then displays forecasted series of water level for the next 6 months associated with various return periods. Due to its simplicity of use, the MeteEau Nappes tool is an appreciated forecasted tool for decisionmaking. In the management of a MAR scheme, it may be an interesting monitoring tool to control injection and discharge operations</p
CARTE D'ANOMALIES GEOCHIMIQUES - Outil national de valorisation des terres excavées
No full textInternational audienceDans le cadre de la mise à jour de la méthodologie nationale de valorisation des terres excavées de 2020, une carte nationale d'anomalies géochimique a été mise en place afin d'aider les parties prenantes à valoriser certaines terres excavées non issues de sites et sols pollués sans caractérisation préalable. En 2021, un premier retour d'expérience a permis d'identifier les besoins d'évolution de la carte, qui ont été mis en oeuvre en 2022 et 2023. Cette carte a été mise en place à partir de plusieurs sources de données, et un traitement statistique particulier a été déployé afin d'aboutir à une carte dont la résolution est de 3 km. Cette carte est actuellement diffusée sur le visualiseur Infoterre du BRGM et sur l'application TERRASS
Microbial biodiversity and geochemistry of the Atlantis MAR site: initial developments towards a biosurveillance toolkit
No full textInternational audienceAquifers contain around 97% of all liquid freshwater on Earth and are one of the largest habitats for microorganisms (comprising ~25% of the total prokaryotic biomass). Aquifer ecosystems provide services that are of immense societal and economic value (water purification, active biodegradation of anthropogenic contaminants, nutrient recycling, mitigation of floods and droughts, support to dependent surface ecosystems). Thus, a healthy aquifer contains a functioning microbial ecosystem (microbiome) which contributes to and maintains water quality by removing, for example, nitrate and certain organic contaminants. This invisible biodiversity is of critical importance to successful aquifer management. Managed aquifer recharge (MAR) involves the mass input of exogenous water (either through infiltration mechanisms or via direct injection) into an established aquifer system. This may be, for example, treated wastewater or stormwater, and may involve the application of some form of wetland or other nature-based solution to further improve incoming water quality before it infiltrates the aquifer proper. This incoming water may nevertheless contain contaminants which may impact aquifer health and water quality, but also its own microbial community. In the continuity of a project conducted by the City of Cape Town, UCT’s Future Water Institute and BRGM in the Cape Flats Aquifer (CFA) on the impacts of stormwater infiltration, a new project, MAReM, involving the same partners, aims to pursue the research on the impact of both stormwater and treated wastewater on aquifer microbial diversity and functions at the Atlantis city installation which has been running for 40 years. This project also aims to identify new management indicators, define education needs and deliver coherent educational material in relation to groundwater. Here we shall present the first results from samples collected up and downstream of Atlantis from a biological and geochemical angle as well as from the continued monitoring of the CFA site
Forms and fluxes of carbon: Surface to deep
No full textInternational audienceCarbon is an essential element for the coevolution of Earth and life, and its largest fraction is stored in the deep Earth. The availability of carbon at the Earth's surface or shallow subsurface over the past 3.8 billion years has been modulated by this reservoir of deep carbon, which has played a fundamental role in the emergence and diversification of life. In turn, major geobiological changes at the Earth's surface have profoundly affected the chemical fingerprints and morphological forms of carbon entering the deep Earth through subduction. This chapter presents an overview of the geology of deep carbon, from its origins and forms throughout the Earth's history, to its movements and fluxes between shallow and deep reservoirs.</div
Observations and Modelling of Coastal Dune Dynamics Along the Gironde Coast, France
No full textInternational audienceConsidered as reservoirs of biodiversity, coastal dunes also represent natural barrier against coastal flooding and large source of sediment to mitigate coastal erosion. Dynamics of coastal dunes are forced and controlled by marine, aeolian and biological processes. A better understanding of the interactions between all these processes based on field observations or numerical modelling is crucial to define management strategies that aim to develop the resilience of coastal dune against sea level rise. The analysis of multi-annual topographic data collected along the Gironde coast in SW France show a strong landward migration of the coastal dunes caused by strong wind events and a decrease in vegetation cover. The same data were also used to calibrate and validate a numerical model, AeoLiS, that simulated Aeolian sediment transport. This model showed good performance to reproduce the landward migration of non-vegetated dune
