Tracking selenium in the Chalk aquifer of northern France: Sr isotope constraints

International audienceGroundwater at the southern and eastern edges of France's Paris Basin has a selenium content that at times exceeds the European Framework Directive's drinking-water limit value of 10 µg/L. To better understand the dynamics of the Chalk groundwater being tapped to supply the city of Lille and the Se origins, we used a combination of geochemical and isotopic tools. Strontium isotopes, coupled with Ca/Sr, Mg/Sr and Se/Sr ratios, were used to identify the main groundwater bodies and their mixings, with the Mg/Sr ratio constraining a ternary system. Groundwater in the agricultural aquifer-recharge zone represents a first end-member and displays the youngest water ages of the catchment along with the highest strontium isotopic signature (0.7084) and low Se contents. Anaerobic groundwater constitutes a second major end-member affected by water-rock interactions over a long residence time, with the lowest strontium isotopic signature (0.7079) and the lowest Se content, its low SF6 content confirming the contribution of old water. Se-rich groundwater containing up to 30 µg/L of Se represents a third major end-member, with an intermediate Sr isotopic ratio, and is mainly constrained by the clayey Se-rich formation overlying the Chalk aquifer. The spatial and temporal Se variability in the groundwater is clearly linked to the presence of this formation identified as Tertiary and also to the hydrological conditions; saturation of the Se-rich clays by oxygenated groundwater enhances Se mobility and also Sr adsorption onto the clays. This multi-tool study including Sr isotopes successfully identified the Se origins in the aquifer and has led to a better understanding of the regional mixing and processes affecting the Chalk groundwater

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

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

How global changes impact water resources in a southern coastal metropolis? Case of Recife (Brazil)

International audienceThe Recife Metropolitan Region is a hot spot illustrating the problems of southern countries on water issues where water resources are threatened both for quantity and quality in the context of global changes. Based on a transdisciplinary approach, the purpose of the study was to assess the impact of human activities on coastal aquifers, through the analysis of pressure on groundwater resources and their social and structural reasons, the identification of sources and mechanisms of groundwater quality degradation, and the evaluation of the impacts of global changes on water resources at the regional level. The methodology was based on a multi-isotopic fingerprinting of groundwater and surface water, on gas analyses, and on sociological and ethnographic investigations including ethno-photography to characterize the daily life of the Recife inhabitants facing the lack of water or its poor quality. The results revisit the aquifer system functioning. In the deep aquifers, the groundwater displays a residence time over 10 000 y with a residual salinity inherited from the Pleistocene marine transgressions. Their recharge is very limited resulting in large water level decrease. Inversed flow directions due to overexploitation favour leakage from the surficial contaminated aquifers. The access to water and its social perception vary according to the social environment and to the residence location of individuals. Access to water is more a political problem than a technical one. The public authorities tend to deny the difficulties of poor people, especially in times of drought. The discredit of the water and sanitation public actors, and political and institutional rivalries and fragmentation are obstacles to technical solutions implementation. Integrated water management is urgently needed knowing that local climatic scenarios predict a reduction of rainfall volume of 20% together with a sea level increase of 18-59 cm by 2100

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

Inter-laboratory testing of the effect of DNA blocking reagent G2 on DNA extraction from low-biomass clay samples

Here we show that a commercial blocking reagent (G2) based on modifed eukaryotic DNA signifcantly improved DNA extraction efciency. We subjected G2 to an inter-laboratory testing, where DNA was extracted from the same clay subsoil using the same batch of kits. The inter-laboratory extraction campaign revealed large variation among the participating laboratories, but the reagent increased the number of PCR-amplifed16S rRNA genes recovered from biomass naturally present in the soils by one log unit. An extensive sequencing approach demonstrated that the blocking reagent was free of contaminating DNA, and may therefore also be used in metagenomics studies that require direct sequencing

Dédicace

Cet ouvrage est dédié à la mémoire de notre mère, Pierrette Faverais (1952-2015), disparue dans l’année finale du projet. Les circonstances de l’époque ne lui ayant pas permis de mener le projet de thèse qu’elle avait soumis à Claude Lefort au début des années 1970, elle devint professeure de sciences économiques et sociales et transmit sa bienveillance et son esprit critique à de nombreux élèves de Caen et d’Argentan. Nous concevons nos travaux et en particulier cet ouvrage comme des fruits ..

Mobilité des éléments selon les alternances aérobie-anaérobie dans un écosystème rizicole en Camargue

In natural or cultivated soils from wet areas, the oxido-reductive conditions influence the mechanisms of element mobilisation or immobilisation. The mobility of elements is studied in paddy soils in Camargue. Soils are originally constituted of sediments which filled a Rhône paleochannel. Processes controlling the soil solution are analysed through continuously in situ recordings of physicochemical parameters. The impact of aerobic-anaerobic alternances, linked to annual irrigation cycle or to stormy precipitations, is studied at high frequency both in ground waters and in irrigation waters. In the waterlogged soil area, these perturbations generate mineral precipitation or dissolution. Thirty years of irrigation are enough for the annual dynamics to create an enriched horizon containing an oxido-reduction front. A year without irrigation initiates the dissolution of neoformed carbonates and oxides phases from this horizon. In the cultivated horizon, silicon, iron and lead are submitted to seasonal fluctuations linked to reductive conditions and plant activity. The amounts of dissolved iron, manganese and magnesium in waters are not controlled by known mineralogical phases. The acidification at pH 4 during the fourteen days of the pre-reproductive period allows the supply of nutrients from the dissolution of solid phases. Poor-crystallized clays and phytoliths are dissolved and provide silicon to rice.Les variations des conditions d'oxydo-réduction dans les sols des zones humides, cultivés ou en friche, influent sur les mécanismes de mobilisation et d'immobilisation des éléments chimiques. La mobilité des éléments est étudiée dans des rizières de Camargue, dont les sols sont issus du comblement récent d'un paléochenal du Rhône. Les processus qui régissent les eaux de nappe sont déterminés à partir d'une acquisition en continu in situ des paramètres physico-chimiques. L'impact des alternances aérobie-anaérobie, liées au cycle annuel d'irrigation ou à des pluies orageuses, est analysé à haute fréquence dans les eaux de la nappe et de la lame de submersion. Dans la zone saturée du sol, en profondeur, ces perturbations engendrent précipitation et dissolution de phases minérales. Trente années d'irrigation ont suffi pour que la dynamique annuelle génère la création d'un horizon d'accumulation, contenant un front d'oxydo-réduction. Pendant une année sans irrigation, la dissolution des phases néoformées, carbonates et oxydes, de cet horizon, est amorcée. Dans l'horizon submergé de la rizière, silice, fer et plomb sont soumis à des fluctuations saisonnières, liées aux conditions réductrices et à l'activité du riz. Les teneurs en fer, manganèse et magnésium des eaux ne sont pas contrôlées par les phases minéralogiques connues. L'acidification à pH 4 par le riz pendant les quatorze jours de la phase de pré-reproduction fournit des éléments nutritifs à partir de la dissolution de phases solides. Les argiles mal cristallisées et les phytolithes sont altérés pour fournir du silicium à la plante

Connections between surface- and groundwater compartments in the Critical Zone through isotopic tracing

International audienc