Life history traits of Aporrectodea caliginosa in a 8-year laboratory experiment
No full textInternational audienceEarthworms are key soil organisms used as bioindicators of soil health and in population dynamics modeling approaches in soil ecology and ecotoxicology. However, very little is known about the effects of environmental conditions on their life history traits such as their life span, growth and reproductive capabilities on long terms. The aim of this study was to bring new data and knowledge on the effects of different conditions of food resource and earthworm density on life history traits (i.e., survival, growth, reproduction) of Aporrectodea caliginosa individuals during a 8-year laboratory experiment conducted in France. For that, a cohort of 20 hatchlings A. caliginosa were synchronized from a breeding culture, fed with normal food conditions or ad libitum (three times the normal food conditions) and monitored individually once a month. Once adults, they were kept isolated or pooled at different densities. Individual weight and cocoons were assessed every 14 days the first year and then once a month. We here highlighted that the lifespan of A. caliginosa can exceed 8 years under laboratory conditions. We also put evidence on a longer life span of individuals pooled with congeners among those kept isolated. An individual kept alone and fed ad libitum from its birth kept growing continuously, with individuals reaching 10 times the regular body mass known for A. caliginosa. Pooled earthworms stopped growing and allocate their energy to reproduction. Pooled or isolated, individual body mass of earthworms was found to be a precise proxy of the amount of organic matter in the soil. These data on earthworms’ biology and ecology can have implications in life cycle modelling and functional ecology
Le rôle de la polarisabilité explicite dans la modélisation moléculaire des interfaces argile–eau selon l’état d’hydratation
No full textInternational audienceClay minerals play a central role in soils by controlling fluid structure and transport at the nanoscale. Molecular dynamics (MD) simulations are powerful tools to investigate these processes, but their predictive capability strongly depends on the chosen force field. This work presents a comparative MD study of a montmorillonite mesopore at varying water saturation levels, from fully saturated systems to conditions with only an adsorbed water layer. Simulations were performed using both the polarizable ion model (PIM) force field and the non-polarizable ClayFF reference.The results reveal significant differences in ion distribution, water structuring, and interfacial dynamics between the two approaches. Under saturated conditions, PIM predicts enhanced ion pairing and long-range organization within the diffuse layer, whereas ClayFF yields smoother profiles consistent with continuum descriptions. At low saturation, polarizability-induced surface deformation in PIM leads to a more homogeneous hydration and partial exclusion of cations from adsorption sites, while ClayFF favors localized cation trapping and heterogeneous surface dehydration. Although average in-plane water diffusion coefficients are similar, ClayFF exhibits strong diffusion anisotropy under desaturated conditions, in contrast to the more isotropic dynamics observed with PIM. These results highlight the importance of explicit polarizability for accurately describing clay–water interfaces under environmentally relevant unsaturated conditions.Les minéraux argileux jouent un rôle central dans les sols en contrôlant la structure et le transport des fluides à l’échelle nanométrique. Les simulations de dynamique moléculaire (DM) constituent des outils puissants pour étudier ces processus, mais leur capacité prédictive dépend fortement du choix du champ de force. Ce travail présente une étude comparative par DM d’un mésopore de montmorillonite à différents niveaux de saturation en eau, allant de systèmes entièrement saturés à des conditions ne comportant qu’une couche d’eau adsorbée. Les simulations ont été réalisées à l’aide du champ de force polarisable basé sur le modèle ionique (PIM) et du champ de force de référence non polarisable ClayFF.Les résultats mettent en évidence des différences marquées entre les deux approches en termes de distribution ionique, de structuration de l’eau et de dynamique interfaciale. En conditions saturées, le modèle PIM prédit un appariement ionique renforcé ainsi qu’une organisation à longue portée au sein de la couche diffuse, tandis que ClayFF conduit à des profils plus lisses, en accord avec les descriptions continues. À faible saturation, les déformations de surface induites par la polarisabilité dans les simulations PIM entraînent une hydratation plus homogène et une exclusion partielle des cations des sites d’adsorption, alors que ClayFF favorise un piégeage localisé des cations et une déshydratation hétérogène de la surface. Bien que les coefficients de diffusion moyens de l’eau dans le plan soient similaires, ClayFF présente une forte anisotropie de diffusion en conditions désaturées, contrairement à la dynamique plus isotrope observée avec PIM. Ces résultats soulignent l’importance de la prise en compte explicite de la polarisabilité pour décrire avec précision les interfaces argile–eau dans des conditions non saturées pertinentes pour l’environnement
Modélisation numérique des processus thermo-hydro-mécaniques dans les milieux poreux fracturés
No full textThis thesis develops and analyzes advanced discretization methods for coupled flow and geomechanics in fractured porous media, accounting for frictional contact at matrix-fracture interfaces. Fractures are modeled as co-dimension one surfaces, leading to mixed-dimensional formulations critical for applications like fault reactivation in geological storage and hydraulic stimulation in geothermal systems. A core challenge addressed is the robust discretization of contact mechanics on complex fracture networks (including tips, corners, and intersections) and polytopal meshes. We investigate three complementary formulations: a mixed formulation using face-wise constant Lagrange multipliers for tractions, enabling local contact conditions and efficient semi-smooth Newton solvers; a stabilized mixed formulation mitigating the discrete compatibility constraints of the standard approach; and a Nitsche's method, providing a consistent weak enforcement of contact conditions, extended here to Coulomb friction with normal compliance and its no-penetration limit. A key contribution of this thesis lies in the numerical analysis of these discretization techniques and their theoretical connections. We further extend these discretizations to mixed-dimensional Thermo-Hydro-Mechanical (THM) models. Two thermodynamically consistent formulations (energy conservation and entropy balance) are discretized using coercive Finite Volume schemes for non-isothermal Darcy flow, including tailored discretization of convective terms and contact-compatible mechanical solvers preserving dissipative properties. We implement these schemes using conforming Finite Elements (P1, P2) and the first-order Virtual Element Method (VEM) for polytopal meshes. Numerical comparisons evaluate accuracy, nonlinear convergence, robustness to parameters. Validation uses 2D/3D test cases with diverse fracture networks, convection-dominated thermal regimes, and fluid models (weakly compressible liquid, compressible gas), demonstrating the efficacy of the proposed methods for subsurface simulations. These developments are crucial for applications such as predicting fault reactivation in CO2 storage and modeling flow in geothermal systems.Cette thèse développe et analyse des méthodes de discrétisation avancées pour les processus hydro-mécaniques couplés dans les milieux poreux fracturés, en tenant compte du contact frottant aux interfaces matrice-fracture. Les fractures sont modélisées comme des surfaces de codimension un, conduisant à des formulations à dimensions mixtes essentielles pour des applications telles que la réactivation de failles dans le stockage géologique et la stimulation hydraulique dans les systèmes géothermiques. Un défi central abordé concerne la discrétisation robuste de la mécanique du contact sur des réseaux de fractures complexes (incluant extrémités, coins et intersections) et sur des maillages polytopaux. Nous étudions trois formulations complémentaires : une formulation mixte utilisant des multiplicateurs de Lagrange constants par face pour les tractions, permettant des conditions de contact locales et des solveurs de Newton semi-lisses efficaces ; une formulation mixte stabilisée, relâchant les contraintes de compatibilité discrètes ; et une méthode de Nitsche, basée sur une imposition faible et consistante des conditions de contact, étendue ici au frottement de Coulomb avec compliance normale et à sa limite de non-pénétrabilité. Cette thèse contribue à l'analyse numérique de ces méthodes de discrétisation et de leurs liens. Nous étendons ensuite ces discrétisations aux modèles thermo-hydro-mécaniques (THM) à dimensions mixtes. Deux formulations thermodynamiquement consistantes, basées soit sur la conservation de l'énergie soit sur le bilan d'entropie, sont discrétisées à l'aide de schémas de volumes finis coercifs pour l'écoulement de Darcy non isotherme, incluant une discrétisation adaptée des termes convectifs et préservant les propriétés dissipatives. Nous implémentons ces schémas en utilisant les éléments finis conformes (P1, P2) ainsi que la méthode des éléments virtuels (VEM) d'ordre un pour les maillages polytopaux. Des comparaisons numériques évaluent la précision, la convergence non linéaire et la robustesse vis-à-vis des paramètres. La validation repose sur des cas tests 2D/3D avec divers réseaux de fractures, des régimes thermiques dominés par la convection et différents modèles de fluide (liquide faiblement compressible, gaz compressible), démontrant l'efficacité des méthodes proposées pour les simulations en subsurface critiques, notamment pour la prédiction de la réactivation de failles lors du stockage du CO2 et pour la modélisation des écoulements dans les systèmes géothermiques